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OPERATION  
MANUAL  
E2005 Lennox Industries Inc.  
Dallas, Texas, USA  
51M37 Merit® Series  
5/2 Day Programmable Thermostat  
CONTROLS  
505,050M  
05/05  
Supersedes 03/05  
Litho U.S.A.  
Table of Contents  
51M37 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .  
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .  
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .  
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .  
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .  
PROG − Thermostat Programming . . . . . . . . . . . . . . .  
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .  
SETTINGS − Filter and Maintenance Reminders . . .  
SETTINGS − Balance Point . . . . . . . . . . . . . . . . . . . . .  
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .  
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .  
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .  
RETAIN THESE INSTRUCTIONS  
FOR FUTURE REFERENCE  
51M37 Merit® Series Thermostat  
®
The Lennox Merit Series 5/2 day programmable  
electronic thermostat 51M37 provides excellent  
temperature control and a large, easy-to-read display. It  
includes a programmable filter change reminder, an  
equipment maintenance reminder, and a system check  
indicator to notify the user when the equipment requires  
service.  
The 51M37 is suitable for heat pump, 3-stage heat/2-stage  
cool applications using a gas or electric auxiliary heat  
source. An optional outdoor temperature sensor provides  
auxiliary heat lockout, balance point operation, and  
dual−fuel compatibility.  
General  
Initial Power−Up Display  
MAINT  
FILTER  
MO TU WE TH FR SA SU  
These instructions are intended as a general guide and do  
not supersede local codes in any way. Consult authorities  
having jurisdiction before installation.  
SERVICE  
AM  
PM I8:88  
Check equipment for shipping damage. If you find any  
damage, immediately contact the last carrier.  
FAN  
ON  
AUTO  
PROG A | B | C | D HOLD  
Introduction  
SYSTEM  
COOL  
OFF  
EMG.  
HEAT  
ROOM  
SETPOINT  
°
This document describes the operation of Lennox thermostat  
51M37. Refer to the installation manual for instructions re-  
garding installation and wiring of the thermostat.  
88°  
Hi  
Lo  
88  
Initial Thermostat Power-up  
Figure 1  
When power is initially applied to the thermostat, the dis-  
All display segments are momentarily activated. This oc-  
curs as a normal part of thermostat initialization.  
05/05  
505,050M  
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Within a few seconds, the Home Screen appears (see fig-  
ure 2) with default settings as shown. After about 1 minute  
of initialization time, the actual room temperature will be  
displayed.  
Thermostat Buttons  
Heat Cool  
Prog Day/Time  
Fan Hold  
programming defaults.  
Home Screen  
MAINT  
(DOOR  
OPEN)  
MO TU WE TH FR SA SU  
FILTER  
Settings Enter  
SERVICE  
AM  
PM I2:00  
FAN  
ON  
AUTO  
PROG A B C D HOLD  
Figure 3  
ROOM  
70°  
SYSTEM  
COOL  
OFF  
When an Arrow, HOLD, HEAT, or COOL button is pressed,  
SETPOINT and the temperature setting appears for 15  
seconds. If desired, start making changes within 15 sec-  
onds or the HOME screen will return. The backlight will  
turn off 15 seconds after the HOME screen reappears.  
HEAT  
Figure 2  
At this point, the thermostat will be fully functional; its de-  
fault temperature setpoint (not shown) is 70°F. At this  
point, if the equipment has been fully powered and if a heat  
demand were present, the system would begin operating.  
DAY/TIME − Setting the Day and Time  
Press the DAY/TIME button and set the CURRENT hour,  
minute, and day of week as follows:  
NOTE − Temperature scale default is Fahrenheit units but  
may be reset to show Celsius, if desired. See page 8.  
1. ꢀAM12" will flash on the screen. Press the Up/Down ar-  
rows buttons to change the hour. (ꢀAM" or ꢀPM" must  
correspond to time of day.) Press DAY/TIME OR, if ad-  
justing for daylight savings time, pressing ENTER  
stores the single change and exits to the HOME  
screen, bypassing minutes and day of week.  
Buttons, Backlight, Timers & Settings  
Buttons are located behind the small door on the right−  
hand side of the thermostat (see figure 3).  
IMPORTANT  
Do NOT begin pressing buttons until after you read  
the following section describing each button.  
2. Minutes will flash. Use the Up/Down arrow buttons to  
display the minutes past the hour. Press DAY/TIME.  
3. Day ꢀMO" (Monday) will flash. Use Up/Down arrow  
buttons to display the current day. Day selections are  
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",  
and ꢀSU". Press DAY/TIME.  
A pale blue display backlight illuminates for 30 seconds  
each time any button is pressed.  
When PROG or DAY/TIME is pressed, a field begins flash-  
ing, expecting another input. Start making changes within  
15 seconds or the HOME screen will return.  
4. The HOME screen reappears; confirm day and time  
are correct. This completes day and time setting.  
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HEAT − Using the Heat Mode  
Heat Mode Disabled  
SYSTEM  
COOL  
OFF  
ROOM  
75°  
Normal Heat Mode  
SETPOINT  
In normal heat mode, both the heat pump and the backup  
heat source are used to provide heat. If the thermostat de-  
tects that the heat pump is not able to provide enough heat  
(as may be the case in very cold weather), then the backup  
heat source is activated.  
75°  
HEAT  
Figure 6  
Heating Demand  
Emergency Heat Mode  
The thermostat must be in either normal or emergency  
heat mode in order to properly control the heating equip-  
ment. In either heat mode, when the actual temperature is  
the thermostat detects a heating demand and activates the  
heating equipment to satisfy the demand. Heating opera-  
tion is indicated by a flame icon in the SYSTEM box.  
In emergency heat mode, only the backup heat source  
provides heat ꢁthe heat pump is disabled. The backup  
heat source is activated only when there is a heat demand.  
Enabling Normal Heat Mode  
Use the HEAT button to select normal heat mode, emer-  
gency heat mode, or to disable heat modes as desired. If  
the thermostat is in OFF or COOL mode, pressing the Heat  
button enables Heat mode. This is indicated by HEAT in  
Heating Demand  
SYSTEM  
ROOM  
SETPOINT  
COOL  
OFF  
Normal Heat Mode Enabled  
72°  
75°  
Figure 7  
Heat Cool  
HEAT  
MO  
AM  
PM 5:30  
FAN  
Prog Day/Time  
HOLD  
AUTO  
SYSTEM  
When the actual temperature rises above the temperature  
setpoint, the flame icon will disappear. This indicates that  
the heating demand has been satisfied and that the heat-  
ing equipment has been turned off.  
ROOM  
SETPOINT  
66°  
Fan Hold  
°
HEAT  
70  
Settings Enter  
Heat pump operation is locked out for 5 minutes after a de-  
mand has been satisfied. If another heat pump demand  
occurs during this 5−minute interval, the flame icon will  
flash; however, the heat pump will not run until the 5−min-  
ute delay has elapsed.  
Figure 4  
NOTE − The heat pump is activated for at least 4 minutes if  
no buttons are pressed during the demand interval. The  
backup heat source is activated for at least 3 minutes if no  
buttons are pressed during the demand interval.  
Enabling Emergency Heat Mode  
If the thermostat is in heat mode, pressing the Heat button  
enables EMG. HEAT mode (see figure 5). If the thermostat  
is in normal heat mode when the HEAT button is pressed,  
then emergency heat mode is enabled.  
If your system supports 3−stage heating (as does the  
51M37 thermostat), you may notice various heating levels  
being delivered during a demand.  
Emergency Heat Mode Enabled  
SYSTEM  
ROOM  
If a small heat pump heating demand is present, "Lo" is dis-  
played in the SYSTEM box; if a large heat pump heating  
SETPOINT  
COOL  
OFF  
EMG.  
HEAT  
62°  
Disabling Heat Mode  
75°  
Large Heat Demand  
Figure 5  
SYSTEM  
COOL  
OFF  
ROOM  
65°  
SETPOINT  
If the thermostat is in emergency heat mode when the  
HEAT button is pressed, then heat modes are disabled.  
This is indicated by OFF in the SYSTEM box as shown in  
75°  
Hi  
HEAT  
Figure 8  
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Cooling Demand  
COOL − Using the Cool Mode  
Enabling and Disabling Cool Mode  
Set the thermostat to cool mode to control the cooling  
equipment. Then, if the room temperature is higher than  
the temperature setpoint, as shown in figure 11, the ther-  
mostat detects a cooling demand and will activate the cool-  
ing equipment to satisfy the demand.  
Use the COOL button to enable or disable cool mode as  
desired. If the thermostat is in HEAT or OFF mode, cool  
mode is enabled when the COOL button is pressed. This is  
indicated by COOL in the SYSTEM box (see figure 9).  
Cooling Demand  
MO TU WE TH FR SA SU  
AM  
PM i0:00  
ROOM  
FAN  
ON  
AUTO  
Turn Cool ON/OFF  
PROG A B C D HOLD  
Heat Cool  
MO  
SYSTEM  
COOL  
OFF  
AM  
PM  
FAN  
I0:30  
Prog Day/Time  
SETPOINT  
75°  
HOLD  
AUTO  
70°  
SYSTEM  
COOL  
ROOM  
Hi  
SETPOINT  
°70°  
Fan Hold  
HEAT  
75  
Figure 11  
Settings Enter  
Cooling operation is indicated by flashing ꢀsnowflake"  
icons in the SYSTEM box. When the actual temperature  
drops below the temperature setpoint, the snowflake icons  
will disappear. This indicates that the cooling demand has  
been satisfied and that the cooling equipment has been  
turned off.  
Figure 9  
If your system supports 2−stage cooling (as does the  
51M37 thermostat), you may notice various cooling levels  
being delivered during a demand. Also, if a small cooling  
demand is present, "Lo" will be displayed in the SYSTEM  
box. However, if a large cooling demand is present, "Hi" will  
be displayed in the SYSTEM box (shown in figure 11).  
If the thermostat is in cool mode, pressing the Cool button  
disables COOL mode (indicated by OFF in the SYSTEM  
box − see figure 10).  
Cool Mode Disabled  
NOTE − If no buttons are pressed during a demand for  
cooling, the equipment must operate for at least 4 minutes.  
After a demand has been satisfied, cooling equipment op-  
eration is locked out for 5 minutes. If another cooling de-  
mand occurs during this 5−minute interval, ꢀCOOL" and the  
snowflakes will flash; however, the cooling equipment will  
not operate until the 5−minute delay has elapsed.  
SYSTEM  
ROOM  
SETPOINT  
COOL  
75°  
OFF  
75°  
HEAT  
Figure 10  
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PROG − Thermostat Programming  
HOLD − Using Temperature Hold Modes  
The 51M37 thermostat can be programmed to perform a  
set of either heating or cooling events (but not a  
combination of heat and cool) for 5 consecutive days using  
a set of 4 unique events per day. The remaining 2 days can  
then be set for a different set of 4 unique events per day.  
Both the consecutive days and the events/temperature are  
set by the homeowner.  
When HOLD is displayed at the HOME screen, the ther-  
mostat is in a temperature hold condition. This means that  
the temperature program data is ignored and the thermo-  
stat functions much like a non−programmable thermostat.  
Adjusting Temperature Setpoint in Hold Mode  
The temperature setpoint represents the desired tempera-  
ture of the space around the thermostat. The default tem-  
perature setpoint in Hold mode is 70°F.  
To Change Consecutive Days...  
To alter the 5 consecutive days, press and hold the  
PROG button for 5 seconds. The 5 consecutive day period  
is then displayed (default is MOnday thru FRiday). To  
change to a different 5-consecutive days, use the Up/  
Down arrow buttons. Any 5 consecutive−day span may be  
selected, for example, in figure 14, Saturday through  
Wednesday is defined as the 5−day programming (Thurs-  
day and Friday would constitute the 2−day programming).  
Press the PROG button when finished.  
To adjust the setpoint, press the UP or DOWN (YB) arrow  
buttons (see figure 12); the existing setpoint is displayed to  
the right of the actual room temperature. Each button  
press adjusts the setpoint up or down by 1 degree.  
Hold Temperature Mode  
Heat Cool  
MO  
AM  
PM  
FAN  
Day/Time  
Prog  
I0:36  
HOLD  
AUTO  
SYSTEM  
COOL  
Change Consecutive days  
ROOM  
SETPOINT  
°68°  
Fan Hold  
MO TU WE TH FR SA SU  
72  
FILTER  
Settings Enter  
Figure 14  
To Set Program Events and Temperatures...  
Figure 15 gives an example of how the two sets of pro-  
grams can be set for a normal workweek and weekend.  
Figure 12  
After the desired setpoint is reached, the HOME screen  
will reappear after about 15 seconds.  
In the 5−day bar graph, note how programs A and C reflect  
the desired warmth while the home IS occupied (72°); B  
allows less heating while the home is NOT occupied; D re-  
flects a cooler sleeping temperature. The 2−day bar graph  
would support day−long occupancy and, because the first  
program begins later, a less−demanding time schedule.  
Permanent Hold Mode  
At any time the program is running, from the HOME  
screen, set a permanent hold (program override) by press-  
ing the HOLD button (see figure 12). The thermostat now  
functions much like a non−programmable thermostat. Use  
the Up/Down arrow buttons to adjust the hold setpoint. To  
return to the program, press HOLD again.  
5/2 Program Example  
|
5−DAY PROGRAM: | A  
|
B
|
C
D
|
72°  
68°  
64°  
6AM  
Temporary (2-Hour) Hold  
At any time the program is running, from the HOME  
screen, set a temporary 2-hour hold by pressing the Up/  
Down arrow buttons until the desired setpoint is displayed;  
ꢀHOLD" flashes (see figure 13). This overrides the pro-  
gram for 2 hours from the last button press, then returns to  
the program.  
NOON  
6PM  
|
MID  
NIGHT  
6AM  
2−DAY PROGRAM:  
| A |  
B
C
|
D
|
72°  
68°  
64°  
6AM  
NOON  
6PM  
MID  
NIGHT  
6AM  
Temporary Hold Temperature Mode  
Figure 15  
Heat Cool  
MO  
NOTE − Pressing ENTER during the following program-  
ming steps, saves and exits to the HOME screen.  
AM  
PM  
FAN  
Day/Time  
Prog  
I0:36  
AUTO  
PROG  
B
HOLD  
SYSTEM  
COOL  
To program events and temperatures, perform the follow-  
ing steps, once with Cool selected and once with Heat se-  
lected.  
ROOM  
SETPOINT  
°68°  
Fan Hold  
72  
Settings Enter  
1. Press and release PROG. AM 6:00", period ꢀA", and  
the 5 consecutive days are displayed; ꢀAM 6" flashes.  
2. Use the Up/Down arrow buttons to select the desired  
hour; press PROG when the desired hour is reached.  
Figure 13  
3. Use the Up/Down arrow buttons to select the desired  
minute; press PROG.  
While in Temporary Hold, press HOLD once to switch to  
Permanent Hold (HOLD displays solid; PROG not dis-  
played); press HOLD again to return to the program  
(PROG displays; HOLD not displayed).  
4. Use the Up/Down arrow buttons to select the desired  
temperature setpoint; press PROG.  
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51M37 SERIES 5/2  
 
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5. Repeat steps 2 through 4 for periods B, C, and D.  
6. Repeat steps 1 through 5 for the 2−day program.  
Reminder Settings Display  
For MAINT,  
press Settings  
button 2 times  
MAINT  
FILTER  
NOTE − This thermostat will NOT automatically switch  
from heating to cooling, or vice versa; operator involve-  
ment is required. At the change of seasons, or to accomo-  
date abnormal seasonal temperature swings, you must  
manually select to the opposite conditioning (Heat or Cool)  
program.  
For FILTER,  
press Settings  
button 1 time  
OFF  
Figure 18  
FAN − Controlling the Fan Operation  
The default setting for the reminders is OFF (disabled).  
Press Up/Down arrow buttons to select the desired re-  
minder intervals.  
Use the FAN button to select either continuous fan mode or  
auto fan mode.  
To change from continuous to auto fan mode (or vice ver-  
sa), press the FAN button. Note whether a fan icon in the  
FAN box is present (indicating that the fan is running) or not  
(fan not running).  
Table 1  
Filter and Maintenance Reminders  
Buttons to  
Use  
Available Settings and  
How to Use  
If continuous fan mode is enabled (ON displayed in FAN  
box − see figure 16), the fan will run continuously regard-  
less of whether the heating or cooling equipment is run-  
ning.  
Reminder  
FILTER  
Settings  
(1st press)  
then Arrows  
to scroll  
Total fan run time expressed in  
months (Off, 1, 3, 6, 12); for  
example, if fan runs 12 hours a  
day, 1 month reminder displays  
in 2 calendar months.  
selections  
Using Fan ON  
Heat Cool  
MO  
Settings  
(2nd press)  
then Arrows  
to scroll  
Elapsed chronological time in  
months (Off, 6, 12). Use this,  
for example, to remind yourself  
when to perform routine checks  
or when to call a technician for  
perodic preventive  
MAINT  
AM  
PM I0:I0  
FAN  
ON  
Day/Time  
Prog  
PROG  
B
SYSTEM  
HEAT  
ROOM  
67°  
selections  
Fan Hold  
maintenance.  
Settings Enter  
Enter  
Stores settings.  
NOTE − The HOME screen will reappear about 15 seconds  
after the final arrow button press. OR, press ENTER at any  
time to store any changes and exit to the HOME screen.  
Figure 16  
If auto fan mode is selected (AUTO displayed in FAN box −  
see figure 17), the fan will only run when the heating or  
cooling equipment is running.  
After either programmed interval has elapsed, the remind-  
Using Fan Auto  
Heat Cool  
Reminders  
MO  
MAINT  
AM  
PM  
FAN  
MO TU WE TH FR SA SU  
FILTER  
Day/Time  
Prog  
I0:36  
PROG  
B
AUTO  
SERVICE  
SYSTEM  
COOL  
ROOM  
Fan Hold  
72°  
AM  
PM  
FAN  
ON  
AUTO  
I0:i2  
Settings Enter  
PROG A B C D HOLD  
ROOM  
SYSTEM  
COOL  
OFF  
Figure 17  
78°  
SETTINGS − Filter/Maintenance Reminders  
HEAT  
The 51M37 thermostat is designed to remind you when the  
filter needs changing or when routine maintenance is re-  
quired, as (and if) defined, by you. These optional remind-  
ers are not enabled until you activate them. To do so, press  
the Settings button (shown below the Fan button in figure  
17) once or twice for the desired reminder as shown in fig-  
ure 18 and as described in table 1.  
Figure 19  
After the filter has been changed or maintenance per-  
formed, reset the reminder by pressing the SETTINGS  
button for 4 seconds. The screen will blink for a few mo-  
ments to indicate that the timer has been reset.  
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Service Indicator Flashing  
SETTINGS − Balance Point  
MO TU WE TH FR SA SU  
FILTER  
SERVICE  
NOTE − The balance point is only available with the out-  
door sensor.  
AM  
PM I0:i4  
If the optional outdoor sensor (X2658) is connected to the  
thermostat, balance point adjustment is available. The bal-  
ance point feature allows the measured outdoor tempera-  
ture to govern operation of the heat pump and backup heat  
source.  
FAN  
ON  
AUTO  
PROG A B C D HOLD  
ROOM  
SYSTEM  
COOL  
OFF  
79°  
Use the SETTINGS button to access the balance point  
setting. If the outdoor sensor is attached to the thermostat,  
press the SETTINGS button three times to access the Bal-  
HEAT  
Figure 21  
Balance Point Setting  
ROOM  
Thermostat RESET  
SETPOINT  
Under some abnormal conditions, it may be necessary to  
ꢀreset" the thermostat to its default condition. Such a RE-  
SET would delete all programming and settings and there-  
fore should only be used on rare occasions when the the  
thermostat fails to function as designd and/or as pro-  
grammed. Such an instance can occur as a result of a pow-  
er surge or similar electrical disturbance (e.g. after an elec-  
trical storm or power outage). The RESET button can be  
used to recover from this situation.  
55°  
6p  
Figure 20  
The balance point can be set at any temperature between  
1°F and 55°F (−17°C to 13°C). A balance point setting of  
zero disables the balance point function (in this case, the  
thermostat behaves as if no outdoor sensor is attached).  
The default balance point setting is 55°F.  
To adjust the balance point up or down, press the Up/Down  
arrow buttons to the right of the display screen. The bal-  
ance point setting increases by 1°F each time the Up but-  
ton is pressed and decreases by 1°F each time the Down  
button is pressed.  
CAUTION  
When the RESET button is pressed, ALL settings re-  
vert back to the defaults, including the default pro-  
gram (see tables 2 and 3).  
After the desired balance point is reached, the HOME  
screen will reappear after about 15 seconds.  
The RESET button is an unlabeled, recessed button lo-  
cated behind the door, on the right−hand side of the ther-  
paper clip or small pencil to press the RESET button; ALL  
thermostat settings will be reset to the defaults listed in the  
Default Thermostat Settings section.  
NOTE − The balance point feature allows the outdoor tem-  
perature to rise or fall 3°F above or below the balance  
point. This prevents excessive cycling of the equipment  
when the outdoor temperature is near the balance point.  
For example, if the balance point is 40°F and the actual  
outdoor temperature is 35°F, the outdoor temperature  
must rise to 43°F before equipment adjustment occurs.  
Conversely, if the balance point is 40°F and the actual out-  
door temperature is 45°F, the outdoor temperature must  
drop to 37°F before equipment adjustment occurs.  
Removing/Installing Thermostat  
The thermostat hinges on tabs on the top of the subbase;  
no tool is needed to remove the thermostat from the sub-  
base. Pivot the bottom of the thermostat outward (releas-  
ing the snaps), then lift up to remove.  
Service Indicator  
When abnormal equipment operation is detected, the  
This indicates that the equipment requires service from a  
qualified service technician.  
To replace it, first position the top tilted toward the wall  
bracket and align it until you feel the tabs and slots engage;  
then, while the top is in place, pivot the bottom toward the  
wall until the thermostat snaps into place.  
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Default Thermostat Settings  
CAUTION  
24VAC is present on the terminals of the thermostat  
bracket. If removing the thermostat from the wall,  
use caution and avoid touching any of the connec-  
tor terminals on the wall bracket.  
Default thermostat settings are shown in table 2 and the  
default program is shown in table 3.  
Table 2  
Also, when working with the thermostat dip  
switches, use a non−conductive tool and take cau-  
tion to avoid making any contact with the circuit  
board, its imprinted circuitry and its connector  
prongs.  
Default Thermostat Settings  
Mode  
Heat (Permanent Hold  
Mode)  
Setpoint  
70°F (or 21°C)  
Fan  
Auto  
Filter Reminder  
Maintenance Reminder  
OFF  
Temperature Display  
Display Scale: Fahrenheit or Celsius user selectable (via  
Display range: 35°F (2°C) to 99°F (37°C)  
Display resolution: 1°F (1°C)  
Display Accuracy: +/−1°F  
OFF  
Equipment Protection  
Timers  
Reset Back to Zero  
Table 3  
Default Program Settings  
If the Fahrenheit/Celsius display must be changed, use a  
plastic, non−conductive tool to push the dip switch to the  
Temperature Temperature  
(Heat) (Cool)  
Programs  
Time  
Changing Fahrenheit/Celsius Setting  
Weekday − A 6:00am 70_F / 21°C 78_F / 26°C  
Weekday − B 8:00am 62_F / 17°C 85_F / 29°C  
Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C  
Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C  
Weekend − A 6:00am 70_F / 21°C 78_F / 26°C  
Weekend − B 8:00am 62_F / 17°C 85_F / 29°C  
Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C  
Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C  
1
2
3
4
5
PLUG IN (Use Care To Avoid Bending Prongs.)  
6
Weekday  
Monday, Tuesday, Wednesday,  
Thursday, Friday  
F/C (Fahrenheit/Celsius) switch  
shown in Celsius position  
Weekend  
Saturday, Sunday  
Figure 22  
Temperature Measurement Range  
Measurement Scale: Fahrenheit  
Measurement Range: 35°F to 99°F  
Measurement Resolution: 0.5°F  
Measurement Accuracy: +/−1°F  
Field Offset: via DIP switches to +/−3°F  
Sampling Method: temperature measurements sampled  
every 15 seconds. Displayed temperature is the average  
of the last four measurements.  
Technical Specifications  
Thermostat Type  
Electronic programmable thermostat for heat pump,  
3-stage heat/2-stage cool.  
NOTE − 2 stages of heat pump heat and one stage of auxil-  
iary (gas or electric) backup heat are supported.  
Temperature Setpoint Range  
Setting range: 50°F (10°C) to 90°F (32°C)  
Setting resolution: 1°F (1°C)  
Power Supply Range  
18VAC − 30VAC (24VAC nominal), 60Hz  
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505050M 5/1/05  
 
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Smart Setback Recovery (via DIP switch #6)  
Smart Setback Recovery (SSR) affects the way the ther-  
mostat responds to program events. If SSR is disabled, the  
thermostat will react to a program event at the time the  
event occurs. However, if SSR is enabled, the thermostat  
will react to a program event before the event occurs such  
that the desired temperature is reached at the time of the  
event, not after.  
Equipment Protection Override  
Both the minimum compressor OFF timer and the mini-  
mum equipment cycle timer can be over−ridden by press-  
ing and holding either the HEAT or COOL button down for 4  
seconds.  
Over−Temperature Protection  
Thermal-mechanical switch opens W1, Y1, Y2, and E at  
93°F+/−6°F.  
Fan Control  
AUTO or ON modes.  
Filter Reminder  
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are  
available. When programmed time has elapsed, a FILTER  
indicator is displayed.  
I/O Relays  
All thermostat relays are latching type to minimize power  
consumption.  
Maintenance Reminder  
Table 4  
51M37 Terminal Designations  
Term. Description  
Settings of Off, 6 or 12 (months of chronological time) are  
available. When programmed time has elapsed, a mainte-  
nance indicator ꢀMAINT" is displayed.  
B
Reversing valve, heat active  
Reversing valve, cool active  
24VAC  
Service Reminder  
O
R
The SERVICE indicator is displayed only under the follow-  
ing conditions:  
Y1  
First-stage cooling/heating, compressor  
energized  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the R terminal;  
W1*  
Y2  
Auxiliary heating, furnace energized  
Second-stage cooling/heating, compressor  
energized  
OR  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the C terminal.  
E*  
G
L
Emergency heat  
Fan control  
Service Indicator  
Power Loss/Recovery  
C
T
24VAC common  
Thermostat memory is retained for a minimum of 24 hours  
during a power loss (includes retention of program infor-  
mation, HOLD status, programmed temperature setpoint,  
heat/cool and fan mode settings, filter reminder status,  
maintenance reminder status, and equipment protection  
timers). After 24 hours of power loss, programmed settings  
will be lost and replaced with default settings.  
Outdoor temperature sensor connection 1  
Outdoor temperature sensor connection 2  
T
* For most applications, E will be jumpered to W1. If sep-  
arate wires are not provided for both E and W1, jumper  
the E terminal to the W1 terminal on the thermostat sub-  
base. For applications involving the use of a balance  
point (whereby the outdoor temperature is to be used to  
restrict either heat pump operation or backup heat  
source operation), the optional outdoor sensor (part  
number X2658) MUST be installed.  
IMPORTANT  
Power must be applied for at least six consecutive  
hours prior to a power loss in order for memory to  
be retained for the specified time.  
Equipment Protection Timers  
Minimum Compressor OFF time: 5 minutes  
Minimum Compressor ON time: 4 minutes  
Minimum Furnace ON time: 3 minutes  
LCD Backlight  
Activated for 30 seconds when any button is pressed.  
Minimum furnace cycle time (elapsed time between any  
furnace activation and the next furnace activation): 6 min-  
utes.  
NOTE − During an electrical storm or similar disturbance,  
the backlight may activate for a few seconds. This is nor-  
mal and will no longer occur after the electrical disturbance  
has passed.  
Minimum elapsed time between any compressor activa-  
tion and the next compressor activation: 6 minutes.  
Thermostat Operating Conditions  
NOTE − All protection timers (except the compressor OFF  
timer) can be over−ridden if a heating or cooling demand is  
initiated or terminated using the UP, DOWN, HEAT, or  
COOL buttons.  
35°F to 105°F, 5% to 90% RH  
Thermostat Storage Conditions  
−40°F to 185°F, 5% to 95% RH  
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51M37 SERIES 5/2  
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Table 5  
Thermostat Output Table  
THERMOSTAT OUTPUTS  
Table5 depicts the 51M37 thermostat output states for var-  
ious input conditions. The following notes described terms  
used in the table.  
W1  
E
Y1 Y2  
G
O
Demand Condition  
Cooling Demands  
SMALL  
X
X
X
X
X
X
NOTES:  
LARGE  
X
X
S X = output is activated with 24VAC.  
S BBP: outdoor temperature is below balance point  
S ABP: outdoor temperature is above balance point  
No Demand  
Heating Demands  
w/Electric Backup (no outdoor sensor)  
S In all cases, the state of the B terminal is opposite that  
SMALL  
LARGE  
X
X
X
X
X
X
X
of the O terminal  
X
X
Very LARGE  
Emergency  
No Demand  
X
S Data are tabulated for AUTO fan setting. If the fan set-  
ting is ON, the G output is activated in all cases  
X
S Upstage timers:  
w/Gas Backup (dual fuel; no outdoor sensor)  
30 minutes (when upstaging from small to large  
demand)  
SMALL  
X
X
X
X
LARGE  
X
15 minutes (HEAT MODE ONLY − when upstaging  
from large to very large demand)  
Very LARGE  
Emergency  
No Demand  
X
X
S The temperature ranges expressed in the following  
definitions of ꢀSMALL/LARGE/VERY LARGE" de-  
mands are for guidance only; actual temperatures  
may vary:  
w/Electric Backup (outdoor sensor attached)  
SMALL (Above Balance Point)  
LARGE (ABP)  
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
With a small HEAT demand, temperature is:  
below setpoint –0.5F but above setpoint –1.5F  
AND 30-minute upstage timer HAS NOT expired.  
Very LARGE (ABP)  
SMALL (Below Balance Point)  
LARGE (BBP)  
With a large HEAT demand, temperature is:  
less than setpoint –1.5F  
OR 30-minute upstage timer HAS expired.  
X
X
Very LARGE (BBP)  
Emergency  
X
X
With a very large heat demand, temperature is:  
below setpoint –2.5F  
OR 15−minute 2 upstage timer HAS expired.  
No Demand  
w/Gas Backup (dual fuel; outdoor sensor attached)  
nd  
SMALL (ABP)  
LARGE (ABP)  
Very LARGE (ABP)  
SMALL (BBP)  
LARGE (BBP)  
Very LARGE (BBP)  
Emergency  
X
X
X
X
Emergency heat demand (only if emergency heat is  
enabled): temperature is below setpoint –0.5F.  
X
X
X
X
X
With a small COOL demand, temperature is:  
above setpoint +0.5F but below setpoint +1.5F  
AND 30-minute upstage timer HAS NOT expired.  
With a large COOL demand, temperature is:  
above setpoint +1.5F  
OR 30-minute upstage timer HAS expired.  
X
No Demand  
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505050M 5/1/05  
 
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OPERATION  
MANUAL  
E2005 Lennox Industries Inc.  
Dallas, Texas, USA  
51M35 Merit® Series  
5/2 Day Programmable Thermostat  
CONTROLS  
505,049M  
05/05  
Supersedes 03/05  
Litho U.S.A.  
Table of Contents  
51M35 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .  
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .  
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .  
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .  
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .  
PROG − Thermostat Programming . . . . . . . . . . . . . . .  
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .  
SETTINGS − Filter /Maintenance Reminder . . . . . . .  
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .  
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .  
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .  
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .  
RETAIN THESE INSTRUCTIONS  
FOR FUTURE REFERENCE  
51M35 Merit® Series Thermostat  
®
The Lennox Merit Series 5/2 day programmable  
electronic thermostat 51M35 provides excellent  
temperature control and a large, easy-to-read display. This  
product includes a programmable filter change reminder,  
an equipment maintenance reminder, and a system check  
indicator to notify the user when the equipment requires  
service.  
Thermostat 51M35 is suitable for non-heat pump, two-  
stage heat/two-stage cool applications using a gas or elec-  
tric furnace.  
General  
Initial Power−Up Display  
MAINT  
FILTER  
MO TU WE TH FR SA SU  
These instructions are intended as a general guide and do  
not supersede local codes in any way. Consult authorities  
having jurisdiction before installation.  
SERVICE  
AM  
PM I8:88  
Check equipment for shipping damage. If you find any  
damage, immediately contact the last carrier.  
FAN  
ON  
AUTO  
PROG A | B | C | D HOLD  
Introduction  
SYSTEM  
COOL  
OFF  
EMG.  
HEAT  
ROOM  
SETPOINT  
°
This document describes the operation of Lennox thermostat  
51M35. Refer to the installation manual for instructions re-  
garding installation and wiring of the thermostat.  
88°  
Hi  
Lo  
88  
Initial Thermostat Power-up  
Figure 1  
When power is initially applied to the thermostat, the dis-  
All display segments are momentarily activated. This oc-  
curs as a normal part of thermostat initialization.  
05/05  
505,049M  
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Within a few seconds, the Home Screen appears (see fig-  
ure 2) with default settings as shown. After about 1 minute  
of initialization time, the actual room temperature will be  
displayed.  
Thermostat Buttons  
Heat Cool  
Prog Day/Time  
Fan Hold  
programming defaults.  
MAINT  
Home Screen  
MO TU WE TH FR SA SU  
FILTER  
(DOOR  
OPEN)  
Settings Enter  
SERVICE  
AM  
PM I2:00  
FAN  
ON  
AUTO  
PROG A B C D HOLD  
Figure 3  
ROOM  
70°  
SYSTEM  
COOL  
OFF  
When an Arrow, HOLD, HEAT, or COOL button is pressed,  
SETPOINT and the temperature setting appears for 15  
seconds. If desired, start making changes within 15 sec-  
onds or the HOME screen will return. The backlight will  
turn off 15 seconds after the HOME screen reappears.  
HEAT  
Figure 2  
At this point, the thermostat will be fully functional; its de-  
fault temperature setpoint (not shown) is 70°F. At this  
point, if the equipment has been fully powered and if a heat  
demand were present, the system would begin operating.  
DAY/TIME − Setting the Day and Time  
Press the DAY/TIME button and set the CURRENT hour,  
minute, and day of week as follows:  
NOTE − Temperature scale default is Fahrenheit units but  
may be reset to show Celsius, if desired. See page 7.  
1. ꢀAM12" will flash on the screen. Press the UP/DOWN  
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-  
spond to time of day.) Press DAY/TIME OR, if adjust-  
ing for daylight savings time, pressing ENTER stores  
the single change and exits to the HOME screen, by-  
passing minutes and day of week.  
Buttons, Backlight, Timers & Settings  
Buttons are located behind the small door on the right−  
hand side of the thermostat (see figure 3).  
IMPORTANT  
Do NOT begin pressing buttons until after you read  
the following section describing each button.  
2. Minutes will flash. Use the UP/DOWN arrow button to  
display the minutes past the hour. Press DAY/TIME.  
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow  
buttons to display the current day. Day selections are  
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",  
and ꢀSU". Press DAY/TIME.  
A pale blue display backlight illuminates for 30 seconds  
each time any button is pressed.  
When PROG or DAY/TIME is pressed, a field begins flash-  
ing, expecting another input. Start making changes within  
15 seconds or the HOME screen will return.  
4. The HOME screen reappears; confirm day and time  
are correct. This completes day and time setting.  
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HEAT − Using the Heat Mode  
COOL − Using the Cool Mode  
Enabling and Disabling Cool Mode  
Enabling and Disabling Heat Mode  
Use the COOL button to enable or disable cool mode as  
desired. If the thermostat is in HEAT or OFF mode, cool  
mode is enabled when the COOL button is pressed. This is  
indicated by COOL in the SYSTEM box (see figure 8).  
The thermostat must be in the HEAT mode to control the  
heating equipment. Press the Heat button to enable or dis-  
able HEAT mode. If the thermostat is in COOL or OFF  
mode, pressing the Heat button enables the HEAT mode  
(indicated by HEAT in the SYSTEM box − see figure 4).  
Turn Cool ON/OFF  
Turn Heat ON/OFF  
Heat Cool  
MO  
Heat Cool  
AM  
PM  
FAN  
I0:30  
Prog Day/Time  
MO  
AM  
PM 5:30  
FAN  
HOLD  
AUTO  
SYSTEM  
COOL  
Prog Day/Time  
ROOM  
SETPOINT  
°70°  
HOLD  
AUTO  
SYSTEM  
Fan Hold  
ROOM  
75  
SETPOINT  
66°  
Fan Hold  
°
HEAT  
70  
Settings Enter  
Settings Enter  
Figure 7  
Figure 4  
If the thermostat is in cool mode, pressing the Cool button  
disables COOL mode (indicated by OFF in the SYSTEM  
box − see figure 5).  
If the thermostat is in heat mode, pressing the Heat button  
disables HEAT mode (indicated by OFF in the SYSTEM  
Cooling Demand  
Set the thermostat to cool mode to control the cooling  
equipment. Then, if the room temperature is higher than  
the temperature setpoint, as shown in figure 8, the thermo-  
stat detects a cooling demand and will activate the cooling  
equipment to satisfy the demand.  
Heat/Cool Mode Disabled  
SYSTEM  
COOL  
OFF  
ROOM  
75°  
Heating Demand  
SETPOINT  
75°  
HEAT  
Cooling Demand  
Figure 5  
SYSTEM  
COOL  
OFF  
ROOM  
SETPOINT  
75°  
Set the thermostat to heat mode to control the heating  
equipment. Then, if the room temperature is lower than the  
temperature setpoint, as shown in figure 6, the thermostat  
detects a heating demand and will activate the heating  
equipment to satisfy the demand.  
70°  
Hi  
HEAT  
Figure 8  
Cooling operation is indicated by flashing ꢀsnowflake"  
icons in the SYSTEM box. When the actual temperature  
drops below the temperature setpoint, the snowflake icons  
will disappear. This indicates that the cooling demand has  
been satisfied and that the cooling equipment has been  
turned off.  
Heating Demand  
SYSTEM  
ROOM  
SETPOINT  
COOL  
72°  
OFF  
75°  
If your system supports 2−stage cooling (as does the  
51M35 thermostat), you may notice various cooling levels  
being delivered during a demand. Also, if a small cooling  
demand is present, "Lo" will be displayed in the SYSTEM  
box. However, if a large cooling demand is present, "Hi" will  
be displayed in the SYSTEM box (shown in figure 8).  
HEAT  
Figure 6  
Heating operation is indicated by a flame icon in the SYS-  
TEM box. When the actual temperature rises above the  
temperature setpoint, the flame icon will disappear. This  
indicates that the heating demand has been satisfied and  
that the heating equipment has been turned off.  
NOTE − If no buttons are pressed during a demand for  
cooling, the equipment must operate for at least 4 minutes.  
After a demand has been satisfied, cooling equipment op-  
eration is locked out for 5 minutes. If another cooling de-  
mand occurs during this 5−minute interval, ꢀCOOL" and the  
snowflakes will flash; however, the cooling equipment will  
not operate until the 5−minute delay has elapsed.  
If your system supports 2−stage heating (as does the  
51M35 thermostat), you may notice various heating levels  
being delivered during a demand.  
NOTE − Heating equipment is activated for at least 3 min-  
utes if no buttons are pressed during the demand interval.  
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HOLD − Using Temperature Hold Modes  
PROG − Thermostat Programming  
The 51M35 thermostat can be programmed to perform a  
set of either heating or cooling events (but not a  
combination of heat and cool) for 5 consecutive days using  
a set of 4 unique events per day. The remaining 2 days can  
then be set for a different set of 4 unique events per day.  
Both the consecutive days and the events/temperature are  
set by the homeowner.  
When HOLD is displayed at the HOME screen, the ther-  
mostat is in a temperature hold condition. This means that  
the temperature program data is ignored and the thermo-  
stat functions much like a non−programmable thermostat.  
Adjusting Temperature Setpoint in Hold Mode  
The temperature setpoint represents the desired tempera-  
ture of the space around the thermostat. The default tem-  
perature setpoint in Hold mode is 70°F.  
To Change Consecutive Days...  
To alter the 5 consecutive days, press and hold the PROG  
button for 5 seconds. The existing 5 consecutive day peri-  
od is then displayed (default is MOnday thru FRiday). To  
change to a different 5-consecutive days, use the Up/  
Down arrow buttons. Any 5 consecutive−day span may be  
selected, for example, in figure 11, Saturday through  
Wednesday is defined as the 5−day programming (Thurs-  
day and Friday would constitute the 2−day programming).  
Press the PROG button when finished.  
To adjust the setpoint, press the UP or DOWN (YB) arrow  
the right of the actual room temperature. Each button  
press adjusts the setpoint up or down by 1 degree.  
Hold Temperature Mode  
Heat Cool  
MO  
AM  
PM  
Change Consecutive days  
FAN  
Day/Time  
Prog  
I0:36  
HOLD  
AUTO  
MO TU WE TH FR SA SU  
SYSTEM  
COOL  
FILTER  
ROOM  
SETPOINT  
°68°  
Fan Hold  
72  
Figure 11  
Settings Enter  
To Set Program Events and Temperatures...  
Figure 12 gives an example of how the two sets of pro-  
grams can be set for a normal workweek and weekend.  
In the 5−day bar graph, note how programs A and C reflect  
the desired warmth while the home IS occupied (72°); B  
allows less heating while the home is NOT occupied; D re-  
flects a cool sleeping temperature. The 2−day bar graph  
would support day−long occupancy and, because the first  
program begins later, a less−demanding time schedule.  
Figure 9  
After the desired setpoint is reached, the HOME screen  
will reappear after about 15 seconds.  
Permanent Hold Mode  
At any time the program is running, from the HOME  
screen, set a permanent hold (program override) by press-  
ing the HOLD button (see figure 9). The thermostat now  
functions much like a non−programmable thermostat. Use  
the Up/Down arrow buttons to adjust the hold setpoint. To  
return to the program, press HOLD again.  
5/2 Program Example  
5−DAY PROGRAM: | A  
|
B
|
C
|
D
|
72°  
68°  
64°  
6AM  
Temporary (2-Hour) Hold  
NOON  
6PM  
|
MID  
NIGHT  
6AM  
At any time the program is running, from the HOME  
screen, set a temporary 2-hour hold by pressing the Up/  
Down arrow buttons until the desired setpoint is displayed;  
ꢀHOLD" flashes (see figure 10). This overrides the pro-  
gram for 2 hours from the last button press, then returns to  
the program.  
2−DAY PROGRAM:  
| A |  
B
C
|
D
|
72°  
68°  
64°  
6AM  
NOON  
6PM  
MID  
NIGHT  
6AM  
Figure 12  
Temporary Hold Temperature Mode  
NOTE − Pressing ENTER during the following program-  
ming steps, saves and exits to the HOME screen.  
Heat Cool  
MO  
AM  
PM  
FAN  
To program events and temperatures, perform the follow-  
ing steps, once with Cool selected and once with Heat se-  
lected.  
Day/Time  
Prog  
I0:36  
AUTO  
PROG  
B
HOLD  
SYSTEM  
COOL  
ROOM  
SETPOINT  
°68°  
Fan Hold  
72  
1. Press and release PROG. AM 6:00", period ꢀA", and  
the 5 consecutive days are displayed; ꢀAM 6" flashes.  
2. Use the Up/Down arrow buttons to select the desired  
hour; press PROG when the desired hour is reached.  
3. Use the Up/Down arrow buttons to select the desired  
minute; press PROG.  
Settings Enter  
Figure 10  
4. Use the Up/Down arrow buttons to select the desired  
temperature setpoint; press PROG.  
While in Temporary Hold, press HOLD once to switch to  
Permanent Hold (HOLD displays solid; PROG not dis-  
played); press HOLD again to return to the program  
(PROG displays; HOLD not displayed).  
5. Repeat steps 2 through 4 for periods B, C, and D.  
6. Repeat steps 1 through 5 for the 2−day program.  
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NOTE − This thermostat will NOT automatically switch  
from heating to cooling, or vice versa; operator involve-  
ment is required. At the change of seasons, or to accomo-  
date abnormal seasonal temperature swings, you must  
manually select to the opposite conditioning (Heat or Cool)  
program.  
Reminder Settings Display  
For MAINT,  
press Settings  
button 2 times  
MAINT  
FILTER  
For FILTER,  
press Settings  
button 1 time  
OFF  
FAN − Controlling the Fan Operation  
Use the FAN button to select either continuous fan mode or  
auto fan mode.  
Figure 15  
The default setting for the reminders is OFF (disabled).  
Press Up/Down arrow buttons to select the desired re-  
minder intervals.  
To change from continuous to auto fan mode (or vice ver-  
sa), press the FAN button. Note whether a fan icon in the  
FAN box is present (indicating that the fan is running) or not  
(fan not running).  
Table 1  
Filter and Maintenance Reminders  
If continuous fan mode is enabled (ON displayed in FAN  
box − see figure 13), the fan will run continuously regard-  
less of whether the heating or cooling equipment is run-  
ning.  
Buttons to  
Use  
Available Settings and  
How to Use  
Reminder  
FILTER  
Settings  
(1st press)  
then Arrows  
to scroll  
Total fan run time expressed in  
months (Off, 1, 3, 6, 12); for  
example, if fan runs 12 hours a  
day, 1 month reminder displays  
in 2 calendar months.  
Using Fan ON  
Heat Cool  
MO  
AM  
PM I0:I0  
FAN  
ON  
selections  
Day/Time  
Prog  
PROG  
B
SYSTEM  
HEAT  
Settings  
(2nd press)  
then Arrows  
to scroll  
Elapsed chronological time in  
months (Off, 6, 12). Use this,  
for example, to remind yourself  
when to perform routine checks  
or when to call a technician for  
perodic preventive  
MAINT  
ROOM  
67°  
Fan Hold  
Settings Enter  
selections  
maintenance.  
Figure 13  
Enter  
Stores settings.  
If auto fan mode is selected (AUTO displayed in FAN box −  
see figure 14), the fan will only run when the heating or  
cooling equipment is running.  
NOTE − The HOME screen will reappear about 15 seconds  
after the final arrow button press. OR, press ENTER at any  
time to store any changes and exit to the HOME screen.  
After either programmed interval has elapsed, the remind-  
Using Fan Auto  
Heat Cool  
MO  
Reminders  
MAINT  
AM  
PM  
FAN  
Day/Time  
Prog  
I0:36  
MO TU WE TH FR SA SU  
FILTER  
PROG  
B
AUTO  
SYSTEM  
COOL  
ROOM  
SERVICE  
Fan Hold  
72°  
AM  
PM  
FAN  
ON  
AUTO  
I0:i2  
Settings Enter  
PROG A B C D HOLD  
ROOM  
SYSTEM  
COOL  
OFF  
Figure 14  
78°  
SETTINGS − Filter/Maintenance Reminders  
HEAT  
The 51M35 thermostat is designed to remind you when the  
filter needs changing or when routine maintenance is re-  
quired, as (and if) defined, by you. These optional remind-  
ers are not enabled until you activate them. To do so, press  
the Settings button (shown below the Fan button in figure  
14) once or twice for the desired reminder as shown in fig-  
ure 15 and as described in table 1.  
Figure 16  
After the filter has been changed or maintenance per-  
formed, reset the reminder by pressing the SETTINGS  
button for 4 seconds. The screen will blink for a few mo-  
ments to indicate that the timer has been reset.  
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51M35 SERIES 5/2  
 
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Service Indicator  
Removing and Installing Thermostat  
The thermostat hinges on tabs on the top of the subbase;  
no tool is needed to remove the thermostat from the sub-  
base. Pivot the bottom of the thermostat outward (releas-  
ing the snaps), then lift up to remove.  
When abnormal equipment operation is detected, the  
This indicates that the equipment requires service from a  
qualified service technician.  
To install it, first position the top tilted toward the wall brack-  
et and align it until you feel the tabs and slots engage; then,  
while the top is in place, pivot the bottom toward the wall  
until the thermostat snaps into place.  
Service Indicator Flashing  
MO TU WE TH FR SA SU  
FILTER  
SERVICE  
Default Thermostat Settings  
AM  
FAN  
PM I0:i4  
ON  
Default thermostat settings are in table 2 and the default  
program is shown in table 3.  
AUTO  
PROG A B C D HOLD  
ROOM  
Table 2  
SYSTEM  
COOL  
OFF  
Default Thermostat Settings  
79°  
Mode  
Heat (Permanent Hold  
Mode)  
HEAT  
Setpoint  
70°F (or 21°C)  
Figure 17  
Fan  
Auto  
Filter Reminder  
Maintenance Reminder  
OFF  
Thermostat RESET  
OFF  
Under some abnormal conditions, it may be necessary to  
ꢀreset" the thermostat to its default condition. Such a RE-  
SET would delete all programming and settings and there-  
fore should only be used on rare occasions when the the  
thermostat fails to function as designd and/or as pro-  
grammed. Such an instance can occur as a result of a pow-  
er surge or similar electrical disturbance (e.g. after an elec-  
trical storm or power outage). The RESET button can be  
used to recover from this situation.  
Equipment Protection  
Timers  
Reset Back to Zero  
Table 3  
Default Program Settings  
Temperature Temperature  
(Heat) (Cool)  
Programs  
Time  
Weekday − A 6:00am 70_F / 21°C 78_F / 26°C  
Weekday − B 8:00am 62_F / 17°C 85_F / 29°C  
Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C  
Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C  
Weekend − A 6:00am 70_F / 21°C 78_F / 26°C  
Weekend − B 8:00am 62_F / 17°C 85_F / 29°C  
Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C  
Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C  
CAUTION  
When the RESET button is pressed, ALL settings re-  
vert back to the defaults, including the default pro-  
gram (see tables 2 and 3).  
The RESET button is an unlabeled, recessed button lo-  
cated behind the door on the right−hand side of the thermo-  
stat, below the SETTINGS button (see figure 3). Use a pa-  
per clip or small pencil to press the RESET button; ALL  
thermostat settings will be reset to the defaults listed in the  
Default Thermostat Settings section.  
Weekday  
Monday, Tuesday, Wednesday,  
Thursday, Friday  
Weekend  
Saturday, Sunday  
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Temperature Setpoint Range  
Setting range: 50°F (10°C) to 90°F (32°C)  
Setting resolution: 1°F (1°C)  
Technical Specifications  
Thermostat Type  
Electronic programmable thermostat for 2−Stage (gas or  
electric) Heat/2-Stage Cool, non−heat pump, non−power  
robbing applications.  
Smart Setback Recovery (via DIP switch #6)  
Smart Setback Recovery (SSR) affects the way the ther-  
mostat responds to program events. If SSR is disabled, the  
thermostat will react to a program event at the time the  
event occurs. However, if SSR is enabled, the thermostat  
will react to a program event before the event occurs such  
that the desired temperature is reached at the time of the  
event, not after.  
Power Supply Range  
18VAC − 30VAC (24VAC nominal), 60Hz  
CAUTION  
Fan Control  
AUTO or ON modes, gas or electric heat compatible via  
DIP switches (also see Thermostat Output section).  
24VAC is present on the terminals of the thermostat  
bracket. If removing the thermostat from the wall,  
use caution and avoid touching any of the connec-  
tor terminals on the wall bracket.  
I/O Relays  
All thermostat relays are latching type to minimize power  
consumption.  
Also, when working with the thermostat dip  
switches, use a non−conductive tool and take cau-  
tion to avoid making any contact with the circuit  
board, its imprinted circuitry and its connector  
prongs.  
Table 4  
51M35 Terminal Designations  
Terminal  
Description  
R
24VAC  
Temperature Display  
Display Scale: Fahrenheit or Celsius user selectable (via  
Display range: 35°F (2°C) to 99°F (37°C)  
Display resolution: 1°F (1°C)  
Display Accuracy: +/−1°F  
Y1  
W1  
Y2  
W2  
G
First-stage cooling  
First-stage heating  
Second-stage cooling  
Second-stage heating  
Fan control  
If the Fahrenheit/Celsius display must be changed, use a  
plastic, non−conductive tool to push the dip switch to the  
L
Service Indicator  
24VAC common  
C
Changing Fahrenheit/Celsius Setting  
Equipment Protection Timers  
Minimum Compressor OFF time: 5 minutes  
Minimum Compressor ON time: 4 minutes  
Minimum Furnace ON time: 3 minutes  
Minimum furnace cycle time (elapsed time between any  
furnace activation and the next furnace activation): 6 min-  
utes.  
1
2
3
4
5
Minimum elapsed time between any compressor activa-  
tion and the next compressor activation: 6 minutes.  
NOTE − All protection timers (except the compressor OFF  
timer) can be over−ridden if a heating or cooling demand is  
initiated or terminated using the UP, DOWN, HEAT, or  
COOL buttons.  
PLUG IN (Use Care To Avoid Bending Prongs.)  
6
F/C (Fahrenheit/Celsius) switch  
shown in Celsius position  
Equipment Protection Override  
Both the minimum compressor OFF timer and the mini-  
mum equipment cycle timer can be over−ridden by press-  
ing and holding either the HEAT or COOL button down for 4  
seconds.  
Figure 18  
Temperature Measurement Range  
Measurement Scale: Fahrenheit  
Measurement Range: 35°F to 99°F  
Measurement Resolution: 0.5°F  
Measurement Accuracy: +/−1°F  
Over−Temperature Protection  
Thermal-mechanical switch opens W1 and W2 at  
93°F+/−6°F.  
Field Offset: via DIP switches to +/−3°F  
Sampling Method: temperature measurements sampled  
every 15 seconds. Displayed temperature is the average  
of the last four measurements.  
Filter Reminder  
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are  
available. When programmed time has elapsed, a FILTER  
indicator is displayed.  
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51M35 SERIES 5/2  
 
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NOTE − The temperature ranges expressed in the follow-  
ing definitions of ꢀSMALL/LARGE" demands are for guid-  
ance only; actual temperatures may vary.  
Maintenance Reminder  
Settings of Off, 6 or 12 (months of chronological time) are  
available. When programmed time has elapsed, a mainte-  
nance indicator ꢀMAINT" is displayed.  
With a small HEAT demand, temperature is:  
below setpoint –0.5F but above setpoint –1.5F  
AND 30-minute upstage timer HAS NOT expired.  
Service Reminder  
The SERVICE indicator is displayed only under the follow-  
ing conditions:  
With a large HEAT demand, temperature is:  
less than setpoint –1.5F  
OR 30-minute upstage timer HAS expired.  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the R terminal;  
With a small COOL demand, temperature is:  
above setpoint +0.5F but below setpoint +1.5F  
AND 30-minute upstage timer HAS NOT expired.  
OR  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the C terminal.  
With a large COOL demand, temperature is:  
above setpoint +1.5F  
OR 30-minute upstage timer HAS expired.  
Power Loss/Recovery  
Table 5  
Thermostat memory is retained for a minimum of 24 hours  
during a power loss (includes retention of program infor-  
mation, HOLD status, programmed temperature setpoint,  
heat/cool and fan mode settings, filter reminder status,  
maintenance reminder status, and equipment protection  
timers). After 24 hours of power loss, programmed settings  
will be lost and replaced with default settings.  
Thermostat Outputs  
Condition  
W1 W2 Y1 Y2  
G
Gas Heat, Auto Fan  
HEAT Demand, SMALL  
HEAT Demand, LARGE  
COOL Demand, SMALL  
COOL Demand, LARGE  
No Demand  
X
X
X
X
X
X
X
X
X
X
X
X
X
IMPORTANT  
Power must be applied for at least six consecutive  
hours prior to a power loss in order for memory to  
be retained for the specified time.  
Gas Heat, Continuous Fan  
HEAT Demand, SMALL  
HEAT Demand, LARGE  
COOL Demand, SMALL  
COOL Demand, LARGE  
No Demand  
X
X
X
X
X
X
X
LCD Backlight  
X
X
Activated for 30 seconds when any button is pressed.  
NOTE − During an electrical storm or similar disturbance,  
the backlight may activate for a few seconds. This is nor-  
mal and will no longer occur after the electrical disturbance  
has passed.  
Electric Heat, Auto Fan  
HEAT Demand, SMALL  
HEAT Demand, LARGE  
COOL Demand, SMALL  
COOL Demand, LARGE  
No Demand  
X
X
X
X
X
X
Thermostat Operating Conditions  
X
X
35°F to 105°F, 5% to 90% RH  
Thermostat Storage Conditions  
−40°F to 185°F, 5% to 95% RH  
Electric Heat, Continuous Fan  
Thermostat Output Table  
HEAT Demand, SMALL  
HEAT Demand, LARGE  
COOL Demand, SMALL  
COOL Demand, LARGE  
No Demand  
X
X
X
X
X
X
X
X
Table5 depicts the 51M35 thermostat output states for var-  
ious input conditions. The following notes described terms  
used in the table.  
X
X
NOTE − X = output is activated with 24VAC.  
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OPERATION  
MANUAL  
E2005 Lennox Industries Inc.  
Dallas, Texas, USA  
51M34 Merit® Series  
5/2 Day Programmable Thermostat  
CONTROLS  
505,048M  
05/05  
Supersedes 03/05  
Litho U.S.A.  
Table of Contents  
51M34 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .  
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .  
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .  
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .  
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .  
PROG − Thermostat Programming . . . . . . . . . . . . . . .  
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .  
SETTINGS − Filter /Maintenance Reminder . . . . . . .  
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .  
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .  
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .  
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .  
RETAIN THESE INSTRUCTIONS  
FOR FUTURE REFERENCE  
51M34 Merit® Series Thermostat  
®
The Lennox Merit Series 5/2 day programmable  
electronic thermostat 51M34 provides excellent  
temperature control and a large, easy-to-read display. This  
product includes a programmable filter change reminder,  
an equipment maintenance reminder, and a system check  
indicator to notify the user when the equipment requires  
service.  
Thermostat 51M34 is suitable for non−heat pump, single-  
stage heat/single-stage cool applications with either gas or  
electric furnace.  
General  
Initial Power−Up Display  
MAINT  
FILTER  
MO TU WE TH FR SA SU  
These instructions are intended as a general guide and do  
not supersede local codes in any way. Consult authorities  
having jurisdiction before installation.  
SERVICE  
AM  
PM I8:88  
Check equipment for shipping damage. If you find any  
damage, immediately contact the last carrier.  
FAN  
ON  
AUTO  
PROG A | B | C | D HOLD  
Introduction  
SYSTEM  
COOL  
OFF  
EMG.  
HEAT  
ROOM  
SETPOINT  
°
This document describes the operation of Lennox thermostat  
51M34. Refer to the installation manual for instructions re-  
garding installation and wiring of the thermostat.  
88°  
Hi  
Lo  
88  
Initial Thermostat Power-up  
Figure 1  
When power is initially applied to the thermostat, the dis-  
All display segments are momentarily activated. This oc-  
curs as a normal part of thermostat initialization.  
05/05  
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Within a few seconds, the Home Screen appears (see fig-  
ure 2) with default settings as shown. After about 1 minute  
of initialization time, the actual room temperature will be  
displayed.  
Thermostat Buttons  
Heat Cool  
Prog Day/Time  
Fan Hold  
MAINT  
Home Screen  
MO TU WE TH FR SA SU  
FILTER  
SERVICE  
(DOOR  
OPEN)  
AM  
PM I2:00  
Settings Enter  
FAN  
ON  
AUTO  
PROG A B C D HOLD  
ROOM  
70°  
SYSTEM  
COOL  
OFF  
Figure 3  
HEAT  
When an Arrow, HOLD, HEAT, or COOL button is pressed,  
SETPOINT and the temperature setting appears for 15  
seconds. If desired, start making changes within 15 sec-  
onds or the HOME screen will return. The backlight will  
turn off 15 seconds after the HOME screen reappears.  
Figure 2  
At this point, the thermostat will be fully functional; its de-  
fault temperature setpoint (not shown) is 70°F. At this  
point, if the equipment has been fully powered and if a heat  
demand were present, the system would begin operating.  
DAY/TIME − Setting the Day and Time  
NOTE − Temperature scale default is Fahrenheit units but  
may be reset to show Celsius, if desired. See page 7.  
Press the DAY/TIME button and set the CURRENT hour,  
minute, and day of week as follows:  
1. ꢀAM12" will flash on the screen. Press the UP/DOWN  
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-  
spond to time of day.) Press DAY/TIME OR, if adjust-  
ing for daylight savings time, pressing ENTER stores  
the single change and exits to the HOME screen, by-  
passing minutes and day of week.  
Buttons, Backlight, Timers & Settings  
Buttons are located behind the small door on the right−  
hand side of the thermostat (see figure 3).  
IMPORTANT  
Do NOT begin pressing buttons until after you read  
the following section describing each button.  
2. Minutes will flash. Use the UP/DOWN arrow button to  
display the minutes past the hour. Press DAY/TIME.  
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow  
buttons to display the current day. Day selections are  
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",  
and ꢀSU". Press DAY/TIME.  
A pale blue display backlight illuminates for 30 seconds  
each time any button is pressed.  
When PROG or DAY/TIME is pressed, a field begins flash-  
ing, expecting another input. Start making changes within  
15 seconds or the HOME screen will return.  
4. The HOME screen reappears; confirm day and time  
are correct. This completes day and time setting.  
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HEAT − Using the Heat Mode  
COOL − Using the Cool Mode  
Enabling and Disabling Cool Mode  
Enabling and Disabling Heat Mode  
The thermostat must be in the HEAT mode to control the  
heating equipment. Press the Heat button to enable or dis-  
able HEAT mode. If the thermostat is in COOL or OFF  
mode, pressing the Heat button enables the HEAT mode  
(indicated by HEAT in the SYSTEM box − see figure 4).  
Use the COOL button to enable or disable cool mode as  
desired. If the thermostat is in HEAT or OFF mode, cool  
mode is enabled when the COOL button is pressed. This is  
indicated by COOL in the SYSTEM box (see figure 7).  
Turn Heat ON/OFF  
Turn Cool ON/OFF  
Heat Cool  
Heat Cool  
MO  
MO  
AM  
PM 5:30  
FAN  
AM  
PM  
FAN  
I0:30  
Prog Day/Time  
Prog Day/Time  
Fan Hold  
HOLD  
AUTO  
HOLD  
AUTO  
SYSTEM  
SYSTEM  
COOL  
ROOM  
ROOM  
SETPOINT  
°70°  
SETPOINT  
66°  
Fan Hold  
°
HEAT  
75  
70  
Settings Enter  
Settings Enter  
Figure 4  
Figure 7  
If the thermostat is in heat mode, heat mode is disabled  
when the HEAT button is pressed. This is indicated by OFF  
If the thermostat is in cool mode, pressing the Cool button  
disables COOL mode (indicated by OFF in the SYSTEM  
box − see figure 5).  
Heat/Cool Mode Disabled  
Cooling Demand  
SYSTEM  
COOL  
OFF  
ROOM  
75°  
Heating Demand  
Set the thermostat to cool mode to control the cooling  
equipment. Then, if the room temperature is higher than  
the temperature setpoint, as shown in figure 8, the thermo-  
stat detects a cooling demand and will activate the cooling  
equipment to satisfy the demand.  
SETPOINT  
75°  
HEAT  
Figure 5  
Cooling Demand  
Set the thermostat to heat mode to control the heating  
equipment. Then, if the room temperature is lower than the  
temperature setpoint, as shown in figure 6, the thermostat  
detects a heating demand and will activate the heating  
equipment to satisfy the demand.  
SYSTEM  
COOL  
OFF  
ROOM  
75°  
SETPOINT  
70°  
HEAT  
Figure 8  
Heating Demand  
SYSTEM  
ROOM  
Cooling operation is indicated by flashing ꢀsnowflake"  
icons in the SYSTEM box. When the actual temperature  
drops below the temperature setpoint, the snowflake icons  
will disappear. This indicates that the cooling demand has  
been satisfied and that the cooling equipment has been  
turned off.  
SETPOINT  
COOL  
72°  
OFF  
75°  
HEAT  
Figure 6  
NOTE − If no buttons are pressed during a demand for  
cooling, the equipment must operate for at least 4 minutes.  
After a demand has been satisfied, cooling equipment op-  
eration is locked out for 5 minutes. If another cooling de-  
mand occurs during this 5−minute interval, ꢀCOOL" and the  
snowflakes will flash; however, the cooling equipment will  
not operate until the 5−minute delay has elapsed.  
Heating operation is indicated by a flame icon in the SYS-  
TEM box. When the actual temperature rises above the  
temperature setpoint, the flame icon will disappear. This  
indicates that the heating demand has been satisfied and  
that the heating equipment has been turned off.  
NOTE − Heating equipment is activated for at least 3 min-  
utes if no buttons are pressed during the demand interval.  
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51M34 SERIES 5/2  
 
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HOLD − Using Temperature Hold Modes  
PROG − Thermostat Programming  
The 51M34 thermostat can be programmed to perform a  
set of either heating or cooling events (but not a  
combination of heat and cool) for 5 consecutive days using  
a set of 4 unique events per day. The remaining 2 days can  
then be set for a different set of 4 unique events per day.  
Both the consecutive days and the events/temperature are  
set by the homeowner.  
When HOLD is displayed at the HOME screen, the ther-  
mostat is in a temperature hold condition. This means that  
the temperature program data is ignored and the thermo-  
stat functions much like a non−programmable thermostat.  
Adjusting Temperature Setpoint in Hold Mode  
The temperature setpoint represents the desired tempera-  
ture of the space around the thermostat. The default tem-  
perature setpoint in Hold mode is 70°F.  
To Change Consecutive Days...  
To alter the 5 consecutive days, press and hold the PROG  
button for 5 seconds. The existing 5 consecutive day peri-  
od is then displayed (default is MOnday thru FRiday). To  
change to a different 5-consecutive days, use the Up/  
Down arrow buttons. Any 5 consecutive−day span may be  
selected, for example, in figure 11, Saturday through  
Wednesday is defined as the 5−day programming (Thurs-  
day and Friday would constitute the 2−day programming).  
Press the PROG button when finished.  
To adjust the setpoint, press the UP or DOWN (YB) arrow  
the right of the actual room temperature. Each button  
press adjusts the setpoint up or down by 1 degree.  
Hold Temperature Mode  
Heat Cool  
MO  
AM  
PM  
Change Consecutive days  
FAN  
Day/Time  
Prog  
I0:36  
HOLD  
AUTO  
MO TU WE TH FR SA SU  
SYSTEM  
COOL  
FILTER  
ROOM  
SETPOINT  
°68°  
Fan Hold  
72  
Figure 11  
Settings Enter  
To Set Program Events and Temperatures...  
Figure 12 gives an example of how the two sets of pro-  
grams can be set for a normal workweek and weekend.  
In the 5−day bar graph, note how programs A and C reflect  
the desired warmth while the home IS occupied (72°); B  
allows less heating while the home is NOT occupied; D re-  
flects a cool sleeping temperature. The 2−day bar graph  
would support day−long occupancy and, because the first  
program begins later, a less−demanding time schedule.  
Figure 9  
After the desired setpoint is reached, the HOME screen  
will reappear after about 15 seconds.  
Permanent Hold Mode  
At any time the program is running, from the HOME  
screen, set a permanent hold (program override) by press-  
ing the HOLD button (see figure 9). The thermostat now  
functions much like a non−programmable thermostat. Use  
the Up/Down arrow buttons to adjust the hold setpoint. To  
return to the program, press HOLD again.  
5/2 Program Example  
5−DAY PROGRAM: | A  
|
B
|
C
|
D
|
72°  
68°  
64°  
6AM  
Temporary (2-Hour) Hold  
NOON  
6PM  
|
MID  
NIGHT  
6AM  
At any time the program is running, from the HOME  
screen, set a temporary 2-hour hold by pressing the Up/  
Down arrow buttons until the desired setpoint is displayed;  
ꢀHOLD" flashes (see figure 10). This overrides the pro-  
gram for 2 hours from the last button press, then returns to  
the program.  
2−DAY PROGRAM:  
| A |  
B
C
|
D
|
72°  
68°  
64°  
6AM  
NOON  
6PM  
MID  
NIGHT  
6AM  
Figure 12  
Temporary Hold Temperature Mode  
NOTE − Pressing ENTER during the following program-  
ming steps, saves and exits to the HOME screen.  
Heat Cool  
MO  
AM  
PM  
FAN  
To program events and temperatures, perform the follow-  
ing steps, once with Cool selected and once with Heat se-  
lected.  
Day/Time  
Prog  
I0:36  
AUTO  
PROG  
B
HOLD  
SYSTEM  
COOL  
ROOM  
SETPOINT  
°68°  
Fan Hold  
72  
1. Press and release PROG. AM 6:00", period ꢀA", and  
the 5 consecutive days are displayed; ꢀAM 6" flashes.  
2. Use the Up/Down arrow buttons to select the desired  
hour; press PROG when the desired hour is reached.  
3. Use the Up/Down arrow buttons to select the desired  
minute; press PROG.  
Settings Enter  
Figure 10  
4. Use the Up/Down arrow buttons to select the desired  
temperature setpoint; press PROG.  
While in Temporary Hold, press HOLD once to switch to  
Permanent Hold (HOLD displays solid; PROG not dis-  
played); press HOLD again to return to the program  
(PROG displays; HOLD not displayed).  
5. Repeat steps 2 through 4 for periods B, C, and D.  
6. Repeat steps 1 through 5 for the 2−day program.  
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NOTE − This thermostat will NOT automatically switch  
from heating to cooling, or vice versa; operator involve-  
ment is required. At the change of seasons, or to accomo-  
date abnormal seasonal temperature swings, you must  
manually select to the opposite conditioning (Heat or Cool)  
program.  
Reminder Settings Display  
For MAINT,  
press Settings  
button 2 times  
MAINT  
FILTER  
For FILTER,  
press Settings  
button 1 time  
OFF  
FAN − Controlling the Fan Operation  
Figure 15  
Use the FAN button to select either continuous fan mode or  
auto fan mode.  
The default setting for the reminders is OFF (disabled).  
Press Up/Down arrow buttons to select the desired re-  
minder intervals.  
To change from continuous to auto fan mode (or vice ver-  
sa), press the FAN button. Note whether a fan icon in the  
FAN box is present (indicating that the fan is running) or not  
(fan not running).  
Table 1  
Filter and Maintenance Reminders  
If continuous fan mode is enabled (ON displayed in FAN  
box − see figure 13), the fan will run continuously regard-  
less of whether the heating or cooling equipment is run-  
ning.  
Buttons to  
Use  
Available Settings and  
How to Use  
Reminder  
FILTER  
Settings  
(1st press)  
then Arrows  
to scroll  
Total fan run time expressed in  
months (Off, 1, 3, 6, 12); for  
example, if fan runs 12 hours a  
day, 1 month reminder displays  
in 2 calendar months.  
Using Fan ON  
Heat Cool  
MO  
AM  
PM I0:I0  
FAN  
ON  
selections  
Day/Time  
Prog  
PROG  
B
Settings  
(2nd press)  
then Arrows  
to scroll  
Elapsed chronological time in  
months (Off, 6, 12). Use this,  
for example, to remind yourself  
when to perform routine checks  
or when to call a technician for  
perodic preventive  
MAINT  
SYSTEM  
HEAT  
ROOM  
67°  
Fan Hold  
selections  
Settings Enter  
maintenance.  
Enter  
Stores settings.  
Figure 13  
NOTE − The HOME screen will reappear about 15 seconds  
after the final arrow button press. OR, press ENTER at any  
time to store any changes and exit to the HOME screen.  
If auto fan mode is selected (AUTO displayed in FAN box −  
see figure 14), the fan will only run when the heating or  
cooling equipment is running.  
After either programmed interval has elapsed, the remind-  
Using Fan Auto  
Heat Cool  
MO  
Reminders  
MAINT  
AM  
PM  
FAN  
Day/Time  
Prog  
MO TU WE TH FR SA SU  
I0:36  
FILTER  
PROG  
B
AUTO  
SYSTEM  
COOL  
ROOM  
SERVICE  
Fan Hold  
72°  
AM  
PM  
FAN  
ON  
AUTO  
I0:i2  
Settings Enter  
PROG A B C D HOLD  
ROOM  
SYSTEM  
COOL  
OFF  
Figure 14  
78°  
SETTINGS − Filter/Maintenance Reminders  
HEAT  
The 51M34 thermostat is designed to remind you when the  
filter needs changing or when routine maintenance is re-  
quired, as (and if) defined, by you. These optional remind-  
ers are not enabled until you activate them. To do so, press  
the Settings button (shown below the Fan button in figure  
14) once or twice for the desired reminder as shown in fig-  
ure 15 and as described in table 1.  
Figure 16  
After the filter has been changed, reset the filter reminder  
by pressing the SETTINGS button for 4 seconds. The  
screen will blink for a few moments to indicate that the tim-  
er has been reset.  
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51M34 SERIES 5/2  
 
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Service Indicator  
Removing/Installing Thermostat  
The thermostat hinges on tabs on the top of the subbase;  
no tool is needed to remove the thermostat from the sub-  
base. Pivot the bottom of the thermostat outward (releas-  
ing the snaps), then lift up to remove.  
When abnormal equipment operation is detected, the  
This indicates that the equipment requires service from a  
qualified service technician.  
To replace it, first position the top tilted toward the wall  
bracket and align it until you feel the tabs and slots engage;  
then, while the top is in place, pivot the bottom toward the  
wall until the thermostat snaps into place.  
Service Indicator Flashing  
MO TU WE TH FR SA SU  
FILTER  
SERVICE  
Default Thermostat Settings  
AM  
FAN  
PM I0:i4  
ON  
Default thermostat settings are in table 2 and the default  
program is shown in table 3.  
AUTO  
PROG A B C D HOLD  
ROOM  
Table 2  
SYSTEM  
COOL  
OFF  
Default Thermostat Settings  
79°  
Mode  
Heat (Permanent Hold  
Mode)  
HEAT  
Setpoint  
70°F (or 21°C)  
Figure 17  
Fan  
Auto  
Filter Reminder  
Maintenance Reminder  
OFF  
Thermostat RESET  
OFF  
Under some abnormal conditions, it may be necessary to  
ꢀreset" the thermostat to its default condition. Such a RE-  
SET would delete all programming and settings and there-  
fore should only be used on rare occasions when the the  
thermostat fails to function as designd and/or as pro-  
grammed. Such an instance can occur as a result of a pow-  
er surge or similar electrical disturbance (e.g. after an elec-  
trical storm or power outage). The RESET button can be  
used to recover from this situation.  
Equipment Protection  
Timers  
Reset Back to Zero  
Table 3  
Default Program Settings  
Temperature Temperature  
(Heat) (Cool)  
Programs  
Time  
Weekday − A 6:00am 70_F / 21°C 78_F / 26°C  
Weekday − B 8:00am 62_F / 17°C 85_F / 29°C  
Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C  
Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C  
Weekend − A 6:00am 70_F / 21°C 78_F / 26°C  
Weekend − B 8:00am 62_F / 17°C 85_F / 29°C  
Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C  
Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C  
CAUTION  
When the RESET button is pressed, ALL settings re-  
vert back to the defaults, including the default pro-  
gram (see tables 2 and 3).  
The RESET button is an unlabeled, recessed button lo-  
cated behind the door on the right−hand side of the thermo-  
stat, below the SETTINGS button (see figure 3). Use a pa-  
per clip or small pencil to press the RESET button; ALL  
thermostat settings will be reset to the defaults listed in the  
Default Thermostat Settings section.  
Weekday  
Monday, Tuesday, Wednesday,  
Thursday, Friday  
Weekend  
Saturday, Sunday  
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505048M 5/1/05  
 
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Field Offset: via DIP switches to +/−3°F  
Technical Specifications  
Thermostat Type  
Sampling Method: temperature measurements sampled  
every 15 seconds. Displayed temperature is the average  
of the last four measurements.  
Electronic programmable thermostat for 1-Stage-  
Heat/1-Stage Cool, gas or electric heat, non−heat pump,  
non−power robbing applications.  
Temperature Setpoint Range  
Setting range: 50°F (10°C) to 90°F (32°C)  
Setting resolution: 1°F (1°C)  
Power Supply Range  
Smart Setback Recovery (via DIP switch #6)  
Smart Setback Recovery (SSR) affects the way the ther-  
mostat responds to program events. If SSR is disabled, the  
thermostat will react to a program event at the time the  
event occurs. However, if SSR is enabled, the thermostat  
will react to a program event before the event occurs such  
that the desired temperature is reached at the time of the  
event, not after.  
18VAC − 30VAC (24VAC nominal), 60Hz  
CAUTION  
24VAC is present on the terminals of the thermostat  
bracket. If removing the thermostat from the wall,  
use caution and avoid touching any of the connec-  
tor terminals on the wall bracket.  
Fan Control  
AUTO or ON modes, gas or electric heat compatible via  
DIP switches (also see Thermostat Output section).  
Also, when working with the thermostat dip  
switches, use a non−conductive tool and take cau-  
tion to avoid making any contact with the circuit  
board, its imprinted circuitry and its connector  
prongs.  
I/O Relays  
All thermostat relays are latching type to minimize power  
consumption.  
Table 4  
51M34 Terminal Designations  
Temperature Display  
Terminal  
Description  
Display Scale: Fahrenheit or Celsius user selectable (via  
Display range: 35°F (2°C) to 99°F (37°C)  
Display resolution: 1°F (1°C)  
Display Accuracy: +/−1°F  
R
24VAC  
Y1  
W1  
G
First stage cooling  
First stage heating  
Fan control  
L
Service Indicator  
24VAC common  
If the Fahrenheit/Celsius display must be changed, use a  
plastic, non−conductive tool to push the dip switch to the  
C
Equipment Protection Timers  
Minimum Compressor OFF time: 5 minutes  
Minimum Compressor ON time: 4 minutes  
Minimum Furnace ON time: 3 minutes  
Changing Fahrenheit/Celsius Setting  
Minimum furnace cycle time (elapsed time between any  
furnace activation and the next furnace activation): 6 min-  
utes.  
Minimum elapsed time between any compressor activa-  
tion and the next compressor activation: 6 minutes.  
1
2
3
4
5
NOTE − All protection timers (except the compressor OFF  
timer) can be over−ridden if a heating or cooling demand is  
initiated or terminated using the UP, DOWN, HEAT, or  
COOL buttons.  
PLUG IN (Use Care To Avoid Bending Prongs.)  
6
Equipment Protection Override  
Both the minimum compressor OFF timer and the mini-  
mum equipment cycle timer can be over−ridden by press-  
ing and holding either the HEAT or COOL button down for 4  
seconds.  
F/C (Fahrenheit/Celsius) switch  
shown in Celsius position  
Figure 18  
Over−Temperature Protection  
Thermal-mechanical switch opens W1 at 93°F+/−6°F.  
Temperature Measurement Range  
Measurement Scale: Fahrenheit  
Measurement Range: 35°F to 99°F  
Measurement Resolution: 0.5°F  
Measurement Accuracy: +/−1°F  
Filter Reminder  
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are  
available. When programmed time has elapsed, a FILTER  
indicator is displayed.  
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51M34 SERIES 5/2  
 
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Maintenance Reminder  
Thermostat Operating Conditions  
Settings of Off, 6 or 12 (months of chronological time) are  
available. When programmed time has elapsed, a mainte-  
nance indicator ꢀMAINT" is displayed.  
35°F to 105°F, 5% to 90% RH  
Thermostat Storage Conditions  
Service Reminder  
The SERVICE indicator is displayed only under the follow-  
ing conditions:  
−40°F to 185°F, 5% to 95% RH  
Thermostat Output Table  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the R terminal;  
Table5 depicts the 51M34 thermostat output states for var-  
ious input conditions.  
OR  
Table 5  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the C terminal.  
Thermostat Outputs  
Condition  
W1  
Y1  
G
Power Loss/Recovery  
Gas Heat, Auto Fan  
HEAT Demand  
Thermostat memory is retained for a minimum of 24 hours  
during a power loss (includes retention of program infor-  
mation, HOLD status, programmed temperature setpoint,  
heat/cool and fan mode settings, filter reminder status,  
maintenance reminder status, and equipment protection  
timers). After 24 hours of power loss, programmed settings  
will be lost and replaced with default settings.  
X
COOL Demand  
No Demand  
X
X
Gas Heat, Continuous Fan  
HEAT Demand  
X
X
X
X
X
COOL Demand  
No Demand  
X
X
X
IMPORTANT  
Power must be applied for at least six consecutive  
hours prior to a power loss in order for memory to  
be retained for the specified time.  
Electric Heat, Auto Fan  
HEAT Demand  
X
X
COOL Demand  
No Demand  
LCD Backlight  
Activated for 30 seconds when any button is pressed.  
Electric Heat, Continuous Fan  
HEAT Demand  
COOL Demand  
No Demand  
X
X
X
X
NOTE − During an electrical storm or similar disturbance,  
the backlight may activate for a few seconds. This is nor-  
mal and will no longer occur after the electrical disturbance  
has passed.  
NOTE − X = output is activated with 24VAC.  
Page 8  
505048M 5/1/05  
 
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OPERATION  
MANUAL  
E2005 Lennox Industries Inc.  
Dallas, Texas, USA  
51M39 Merit® Series  
7 Day Programmable Thermostat  
CONTROLS  
505,068M  
07/05  
Litho U.S.A.  
Table of Contents  
51M39 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .  
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .  
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .  
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .  
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
AUTO − Using the Autochangeover Mode . . . . . . . . .  
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .  
PROG − Thermostat Programming . . . . . . . . . . . . . . .  
COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .  
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .  
Displaying Outside Temperature . . . . . . . . . . . . . . . . .  
SETTINGS − Filter /Maintenance Reminder . . . . . . .  
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .  
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .  
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .  
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .  
RETAIN THESE INSTRUCTIONS  
FOR FUTURE REFERENCE  
51M39 Merit® Series Thermostat  
®
The Lennox Merit Series 7 day programmable electronic  
thermostat 51M39 provides excellent temperature control  
and a large, easy-to-read display. This product includes a  
programmable filter change reminder, an equipment  
maintenance reminder, and a system check indicator to  
notify the user when the equipment requires service.  
Thermostat 51M39 is suitable for non-heat pump, two-  
stage heat/two-stage cool applications using a gas or elec-  
tric furnace. Also, the thermostat provides autochange-  
over capability.  
General  
Initial Power−Up Display  
Copy tofrom MO TU WE TH FR SA SU  
These instructions are intended as a general guide and do  
not supersede local codes in any way. Consult authorities  
having jurisdiction before installation.  
PROG  
A B C D  
AM  
PM  
I8:88  
HOLD  
Check equipment for shipping damage. If you find any  
damage, immediately contact the last carrier.  
FAN ON  
AUTO  
COOL SETPOINT  
FILTER  
SERVICE  
°
COOL  
OFF  
MAINTENANCE  
Introduction  
AUTO  
EMG.  
HEAT  
OUTROOM  
i88°  
HEAT SETPOINT  
This document describes the operation of Lennox thermo-  
stat 51M39. Refer to the installation manual for instructions  
regarding installation and wiring of the thermostat.  
°
Hi  
Lo  
Initial Thermostat Power-up  
Figure 1  
When power is initially applied to the thermostat, the dis-  
All display segments are momentarily activated. This oc-  
curs as a normal part of thermostat initialization.  
07/05  
505,068M  
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*P505068M*  
 
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Within a few seconds, the HOME screen appears (see fig-  
ure 2) with default settings as shown. After about 1 minute  
of initialization time, the actual room temperature will be  
displayed.  
Thermostat Buttons  
Heat Cool  
Prog Auto  
Copy Day/Time  
Fan Hold  
programming defaults.  
HOME Screen  
(DOOR  
OPEN)  
MO  
PROG  
A B C D  
HOLD  
Settings Enter  
(Outside)  
AM  
PM I0:26  
FAN ON  
AUTO  
COOL  
Figure 3  
OFF  
ROOM  
AUTO  
EMG.  
70°  
HEAT  
When an Arrow, HOLD, HEAT, or COOL button is pressed,  
SETPOINT and the temperature setting appears for 15  
seconds. If desired, start making changes within 15 sec-  
onds or the HOME screen will return. The backlight will  
turn off 15 seconds after the HOME screen reappears.  
Figure 2  
At this point, the thermostat will be fully functional; its de-  
fault temperature setpoint (not shown) is 70°F. At this  
point, if the equipment has been fully powered and if a heat  
demand were present, the system would begin operating.  
DAY/TIME − Setting the Day and Time  
NOTE − Temperature scale default is Fahrenheit units but  
may be reset to show Celsius, if desired. See page 8.  
Press the DAY/TIME button and set the CURRENT hour,  
minute, and day of week as follows:  
Buttons, Backlight, Timers & Settings  
1. ꢀAM12" will flash on the screen. Press the UP/DOWN  
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-  
spond to time of day.) Press DAY/TIME OR, if adjust-  
ing for daylight savings time, pressing the ENTER but-  
ton stores the single change and exits to the HOME  
screen, bypassing minutes and day of week.  
Buttons are located behind the small door on the right−  
hand side of the thermostat (see figure 3).  
IMPORTANT  
Do NOT begin pressing buttons until after you read  
the following section describing each button.  
2. Minutes will flash. Use the UP/DOWN arrow button to  
display the minutes past the hour. Press DAY/TIME.  
A pale blue display backlight illuminates for 30 seconds  
each time any button is pressed.  
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow  
buttons to display the current day. Day selections are  
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",  
and ꢀSU". Press DAY/TIME.  
When PROG or DAY/TIME button is pressed, a field be-  
gins flashing, expecting another input. Start making  
changes within 15 seconds or the HOME screen will re-  
turn.  
4. The HOME screen reappears; confirm day and time  
are correct. This completes day and time setting.  
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HEAT − Using the Heat Mode  
COOL − Using the Cool Mode  
Enabling and Disabling Heat Mode  
Enabling and Disabling Cool Mode  
Use the COOL button to enable or disable cool mode as  
desired. If the thermostat is in heat or off mode, cool mode  
is enabled when the COOL button is pressed. This is indi-  
cated by COOL on the right side of the display (see figure  
The thermostat must be in the heat mode to control the  
heating equipment. Press the HEAT button to enable or  
disable HEAT mode. If the thermostat is in cool or off  
mode, pressing the HEAT button enables the heat mode  
(indicated by HEAT in the lower right corner − see figure 4).  
Turn Heat ON/OFF  
Turn Cool ON/OFF  
Heat Cool  
Heat Cool  
Prog Auto  
Prog Auto  
5:30  
i0:30  
Copy Day/Time  
Copy Day/Time  
82  
70  
Fan Hold  
(DOOR  
OPEN)  
Fan Hold  
70  
66  
(DOOR  
OPEN)  
75  
66  
Settings Enter  
(Outside)  
Settings Enter  
(Outside)  
Figure 4  
Figure 7  
If the thermostat is in heat mode, heat mode is disabled  
when the HEAT button is pressed. This is indicated by OFF  
If the thermostat is in cool mode, pressing the COOL but-  
ton disables cool mode (indicated by OFF in the SYSTEM  
box − see figure 5).  
Heat/Cool Mode Disabled  
ROOM  
Cooling Demand  
OFF  
Set the thermostat to cool mode to control the cooling  
equipment. Then, if the room temperature is higher than  
the temperature setpoint, as shown in figure 7, the thermo-  
stat detects a cooling demand and will activate the cooling  
equipment to satisfy the demand.  
HEAT SETPOINT  
75°  
75°  
Figure 5  
Cooling operation is indicated by flashing ꢀsnowflake"  
icons on the right side of the display. When the actual tem-  
perature drops below the temperature setpoint, the snow-  
flake icons will disappear. This indicates that the cooling  
demand has been satisfied and that the cooling equipment  
has been turned off.  
Heating Demand  
Set the thermostat to heat mode to control the heating  
equipment. Then, if the room temperature is lower than the  
temperature setpoint, as shown in figure 6, the thermostat  
detects a heating demand and will activate the heating  
equipment to satisfy the demand.  
If your system supports 2−stage cooling (as does the  
51M39 thermostat), you may notice various cooling levels  
being delivered during a demand. Also, if a small cooling  
demand is present, "Lo" will be displayed in the SYSTEM  
box. However, if a large cooling demand is present, "Hi" will  
be displayed in the lower right corner (shown in figure 8).  
Heating Demand  
ROOM  
HEAT SETPOINT  
72°  
HEAT  
75°  
Figure 6  
Cooling Demand  
Heating operation is indicated by a flame icon in the lower  
right corner. When the actual temperature rises above the  
temperature setpoint, the flame icon will disappear. This  
indicates that the heating demand has been satisfied and  
that the heating equipment has been turned off.  
COOL SETPOINT  
FAN  
AUTO  
76°  
COOL  
ROOM  
80°  
If your system supports 2−stage heating (as does the  
51M39 thermostat), you may notice various heating levels  
being delivered during a demand.  
HEAT SETPOINT  
68°  
Hi  
Lo  
NOTE − Heating equipment is activated for at least 3 min-  
utes if no buttons are pressed during the demand interval.  
Figure 8  
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51M39 SERIES 5/2  
 
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NOTE − If no buttons are pressed during a demand for  
cooling, the equipment must operate for at least 4 minutes.  
After a demand has been satisfied, cooling equipment op-  
eration is locked out for 5 minutes. If another cooling de-  
mand occurs during this 5−minute interval, ꢀCOOL" and the  
snowflakes will flash; however, the cooling equipment will  
not operate until the 5−minute delay has elapsed.  
After the desired setpoint is reached, the HOME screen  
will reappear after about 15 seconds.  
Permanent Hold Mode  
At any time the program is running, from the HOME  
screen, set a permanent hold (program override) by press-  
ing the HOLD button (see figure 9). The thermostat now  
functions much like a non−programmable thermostat. Use  
the Up/Down arrow buttons to adjust the hold setpoint. To  
return to the program, press the HOLD button again.  
AUTO − Using the Autochangeover Mode  
When in either heat mode or cool mode, autochangeover  
can be enabled or disabled by pressing the AUTO button. If  
enabled, AUTO appears at the lower right corner of the dis-  
play. The thermostat will then automatically change over  
from heating to cooling and vice versa, to keep the room  
temperature in between the heating and cooling setpoints.  
Autochangeover can be used in either of the thermostat  
hold modes, or when the thermostat program is running.  
These modes are described later.  
Temporary (2-Hour) Hold  
At any time the program is running, from the HOME  
screen, set a temporary 2-hour hold by pressing the Up/  
Down arrow buttons until the desired setpoint is displayed;  
ꢀHOLD" flashes (see figure 10). This overrides the pro-  
gram for 2 hours from the last button press, then returns to  
the program.  
While in Temporary Hold, press the HOLD button once to  
switch to Permanent Hold (HOLD displays solid; PROG  
not displayed); press the HOLD button again to return to  
the program (PROG displays; HOLD not displayed).  
HOLD − Using Temperature Hold Modes  
When HOLD is displayed at the HOME screen, the ther-  
mostat is in a temperature hold condition. This means that  
the temperature program data is ignored and the thermo-  
stat functions much like a non−programmable thermostat.  
Temporary Hold Temperature Mode  
Heat Cool  
Adjusting Temperature Setpoint in Hold Mode  
Prog Auto  
The temperature setpoint represents the desired tempera-  
ture of the space around the thermostat. The default heat  
setpoint in hold mode is 70°F; the default cool setpoint in  
hold mode is 78°F.  
i0:30  
Copy Day/Time  
70  
Fan Hold  
(DOOR  
OPEN)  
72  
68  
Hold Temperature Mode  
Settings Enter  
(Outside)  
Heat Cool  
Prog Auto  
i0:30  
Figure 10  
Copy Day/Time  
78  
PROG − Thermostat Programming  
Fan Hold  
(DOOR  
OPEN)  
80  
70  
The 51M39 thermostat can be programmed to perform a  
set of both heating and cooling events for each day of the  
week. Each day can be programmed for 4 unique events  
per day, and each day can be different from any other day.  
Settings Enter  
(Outside)  
To Set Program Events and Temperatures  
Figure 11 gives an example of how up to 7 different pro-  
grams can be set. Programs A and C reflect the desired  
warmth while the home IS occupied (72°); B allows less  
heating while the home is NOT occupied; D reflects a cool  
sleeping temperature.  
Figure 9  
To adjust the setpoint, press the UP or DOWN (YB) arrow  
buttons; the existing setpoint is displayed to the right of the  
actual room temperature. Each button press adjusts the  
setpoint up or down by 1 degree.  
NOTE − This example shows heat setpoints; a similar pro-  
gram may be set for cool setpoints.  
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C
want the same program.  
Program Example  
MONDAY PROGRAM: 72°  
A
C
68°  
64°  
D
Copy a Programmed Day to Another Day  
B
Heat Cool  
TUESDAY PROGRAM: 72°  
68°  
64°  
A
C
D
B
Prog Auto  
Copy Day/Time  
Fan Hold  
Copy from MO  
WEDNESDAY PROGRAM: 72°  
68°  
64°  
A
C
D
B
(DOOR  
OPEN)  
THURSDAY PROGRAM: 72°  
A
A
C
68°  
64°  
D
B
B
Settings Enter  
(outside)  
FRIDAY PROGRAM: 72°  
68°  
64°  
C
D
Figure 12  
SATURDAY PROGRAM: 72°  
C
A
68°  
64°  
B
7. Repeat steps 1 5 to program different times and tem-  
peratures for days not yet programmed.  
D
SUNDAY PROGRAM: 72°  
FAN − Controlling the Fan Operation  
A
C
68°  
64°  
B
D
Use the FAN button to select either continuous fan mode or  
auto fan mode.  
6AM  
NOON  
6PM  
MID  
NIGHT  
6AM  
To change from continuous to auto fan mode (or vice ver-  
sa), press the FAN button. Note whether a fan icon in the  
FAN box is present (indicating that the fan is running) or not  
(fan not running).  
Figure 11  
NOTE − Pressing the ENTER button during the following  
programming steps, saves and exits to the HOME screen.  
If continuous fan mode is enabled (ON displayed in FAN  
box − see figure 13), the fan will run continuously regard-  
less of whether the heating or cooling equipment is run-  
ning.  
To program events and temperatures, perform the follow-  
ing steps.  
1. Press and release the PROG button. "MO" (Monday),  
"AM 6:00", period "A", "COOL SETPOINT 78", and  
"HEAT SETPOINT 70" are displayed. "AM 6" flashes.  
Heat Cool  
Using Fan ON  
2. Use the Up/Down arrow buttons to select the desired  
hour; press the PROG button when the desired hour  
is reached; use the Up/Down arrow buttons again to  
select desired minute. The selected hour/minute is  
when the program will start. Press the PROG button.  
Prog Auto  
i0:30  
Copy Day/Time  
Fan Hold  
72  
3. Use the Up/Down arrow buttons to select the desired  
heat setpoint; press the PROG button.  
4. Use the Up/Down arrow buttons to select the desired  
cool setpoint; press the PROG button.  
Figure 13  
If auto fan mode is selected (AUTO displayed in FAN box −  
see figure 14), the fan will only run when the heating or  
cooling equipment is running.  
5. Repeat steps 1 − 4 for Monday, periods B, C, and D.  
6. COPY A PROGRAM − If the program entered for  
Monday is satisfactory for other days of the week, do  
the following to repeat the same program for other  
days:  
Heat Cool  
Using Fan AUTO  
A
Press the PROG button, then press the COPY  
button. The display clears the screen except for  
ꢀCopy from MOꢀ with ꢀMO" flashing. Press the  
COPY button again to copy Monday’s program.  
The display changes to ꢀCopy to MO". Use the ar-  
row buttons to scroll to the desired day where you  
want the same program as Monday. Press the  
COPY button.  
Prog Auto  
i0:30  
Copy Day/Time  
B
Fan Hold  
72  
Figure 14  
Page 5  
51M39 SERIES 5/2  
 
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After either programmed interval has elapsed, the remind-  
Displaying Outside Temperature  
Pressing the ENTER (Outside) button at the HOME  
screen displays the outside temperature (see figure 15) for  
several seconds, then returns to room temperature.  
Reminders  
MO  
PROG  
A B C D  
HOLD  
AM  
PM I0:26  
Check Outside Temperature  
FAN ON  
AUTO  
Filter &  
Maintenance  
Indicators  
Fan Hold  
FILTER SERVICE  
MAINTENANCE  
ROOM  
COOL  
OFF  
88  
AUTO  
EMG.  
HEAT  
Settings Enter  
(Outside)  
76°  
Figure 15  
FAN ON  
AUTO  
FILTER SERVICE  
MAINTENANCE  
ROOM  
Service  
Indicator  
Flashing  
COOL  
OFF  
SETTINGS − Filter/Maintenance Reminders  
AUTO  
EMG.  
HEAT  
The 51M39 thermostat is designed to remind you when the  
filter needs changing or when routine maintenance is re-  
quired, as (and if) defined, by you. These optional remind-  
ers are not enabled until you activate them. To do so, press  
the SETTINGS button once or twice for the desired re-  
76°  
Figure 17  
After the filter has been changed or maintenance per-  
formed, reset the reminder by pressing the SETTINGS  
button for 4 seconds at the corresponding reminder set-  
tings screen. The screen will blink for a few moments to in-  
dicate that the timer has been reset.  
Reminder Settings Display  
OFF  
Service Indicator  
For FILTER, press SETTINGS  
button 1 time  
When abnormal equipment operation is detected, the  
This indicates that the equipment requires service from a  
qualified service technician.  
FILTER  
For MAINTENANCE, press  
SETTINGS button 2 times  
MAINTENANCE  
Figure 16  
Thermostat RESET  
The default setting for the reminders is OFF (disabled).  
Press Up/Down arrow buttons to select the desired re-  
minder intervals.  
Under some abnormal conditions, it may be necessary to  
ꢀreset" the thermostat to its default condition. Such a RE-  
SET would delete all programming and settings and there-  
fore should only be used on rare occasions when the the  
thermostat fails to function as designed and/or as pro-  
grammed. Such an instance can occur as a result of a pow-  
er surge or similar electrical disturbance (e.g. after an elec-  
trical storm or power outage). The RESET button can be  
used to recover from this situation.  
Table 1  
Filter and Maintenance Reminders  
Buttons to  
Use  
Available Settings and  
How to Use  
Reminder  
FILTER  
Settings  
(1st press)  
then Arrows  
to scroll  
Total fan run time expressed in  
months (Off, 1, 3, 6, 12); for  
example, if fan runs 12 hours a day,  
1 month reminder displays in 2  
calendar months.  
selections  
CAUTION  
When the RESET button is pressed, ALL settings re-  
vert back to the defaults, including the default pro-  
gram (see tables 2 and 3).  
Settings  
(2nd press)  
then Arrows  
to scroll  
Elapsed chronological time in  
months (Off, 6, 12). Use this, for  
example, to remind yourself when to  
perform routine checks or when to  
call a technician for perodic  
MAINTE-  
NANCE  
selections  
preventive maintenance.  
The RESET button is an unlabeled, recessed button lo-  
cated behind the door on the right−hand side of the thermo-  
stat, below the SETTINGS button (see figure 3). Use a pa-  
per clip or small pencil to press the RESET button; ALL  
thermostat settings will be reset to the defaults listed in the  
Default Thermostat Settings section.  
Enter  
Stores settings.  
NOTE − The HOME screen will reappear about 15 seconds  
after the final arrow button press. OR, press the ENTER  
button at any time to store any changes and exit to the  
HOME screen.  
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Table 4  
Removing and Installing Thermostat  
Thermostat Outputs  
Condition  
W1 W2 Y1 Y2  
G
The thermostat hinges on tabs on the top of the subbase;  
no tool is needed to remove the thermostat from the sub-  
base. Pivot the bottom of the thermostat outward (releas-  
ing the snaps), then lift up to remove.  
Gas Heat, Auto Fan  
Heat Demand, SMALL  
Heat Demand, LARGE  
Cool Demand, SMALL  
Cool Demand, LARGE  
No Demand  
X
X
X
X
X
X
X
X
X
To install it, first position the top tilted toward the wall brack-  
et and align it until you feel the tabs and slots engage; then,  
while the top is in place, pivot the bottom toward the wall  
until the thermostat snaps into place.  
X
X
X
X
X
Gas Heat, Continuous Fan  
Heat Demand, SMALL  
Heat Demand, LARGE  
Cool Demand, SMALL  
Cool Demand, LARGE  
No Demand  
X
X
X
X
X
X
X
Default Thermostat Settings  
X
X
Default thermostat settings are in table 2 and the default  
program is shown in table 3.  
Table 2  
Electric Heat, Auto Fan  
Heat Demand, SMALL  
Heat Demand, LARGE  
Cool Demand, SMALL  
Cool Demand, LARGE  
No Demand  
Default Thermostat Settings  
X
X
X
X
X
X
Mode  
Heat (Perm. Hold)  
70°F (or 21°C)  
78°F (or 26°C)  
Auto  
Heat Setpoint (Perm Hold)  
Cool Setpoint (Perm Hold)  
Fan  
X
X
Filter Reminder  
Maintenance Reminder  
OFF  
Electric Heat, Continuous Fan  
OFF  
Heat Demand, SMALL  
Heat Demand, LARGE  
Cool Demand, SMALL  
Cool Demand, LARGE  
No Demand  
X
X
X
X
X
X
X
Equipment Protection Timers Reset Back to Zero  
Autochangeover OFF  
X
X
Table 3  
Default Program Settings (Time & Temp.)  
NOTE − X = output is activated with 24VAC.  
NOTE − The temperature ranges expressed in the follow-  
ing definitions of ꢀSMALL/LARGE" demands are for guid-  
ance only; actual temperatures may vary.  
Programs  
Time  
Temp.− Heat Temp.− Cool  
All days − A 6:00am 70_F / 21°C  
All days − B 8:00am 62_F / 17°C  
All days − C 6:00pm 70_F / 21°C  
All days − D 10:00pm 62_F / 17°C  
78_F / 26°C  
85_F / 29°C  
78_F / 26°C  
82_F / 28°C  
With a SMALL heat demand, temperature is:  
below setpoint –0.5F but above setpoint –1.5F  
AND 30-minute upstage timer HAS NOT expired.  
With a LARGE heat demand, temperature is:  
less than setpoint –1.5F  
OR 30-minute upstage timer HAS expired.  
With a SMALL cool demand, temperature is:  
above setpoint +0.5F but below setpoint +1.5F  
AND 30-minute upstage timer HAS NOT expired.  
Thermostat Output Table  
Table4 depicts the 51M39 thermostat output states for var-  
ious input conditions. The following notes described terms  
used in the table.  
With a LARGE cool demand, temperature is:  
above setpoint +1.5F  
OR 30-minute upstage timer HAS expired.  
Page 7  
51M39 SERIES 5/2  
 
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Display range: 35°F (2°C) to 99°F (37°C)  
Display resolution: 1°F (1°C)  
Technical Specifications  
Display Accuracy: +/−1°F  
Thermostat Type  
Electronic programmable thermostat for 2−Stage (gas or  
electric) Heat/2-Stage Cool, non−heat pump, non−power  
robbing applications.  
If the Fahrenheit/Celsius display must be changed, use a  
plastic, non−conductive tool to push the dip switch to the  
Table 5  
51M39 Terminal Designations  
Terminal Description  
Indoor Temperature Measurement Range  
Measurement Scale: Fahrenheit  
Measurement Range: 35°F to 99°F  
Measurement Resolution: 0.5°F  
Measurement Accuracy: +/−1°F  
Field Offset: via DIP switches to +/−3°F  
Sampling Method: temperature measurements sampled  
every 15 seconds. Displayed temperature is the average  
of the last four measurements.  
R
24VAC  
Y1  
W1  
Y2  
W2  
G
First-stage cooling  
First-stage heating  
Second-stage cooling  
Second-stage heating  
Fan control  
Outdoor Temperature Measurement Range  
Measurement Scale: Fahrenheit  
Measurement Range: −22°F to 122°F  
Measurement Resolution: 1°F  
Measurement Accuracy: +/−2°F  
L
Service Indicator  
C
24VAC common  
T
Outdoor temperature sensor connection 1  
Outdoor temperature sensor connection 2  
T
Temperature Setpoint Range  
Setting range: 50°F (10°C) to 90°F (32°C)  
Setting resolution: 1°F (1°C)  
Power Supply Range  
18VAC − 30VAC (24VAC nominal), 60Hz  
Smart Setback Recovery (via DIP switch #6)  
Smart Setback Recovery (SSR) affects the way the ther-  
mostat responds to program events. If SSR is disabled, the  
thermostat will react to a program event at the time the  
event occurs. However, if SSR is enabled, the thermostat  
will react to a program event before the event occurs such  
that the desired temperature is reached at the time of the  
event, not after.  
CAUTION  
24VAC is present on the terminals of the thermostat  
bracket. If removing the thermostat from the wall,  
use caution and avoid touching any of the connec-  
tor terminals on the wall bracket.  
Also, when working with the thermostat dip  
switches, use a non−conductive tool and take cau-  
tion to avoid making any contact with the circuit  
board, its imprinted circuitry and its connector  
prongs.  
Autochangeover Deadband Selection (via DIP  
switch #7)  
Autochangeover deadband can be set to 4 or 6 degrees.  
When autochangeover is enabled (via the AUTO button),  
the thermostat will automatically change over from heating  
to cooling and vice versa, to keep the room temperature in  
between the heating and cooling setpoints. The deadband  
is the minimum difference between the heating and cooling  
setpoints.  
Temperature Display  
Display Scale: Fahrenheit or Celsius user selectable (via  
Fan Control  
AUTO or ON modes, gas or electric heat compatible via  
DIP switches (also see Thermostat Output section).  
Changing Fahrenheit/Celsius Setting  
I/O Relays  
All thermostat relays are latching type to minimize power  
consumption.  
Equipment Protection Timers  
1
2
3
4
5
Minimum Compressor OFF time: 5 minutes  
Minimum Compressor ON time: 4 minutes  
Minimum Furnace ON time: 3 minutes  
PLUG IN (Use Care To Avoid Bending Prongs.)  
6
7
Minimum furnace cycle time (elapsed time between any  
furnace activation and the next furnace activation): 6 min-  
utes.  
F/C (Fahrenheit/Celsius) switch  
shown in Celsius position  
Minimum elapsed time between any compressor activa-  
tion and the next compressor activation: 6 minutes.  
Figure 18  
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NOTE − All protection timers (except the compressor OFF  
timer) can be over−ridden if a heating or cooling demand is  
initiated or terminated using the UP, DOWN, HEAT, or  
COOL buttons.  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the C terminal.  
Power Loss/Recovery  
Equipment Protection Override  
Thermostat memory is retained for a minimum of 24 hours  
during a power loss (includes retention of clock setting,  
program information, HOLD status, programmed temper-  
ature setpoint, heat/cool and fan mode settings, filter re-  
minder status, maintenance reminder status, and equip-  
ment protection timers). After 24 hours of power loss,  
programmed settings will be lost and replaced with default  
settings.  
Both the minimum compressor OFF timer and the mini-  
mum equipment cycle timer can be over−ridden by press-  
ing and holding either the HEAT or COOL button down for 4  
seconds.  
Over−Temperature Protection  
Thermal-mechanical switch opens W1 and W2 at  
93°F+/−6°F.  
Filter Reminder  
IMPORTANT  
Power must be applied for at least six consecutive  
hours prior to a power loss in order for memory to  
be retained for the specified time.  
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are  
available. When programmed time has elapsed, a FILTER  
indicator is displayed.  
Maintenance Reminder  
LCD Backlight  
Activated for 30 seconds when any button is pressed.  
Settings of Off, 6 or 12 (months of chronological time) are  
available. When programmed time has elapsed, a mainte-  
nance indicator ꢀMAINTENANCE" is displayed.  
NOTE − During an electrical storm or similar disturbance,  
the backlight may activate for a few seconds. This is nor-  
mal and will no longer occur after the electrical disturbance  
has passed.  
Service Reminder  
The SERVICE indicator is displayed only under the follow-  
ing conditions:  
Thermostat Operating Conditions  
35°F to 105°F, 5% to 90% RH  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the R terminal;  
Thermostat Storage Conditions  
−40°F to 185°F, 5% to 95% RH  
OR  
Page 9  
51M39 SERIES 5/2  
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OPERATION  
MANUAL  
E2005 Lennox Industries Inc.  
Dallas, Texas, USA  
51M42 Merit® Series  
7−Day Programmable Thermostat  
CONTROLS  
505,069M  
07/05  
Litho U.S.A.  
Table of Contents  
51M42 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .  
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .  
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .  
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .  
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
AUTO − Using the Autochangeover Mode . . . . . . . . .  
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .  
PROG − Thermostat Programming . . . . . . . . . . . . . . .  
COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .  
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .  
Displaying Outside Temperature . . . . . . . . . . . . . . . . .  
SETTINGS − Filter and Maintenance Reminders . . .  
SETTINGS − Balance Point . . . . . . . . . . . . . . . . . . . . .  
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .  
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .  
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .  
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .  
RETAIN THESE INSTRUCTIONS  
FOR FUTURE REFERENCE  
51M42 Merit® Series Thermostat  
®
The Lennox Merit Series 7−day programmable electronic  
thermostat 51M42 provides excellent temperature control  
and a large, easy-to-read display. It includes a  
programmable filter change reminder, an equipment  
maintenance reminder, and a system check indicator to  
notify the user when the equipment requires service.  
Thermostat 51M42 is suitable for heat pump, three-stage  
heat/two-stage cool applications using a gas or electric  
auxiliary heat source. In addition to displaying outdoor  
temperature, the outdoor temperature sensor provides  
auxiliary heat lockout, balance point operation, and dual−  
fuel compatibility. Also, the thermostat provides auto-  
changeover capability.  
General  
Initial Power−Up Display  
Copy tofrom MO TU WE TH FR SA SU  
These instructions are intended as a general guide and do  
not supersede local codes in any way. Consult authorities  
having jurisdiction before installation.  
PROG  
A B C D  
AM  
PM  
I8:88  
HOLD  
Check equipment for shipping damage. If you find any  
damage, immediately contact the last carrier.  
FAN ON  
AUTO  
COOL SETPOINT  
FILTER  
SERVICE  
°
COOL  
OFF  
MAINTENANCE  
Introduction  
AUTO  
EMG.  
HEAT  
OUTROOM  
i88°  
HEAT SETPOINT  
This document describes the operation of Lennox thermostat  
51M42. Refer to the installation manual for instructions re-  
garding installation and wiring of the thermostat.  
°
Hi  
Lo  
Initial Thermostat Power-up  
Figure 1  
When power is initially applied to the thermostat, the dis-  
All display segments are momentarily activated. This oc-  
curs as a normal part of thermostat initialization.  
07/05  
505,069M  
Page 1  
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*P505069M*  
 
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Within a few seconds, the HOME screen appears (see fig-  
ure 2) with default settings as shown. After about 1 minute  
of initialization time, the actual room temperature will be  
displayed.  
Thermostat Buttons  
Heat Cool  
Prog Auto  
Copy Day/Time  
Fan Hold  
programming defaults.  
HOME Screen  
(DOOR  
OPEN)  
MO  
PROG  
A B C D  
HOLD  
Settings Enter  
(Outside)  
AM  
PM I0:26  
FAN ON  
AUTO  
COOL  
Figure 3  
OFF  
ROOM  
AUTO  
EMG.  
70°  
HEAT  
When an Arrow, HOLD, HEAT, or COOL button is pressed,  
SETPOINT and the temperature setting appears for 15  
seconds. If desired, start making changes within 15 sec-  
onds or the HOME screen will return. The backlight will  
turn off 15 seconds after the HOME screen reappears.  
Figure 2  
At this point, the thermostat will be fully functional; its de-  
fault temperature setpoint (not shown) is 70°F. At this  
point, if the equipment has been fully powered and if a heat  
demand were present, the system would begin operating.  
DAY/TIME − Setting the Day and Time  
NOTE − Temperature scale default is Fahrenheit units but  
may be reset to show Celsius, if desired. See page 9.  
Press the DAY/TIME button and set the CURRENT hour,  
minute, and day of week as follows:  
Buttons, Backlight, Timers & Settings  
1. ꢀAM12" will flash on the screen. Press the Up/Down ar-  
rows buttons to change the hour. (ꢀAM" or ꢀPM" must  
correspond to time of day.) Press DAY/TIME OR, if ad-  
justing for daylight savings time, pressing the ENTER  
button stores the single change and exits to the HOME  
screen, bypassing minutes and day of week.  
Buttons are located behind the small door on the right−  
hand side of the thermostat (see figure 3).  
IMPORTANT  
Do NOT begin pressing buttons until after you read  
the following section describing each button.  
2. Minutes will flash. Use the Up/Down arrow buttons to  
display the minutes past the hour. Press DAY/TIME.  
3. Day ꢀMO" (Monday) will flash. Use Up/Down arrow  
buttons to display the current day. Day selections are  
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",  
and ꢀSU". Press DAY/TIME.  
A pale blue display backlight illuminates for 30 seconds  
each time any button is pressed.  
When PROG or DAY/TIME button is pressed, a field be-  
gins flashing, expecting another input. Start making  
changes within 15 seconds or the HOME screen will re-  
turn.  
4. The HOME screen reappears; confirm day and time  
are correct. This completes day and time setting.  
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HEAT − Using the Heat Mode  
Heat Mode Disabled  
ROOM  
OFF  
Normal Heat Mode  
HEAT SETPOINT  
75°  
In normal heat mode, both the heat pump and the backup  
heat source are used to provide heat. If the thermostat de-  
tects that the heat pump is not able to provide enough heat  
(as may be the case in very cold weather), then the backup  
heat source is activated.  
75°  
Figure 6  
Heating Demand  
Emergency Heat Mode  
The thermostat must be in either normal or emergency  
heat mode in order to properly control the heating equip-  
ment. In either heat mode, when the actual temperature is  
the thermostat detects a heating demand and activates the  
heating equipment to satisfy the demand. Heating opera-  
tion is indicated by a flame icon in bottom right−hand corner of  
the display.  
In emergency heat mode, only the backup heat source  
provides heat ꢁthe heat pump is disabled. The backup  
heat source is activated only when there is a heat demand.  
Enabling Normal Heat Mode  
Use the HEAT button to select normal heat mode, emer-  
gency heat mode, or to disable heat modes as desired. If  
the thermostat is in off or cool mode, pressing the HEAT  
button enables heat mode. This is indicated by HEAT in the  
lower right corner (see figure 4).  
Heating Demand  
ROOM  
Normal Heat Mode Enabled  
HEAT SETPOINT  
72°  
HEAT  
75°  
Heat Cool  
Prog Auto  
Figure 7  
5:30  
When the actual temperature rises above the temperature  
setpoint, the flame icon will disappear. This indicates that  
the heating demand has been satisfied and that the heat-  
ing equipment has been turned off.  
Copy Day/Time  
82  
Fan Hold  
(DOOR  
OPEN)  
70  
70  
Settings Enter  
(Outside)  
Heat pump operation is locked out for 5 minutes after a de-  
mand has been satisfied. If another heat pump demand  
occurs during this 5−minute interval, the flame icon will  
flash; however, the heat pump will not run until the 5−min-  
ute delay has elapsed.  
Figure 4  
Enabling Emergency Heat Mode  
NOTE − The heat pump is activated for at least 4 minutes if  
no buttons are pressed during the demand interval. The  
backup heat source is activated for at least 3 minutes if no  
buttons are pressed during the demand interval.  
If the thermostat is in heat mode, pressing the HEAT button  
mostat is in normal heat mode when the HEAT button is  
pressed, then emergency heat mode is enabled.  
If your system supports 3−stage heating (as does the  
51M42 thermostat), you may notice various heating levels  
being delivered during a demand.  
Emergency Heat Mode Enabled  
ROOM  
If a small heat pump heating demand is present, "Lo" is dis-  
played in the SYSTEM box; if a large heat pump heating  
HEAT SETPOINT  
EMG.  
HEAT  
62°  
75°  
Large Heat Heating Demand  
Figure 5  
ROOM  
65°  
Disabling Heat Mode  
HEAT SETPOINT  
HEAT  
If the thermostat is in emergency heat mode when the  
HEAT button is pressed, then heat modes are disabled.  
This is indicated by OFF near the right side of the display  
75°  
Hi  
Lo  
Figure 8  
Page 3  
51M42 SERIES 5/2  
 
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NOTE − If no buttons are pressed during a demand for  
cooling, the equipment must operate for at least 4 minutes.  
After a demand has been satisfied, cooling equipment op-  
eration is locked out for 5 minutes. If another cooling de-  
mand occurs during this 5−minute interval, ꢀCOOL" and the  
snowflakes will flash; however, the cooling equipment will  
not operate until the 5−minute delay has elapsed.  
COOL − Using the Cool Mode  
Enabling and Disabling Cool Mode  
Use the COOL button to enable or disable cool mode as  
desired. If the thermostat is in heat or off mode, cool mode  
is enabled when the COOL button is pressed. This is indi-  
cated by COOL on the right side of the display (see figure  
AUTO − Using the Autochangeover Mode  
When in either heat mode or cool mode, autochangeover  
can be enabled or disabled by pressing the AUTO button. If  
enabled, AUTO appears at the lower right corner of the dis-  
play. The thermostat will then automatically change over  
from heating to cooling and vice versa, to keep the room  
temperature in between the heating and cooling setpoints.  
Autochangeover can be used in either of the thermostat  
hold modes, or when the thermostat program is running.  
These modes are described later.  
Turn Cool ON/OFF  
Heat Cool  
Prog Auto  
i0:30  
Copy Day/Time  
70  
Fan Hold  
(DOOR  
OPEN)  
75  
66  
HOLD − Using Temperature Hold Modes  
Settings Enter  
(Outside)  
When the HOLD button is displayed at the HOME screen,  
the thermostat is in a temperature hold condition. This  
means that the temperature program data is ignored and  
the thermostat functions much like a non−programmable  
thermostat.  
Figure 9  
If the thermostat is in cool mode, pressing the COOL but-  
ton disables cool mode (indicated by OFF on the right side  
of the display).  
Adjusting Temperature Setpoint in Hold Mode  
The temperature setpoint represents the desired tempera-  
ture of the space around the thermostat. The default heat  
setpoint in hold mode is 70°F; the default cool setpoint in  
hold mode is 78°F.  
Cooling Demand  
Set the thermostat to cool mode to control the cooling  
equipment. Then, if the room temperature is higher than  
the temperature setpoint, as shown in figure 10, the ther-  
mostat detects a cooling demand and will activate the cool-  
ing equipment to satisfy the demand.  
To adjust the setpoint, press the UP or DOWN (YB) arrow  
buttons (see figure 11); the existing setpoint is displayed to  
the right of the actual room temperature. Each button  
press adjusts the setpoint up or down by 1 degree.  
Hold Temperature Mode  
Cooling Demand  
COOL SETPOINT  
Heat Cool  
FAN  
AUTO  
76°  
Prog Auto  
COOL  
i0:30  
Copy Day/Time  
ROOM  
80°  
78  
HEAT SETPOINT  
Fan Hold  
(DOOR  
OPEN)  
68°  
80  
70  
Hi  
Lo  
Settings Enter  
(Outside)  
Figure 10  
Cooling operation is indicated by flashing ꢀsnowflake"  
icons. When the actual temperature drops below the tem-  
perature setpoint, the snowflake icons will disappear. This  
indicates that the cooling demand has been satisfied and  
that the cooling equipment has been turned off.  
Figure 11  
After the desired setpoint is reached, the HOME screen  
will reappear after about 15 seconds.  
Permanent Hold Mode  
If your system supports 2−stage cooling (as does the  
51M42 thermostat), you may notice various cooling levels  
being delivered during a demand. Also, if a small cooling  
demand is present, "Lo" will be displayed in the SYSTEM  
box. However, if a large cooling demand is present, "Hi" will  
be displayed in the lower right side of the display (shown in  
figure 10).  
At any time the program is running, from the HOME  
screen, set a permanent hold (program override) by press-  
ing the HOLD button (see figure 11). The thermostat now  
functions much like a non−programmable thermostat. Use  
the Up/Down arrow buttons to adjust the hold setpoint. To  
return to the program, press the HOLD button again.  
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Temporary (2-Hour) Hold  
Program Example  
At any time the program is running, from the HOME  
screen, set a temporary 2-hour hold by pressing the Up/  
Down arrow buttons until the desired setpoint is displayed;  
ꢀHOLD" flashes (see figure 12). This overrides the pro-  
gram for 2 hours from the last button press, then returns to  
the program.  
MONDAY PROGRAM: 72°  
A
C
68°  
64°  
D
B
TUESDAY PROGRAM: 72°  
68°  
64°  
A
C
D
B
WEDNESDAY PROGRAM: 72°  
68°  
64°  
A
C
Temporary Hold Temperature Mode  
D
B
Heat Cool  
THURSDAY PROGRAM: 72°  
A
A
C
68°  
64°  
Prog Auto  
D
B
B
i0:30  
FRIDAY PROGRAM: 72°  
Copy Day/Time  
70  
68°  
64°  
C
D
Fan Hold  
(DOOR  
OPEN)  
72  
68  
SATURDAY PROGRAM: 72°  
C
A
68°  
64°  
B
Settings Enter  
(Outside)  
D
SUNDAY PROGRAM: 72°  
A
C
68°  
64°  
B
D
Figure 12  
6AM  
NOON  
6PM  
MID  
NIGHT  
6AM  
While in Temporary Hold, press the HOLD button once to  
switch to Permanent Hold (HOLD displays solid; PROG  
not displayed); press the HOLD button again to return to  
the program (PROG displays; HOLD not displayed).  
Figure 13  
5. Repeat steps 1 − 4 for Monday, periods B, C, and D.  
6. COPY A PROGRAM − If the program entered for  
Monday is satisfactory for other days of the week, do  
the following to repeat the same program for other  
days:  
PROG − Thermostat Programming  
The 51M42 thermostat can be programmed to perform a  
set of both heating and cooling events for each day of the  
week. Each day can be programmed for 4 unique events  
per day, and each day can be different from any other day.  
A
B
C
Press the PROG button, then press the COPY  
button. The display clears the screen except for  
ꢀCopy from MOꢀ with ꢀMO" flashing. Press the  
COPY button again to copy Monday’s program.  
The display changes to ꢀCopy to MO". Use the ar-  
row buttons to scroll to the desired day where you  
want the same program as Monday. Press the  
COPY button.  
To Set Program Events and Temperatures  
Figure 13 gives an example of how up to 7 different pro-  
grams can be set. Programs A and C reflect the desired  
warmth while the home IS occupied (72°); B allows less  
heating while the home is NOT occupied; D reflects a cool  
sleeping temperature.  
want the same program.  
similar program may be set for cool setpoints.  
Copy a Programmed Day to Another Day  
1. Press and release the PROG button. "MO" (Monday),  
"AM 6:00", period "A", "COOL SETPOINT 78", and  
"HEAT SETPOINT 70" are displayed. "AM 6" flashes.  
Heat Cool  
Copy from MO  
Prog Auto  
2. Use the Up/Down arrow buttons to select the desired  
hour; press the PROG button when the desired hour  
is reached; use the Up/Down arrow buttons again to  
select desired minute. The selected hour/minute is  
when the program will start. Press the PROG button.  
Copy Day/Time  
Fan Hold  
(DOOR  
OPEN)  
3. Use the Up/Down arrow buttons to select the desired  
heat setpoint; press the PROG button.  
Settings Enter  
4. Use the Up/Down arrow buttons to select the desired  
cool setpoint; press the PROG button.  
(Outside)  
NOTE − Pressing the ENTER button during the following  
programming steps, saves and exits to the HOME screen.  
Figure 14  
To program events and temperatures, perform the follow-  
ing steps.  
7. Repeat steps 1 5 to program different times and tem-  
peratures for days not yet programmed.  
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51M42 SERIES 5/2  
 
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FAN − Controlling the Fan Operation  
SETTINGS − Filter/Maintenance Reminders  
Use the FAN button to select either continuous fan mode or  
auto fan mode.  
The 51M42 thermostat is designed to remind you when the  
filter needs changing or when routine maintenance is re-  
quired, as (and if) defined, by you. These optional remind-  
ers are not enabled until you activate them. To do so, press  
the SETTINGS button (shown below the FAN button in fig-  
ure 16) once or twice for the desired reminder as shown in  
figure 18 and as described in table 1.  
To change from continuous to auto fan mode (or vice ver-  
sa), press the FAN button. Note whether a fan icon in the  
FAN box is present (indicating that the fan is running) or not  
(fan not running).  
If continuous fan mode is enabled (ON displayed in FAN  
box − see figure 15), the fan will run continuously regard-  
less of whether the heating or cooling equipment is run-  
ning.  
Reminder Settings Display  
Heat Cool  
Using Fan ON  
OFF  
Prog Auto  
i0:30  
For FILTER, press SETTINGS  
button 1 time  
Copy Day/Time  
FILTER  
For MAINTENANCE, press  
SETTINGS button 2 times  
Fan Hold  
72  
MAINTENANCE  
Figure 18  
Figure 15  
The default setting for the reminders is OFF (disabled).  
Press Up/Down arrow buttons to select the desired re-  
minder intervals.  
If auto fan mode is selected (AUTO displayed in FAN box −  
see figure 16), the fan will only run when the heating or  
cooling equipment is running.  
Table 1  
Heat Cool  
Using Fan AUTO  
Filter and Maintenance Reminders  
Prog Auto  
i0:30  
Buttons to  
Use  
Available Settings and  
How to Use  
Reminder  
FILTER  
Copy Day/Time  
Settings  
(1st press)  
then Arrows  
to scroll  
Total fan run time expressed in  
months (Off, 1, 3, 6, 12); for  
example, if fan runs 12 hours a  
day, 1 month reminder displays  
in 2 calendar months.  
Fan Hold  
72  
selections  
Figure 16  
Settings  
(2nd press)  
then Arrows  
to scroll  
Elapsed chronological time in  
months (Off, 6, 12). Use this,  
for example, to remind yourself  
when to perform routine checks  
or when to call a technician for  
perodic preventive  
MAINTE-  
NANCE  
Displaying Outside Temperature  
Pressing the ENTER (Outside) button at the HOME  
screen displays the outside temperature (see figure 17) for  
several seconds, then returns to room temperature.  
selections  
maintenance.  
Check Outside Temperature  
Enter  
Stores settings.  
Fan Hold  
88  
NOTE − The HOME screen will reappear about 15 seconds  
after the final arrow button press. OR, press the ENTER  
button at any time to store any changes and exit to the  
HOME screen.  
Settings Enter  
(Outside)  
After either programmed interval has elapsed, the remind-  
Figure 17  
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To adjust the balance point up or down, press the Up/Down  
arrow buttons to the right of the display screen. The bal-  
ance point setting increases by 1°F each time the Up but-  
ton is pressed and decreases by 1°F each time the Down  
button is pressed. After the desired balance point setting is  
reached, the HOME screen will reappear after about 15  
seconds.  
Reminders  
MO  
AM  
PROG  
A B C D  
HOLD  
PM I0:26  
FAN ON  
AUTO  
Filter &  
Maintenance  
Indicators  
FILTER SERVICE  
MAINTENANCE  
ROOM  
COOL  
OFF  
NOTE − The balance point feature allows the outdoor tem-  
perature to rise or fall 3°F above or below the balance  
point. This prevents excessive cycling of the equipment  
when the outdoor temperature is near the balance point.  
For example, if the balance point is 40°F and the actual  
outdoor temperature is 35°F, the outdoor temperature  
must rise to 43°F before equipment adjustment occurs.  
Conversely, if the balance point is 40°F and the actual out-  
door temperature is 45°F, the outdoor temperature must  
drop to 37°F before equipment adjustment occurs.  
AUTO  
EMG.  
HEAT  
76°  
FILTER SERVICE  
MAINTENANCE  
ROOM  
FAN ON  
AUTO  
Service  
Indicator  
Flashing  
COOL  
OFF  
AUTO  
EMG.  
HEAT  
76°  
Service Indicator  
When abnormal equipment operation is detected, the  
service from a qualified service technician.  
Figure 19  
After the filter has been changed or maintenance per-  
formed, reset the reminder by pressing the SETTINGS  
button for 4 seconds at the corresponding reminder set-  
tings screen. The screen will blink for a few moments to in-  
dicate that the timer has been reset.  
Thermostat RESET  
Under some abnormal conditions, it may be necessary to  
ꢀreset" the thermostat to its default condition. Such a RE-  
SET would delete all programming and settings and there-  
fore should only be used on rare occasions when the the  
thermostat fails to function as designed and/or as pro-  
grammed. Such an instance can occur as a result of a pow-  
er surge or similar electrical disturbance (e.g. after an elec-  
trical storm or power outage). The RESET button can be  
used to recover from this situation.  
SETTINGS − Balance Point  
NOTE − The balance point is only available with the out-  
door sensor.  
When the outdoor sensor (X2658) is connected to the  
51M42 thermostat, balance point adjustment is available.  
The balance point feature allows the measured outdoor  
temperature to govern operation of the heat pump and  
backup heat source.  
CAUTION  
When the RESET button is pressed, ALL settings re-  
vert back to the defaults, including the default pro-  
gram (see tables 2 and 3).  
Press the SETTINGS button three times to access the Bal-  
The RESET button is an unlabeled, recessed button lo-  
cated behind the door, on the right−hand side of the ther-  
paper clip or small pencil to press the RESET button; ALL  
thermostat settings will be reset to the defaults listed in the  
Default Thermostat Settings section.  
Balance Point Setting  
SETPOINT  
Removing/Installing Thermostat  
°
6p  
Figure 20  
55  
The thermostat hinges on tabs on the top of the subbase;  
no tool is needed to remove the thermostat from the sub-  
base. Pivot the bottom of the thermostat outward (releas-  
ing the snaps), then lift up to remove.  
The balance point can be set at any temperature between  
1°F and 55°F (−17°C to 13°C). A balance point setting of  
zero disables the balance point function (in this case, the  
thermostat behaves as if no outdoor sensor is attached).  
The default balance point setting is 55°F.  
To replace it, first position the top tilted toward the wall  
bracket and align it until you feel the tabs and slots engage;  
then, while the top is in place, pivot the bottom toward the  
wall until the thermostat snaps into place.  
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51M42 SERIES 5/2  
 
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With a VERY LARGE heat demand, temperature is:  
below setpoint –2.5F  
OR 15−minute 2 upstage timer HAS expired.  
Default Thermostat Settings  
nd  
Default thermostat settings are shown in table 2 and the  
default program is shown in table 3.  
Emergency heat demand (only if emergency heat is  
enabled): temperature is below setpoint –0.5F.  
Table 2  
Default Thermostat Settings  
With a SMALL cool demand, temperature is:  
above setpoint +0.5F but below setpoint +1.5F  
AND 30-minute upstage timer HAS NOT expired.  
Mode  
Heat (Perm. Hold)  
70°F (or 21°C)  
78°F (or 26°C)  
Auto  
Heat Setpoint (Perm Hold)  
Cool Setpoint (Perm Hold)  
Fan  
With a LARGE cool demand, temperature is:  
above setpoint +1.5F  
OR 30-minute upstage timer HAS expired.  
Filter Reminder  
Maintenance Reminder  
OFF  
Table 4  
OFF  
THERMOSTAT OUTPUTS  
Equipment Protection Timers Reset Back to Zero  
Autochangeover OFF  
W1  
E
Y1 Y2  
G
O
Demand Condition  
Cooling Demands  
Table 3  
Default Program Settings − Time & Temp.  
SMALL  
X
X
X
X
X
X
LARGE  
X
X
No Demand  
Programs  
Time  
Temp. − Heat Temp. − Cool  
All days − A 6:00am 70_F / 21°C  
All days − B 8:00am 62_F / 17°C  
All days − C 6:00pm 70_F / 21°C  
All days − D 10:00pm 62_F / 17°C  
78_F / 26°C  
85_F / 29°C  
78_F / 26°C  
82_F / 28°C  
Heating Demands  
With Electric Backup (no outdoor sensor)  
SMALL  
X
X
X
X
X
X
X
LARGE  
X
X
VERY LARGE  
Emergency  
No Demand  
X
Thermostat Output Table  
X
Table4 depicts the 51M42 thermostat output states for var-  
ious input conditions. The following notes described terms  
used in the table.  
With Gas Backup (dual fuel; no outdoor sensor)  
SMALL  
X
X
X
X
NOTES:  
LARGE  
X
S X = output is activated with 24VAC.  
S BBP: outdoor temperature is below balance point  
S ABP: outdoor temperature is above balance point  
VERY LARGE  
Emergency  
No Demand  
X
X
With Electric Backup (outdoor sensor attached)  
S In all cases, the state of the B terminal is opposite that  
SMALL (Above Balance Point)  
LARGE (ABP)  
X
X
X
X
X
X
X
X
X
X
X
X
X
of the O terminal  
X
X
S Data are tabulated for AUTO fan setting. If the fan set-  
VERY LARGE (ABP)  
SMALL (Below Balance Point)  
LARGE (BBP)  
ting is ON, the G output is activated in all cases  
S Upstage timers:  
X
X
30 minutes (when upstaging from small to large  
demand)  
VERY LARGE (BBP)  
Emergency  
X
X
15 minutes (HEAT MODE ONLY − when upstaging  
from large to very large demand)  
No Demand  
With Gas Backup (dual fuel; outdoor sensor attached)  
S The temperature ranges expressed in the following  
definitions of ꢀSMALL/LARGE/VERY LARGE" de-  
mands are for guidance only; actual temperatures  
may vary:  
SMALL (ABP)  
LARGE (ABP)  
VERY LARGE (ABP)  
SMALL (BBP)  
LARGE (BBP)  
VERY LARGE (BBP)  
Emergency  
X
X
X
X
X
X
X
X
X
With a SMALL heat demand, temperature is:  
below setpoint –0.5F but above setpoint –1.5F  
AND 30-minute upstage timer HAS NOT expired.  
With a LARGE heat demand, temperature is:  
less than setpoint –1.5F  
OR 30-minute upstage timer HAS expired.  
X
No Demand  
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Indoor Temperature Measurement Range  
Technical Specifications  
Measurement Scale: Fahrenheit  
Measurement Range: 35°F to 99°F  
Thermostat Type  
Electronic programmable thermostat for heat pump,  
3-stage heat/2-stage cool.  
Measurement Resolution: 0.5°F  
Measurement Accuracy: +/−1°F  
NOTE − 2 stages of heat pump heat and one stage of auxil-  
iary (gas or electric) backup heat are supported.  
Field Offset: via DIP switches to +/−3°F  
Sampling Method: temperature measurements sampled  
every 15 seconds. Displayed temperature is the average  
of the last four measurements.  
Power Supply Range  
18VAC − 30VAC (24VAC nominal), 60Hz  
Outdoor Temperature Measurement Range  
CAUTION  
24VAC is present on the terminals of the thermostat  
bracket. If removing the thermostat from the wall,  
use caution and avoid touching any of the connec-  
tor terminals on the wall bracket.  
Measurement Scale: Fahrenheit  
Measurement Range: −22°F to 122°F  
Measurement Resolution: 1°F  
Measurement Accuracy: +/−2°F  
Also, when working with the thermostat dip  
switches, use a non−conductive tool and take cau-  
tion to avoid making any contact with the circuit  
board, its imprinted circuitry and its connector  
prongs.  
Temperature Setpoint Range  
Setting range: 50°F (10°C) to 90°F (32°C)  
Setting resolution: 1°F (1°C)  
Smart Setback Recovery (via DIP switch #6)  
Smart Setback Recovery (SSR) affects the way the ther-  
mostat responds to program events. If SSR is disabled, the  
thermostat will react to a program event at the time the  
event occurs. However, if SSR is enabled, the thermostat  
will react to a program event before the event occurs such  
that the desired temperature is reached at the time of the  
event, not after.  
Temperature Display  
Display Scale: Fahrenheit or Celsius user selectable (via  
Display range: 35°F (2°C) to 99°F (37°C)  
Display resolution: 1°F (1°C)  
Display Accuracy: +/−1°F  
If the Fahrenheit/Celsius display must be changed, use a  
plastic, non−conductive tool to push the dip switch to the  
Autochangeover Deadband Selection (via DIP  
switch #7)  
Changing Fahrenheit/Celsius Setting  
Autochangeover deadband can be set to 4 or 6 degrees.  
When autochangeover is enabled (via the AUTO button),  
the thermostat will automatically change over from heating  
to cooling and vice versa, to keep the room temperature in  
between the heating and cooling setpoints. The deadband  
is the minimum difference between the heating and cooling  
setpoints.  
1
2
3
4
5
Fan Control  
PLUG IN (Use Care To Avoid Bending Prongs.)  
6
7
AUTO or ON modes.  
I/O Relays  
F/C (Fahrenheit/Celsius) switch  
shown in Celsius position  
All thermostat relays are latching type to minimize power  
consumption.  
Figure 21  
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51M42 SERIES 5/2  
 
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Table 5  
Over−Temperature Protection  
Thermal-mechanical switch opens W1, Y1, Y2, and E at  
93°F+/−6°F.  
51M42 Terminal Designations  
Term. Description  
Filter Reminder  
B
Reversing valve, heat active  
Reversing valve, cool active  
24VAC  
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are  
available. When programmed time has elapsed, a FILTER  
indicator is displayed.  
O
R
Y1  
First-stage cooling/heating, compressor  
energized  
Maintenance Reminder  
Settings of Off, 6 or 12 (months of chronological time) are  
available. When programmed time has elapsed, a mainte-  
nance indicator ꢀMAINTENANCE" is displayed.  
W1*  
Y2  
Auxiliary heating, furnace energized  
Second-stage cooling/heating, compressor  
energized  
Service Reminder  
The SERVICE indicator is displayed only under the follow-  
ing conditions:  
E*  
G
L
Emergency heat  
Fan control  
Service Indicator  
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the R terminal;  
C
T
24VAC common  
Outdoor temperature sensor connection 1  
Outdoor temperature sensor connection 2  
OR  
T
S
if the thermostat Y1 terminal has been activated with  
24VAC for at least 5 minutes, AND the L terminal is  
shorted to the C terminal.  
* For most applications, E will be jumpered to W1. If sep-  
arate wires are not provided for both E and W1, jumper  
the E terminal to the W1 terminal on the thermostat sub-  
base. For applications involving the use of a balance  
point (whereby the outdoor temperature is to be used to  
restrict either heat pump operation or backup heat  
source operation), the outdoor sensor (part number  
X2658) MUST be installed.  
Power Loss/Recovery  
Thermostat memory is retained for a minimum of 24 hours  
during a power loss (includes retention of clock setting,  
program information, HOLD status, programmed temper-  
ature setpoint, heat/cool and fan mode settings, filter re-  
minder status, maintenance reminder status, and equip-  
ment protection timers). After 24 hours of power loss,  
programmed settings will be lost and replaced with default  
settings.  
Equipment Protection Timers  
Minimum Compressor OFF time: 5 minutes  
Minimum Compressor ON time: 4 minutes  
Minimum Furnace ON time: 3 minutes  
IMPORTANT  
Power must be applied for at least six consecutive  
hours prior to a power loss in order for memory to  
be retained for the specified time.  
Minimum furnace cycle time (elapsed time between any  
furnace activation and the next furnace activation): 6 min-  
utes.  
Minimum elapsed time between any compressor activa-  
tion and the next compressor activation: 6 minutes.  
LCD Backlight  
Activated for 30 seconds when any button is pressed.  
NOTE − All protection timers (except the compressor OFF  
timer) can be over−ridden if a heating or cooling demand is  
initiated or terminated using the UP, DOWN, HEAT, or  
COOL buttons.  
NOTE − During an electrical storm or similar disturbance,  
the backlight may activate for a few seconds. This is nor-  
mal and will no longer occur after the electrical disturbance  
has passed.  
Equipment Protection Override  
Thermostat Operating Conditions  
35°F to 105°F, 5% to 90% RH  
Both the minimum compressor OFF timer and the mini-  
mum equipment cycle timer can be over−ridden by press-  
ing and holding either the HEAT or COOL button down for 4  
seconds.  
Thermostat Storage Conditions  
−40°F to 185°F, 5% to 95% RH  
Page 10  
505069M 07/05  
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OPERATION  
MANUAL  
E2005 Lennox Industries Inc.  
Dallas, Texas, USA  
51M38 Merit® Series  
7 Day Programmable Thermostat  
CONTROLS  
505,067M  
07/05  
Litho U.S.A.  
Table of Contents  
51M38 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .  
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .  
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .  
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .  
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
AUTO − Using the Autochangeover Mode . . . . . . . . .  
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .  
PROG − Thermostat Programming . . . . . . . . . . . . . . .  
COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .  
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .  
Displaying Outside Temperature . . . . . . . . . . . . . . . . .  
SETTINGS − Filter /Maintenance Reminder . . . . . . .  
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .  
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .  
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .  
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .  
RETAIN THESE INSTRUCTIONS  
FOR FUTURE REFERENCE  
51M38 Merit® Series Thermostat  
®
The Lennox Merit Series 7 day programmable electronic  
thermostat 51M38 provides excellent temperature control  
and a large, easy-to-read display. This product includes a  
programmable filter change reminder, an equipment  
maintenance reminder, and a system check indicator to  
notify the user when the equipment requires service.  
Thermostat 51M38 is suitable for non−heat pump, single-  
stage heat/single-stage cool applications with either gas or  
electric furnace. Also, the thermostat provides auto-  
changeover capability.  
General  
Initial Power−Up Display  
Copy tofrom MO TU WE TH FR SA SU  
These instructions are intended as a general guide and do  
not supersede local codes in any way. Consult authorities  
having jurisdiction before installation.  
PROG  
A B C D  
AM  
PM  
I8:88  
HOLD  
Check equipment for shipping damage. If you find any  
damage, immediately contact the last carrier.  
FAN ON  
AUTO  
COOL SETPOINT  
FILTER  
SERVICE  
°
COOL  
OFF  
MAINTENANCE  
Introduction  
AUTO  
EMG.  
HEAT  
OUTROOM  
i88°  
HEAT SETPOINT  
This document describes the operation of Lennox thermo-  
stat 51M38. Refer to the installation manual for instructions  
regarding installation and wiring of the thermostat.  
°
Hi  
Lo  
Initial Thermostat Power-up  
Figure 1  
When power is initially applied to the thermostat, the dis-  
All display segments are momentarily activated. This oc-  
curs as a normal part of thermostat initialization.  
07/05  
505,067M  
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Within a few seconds, the HOME screen appears (see fig-  
ure 2) with default settings as shown. After about 1 minute  
of initialization time, the actual room temperature will be  
displayed.  
Thermostat Buttons  
Heat Cool  
Prog Auto  
Copy Day/Time  
Fan Hold  
HOME Screen  
MO  
PROG  
A B C D  
HOLD  
(DOOR  
OPEN)  
AM  
PM I0:26  
Settings Enter  
(Outside)  
FAN ON  
AUTO  
COOL  
OFF  
ROOM  
AUTO  
Figure 3  
EMG.  
70°  
HEAT  
When an Arrow, HOLD, HEAT, or COOL button is pressed,  
SETPOINT and the temperature setting appears for 15  
seconds. If desired, start making changes within 15 sec-  
onds or the HOME screen will return. The backlight will  
turn off 15 seconds after the HOME screen reappears.  
Figure 2  
At this point, the thermostat will be fully functional; its de-  
fault temperature setpoint (not shown) is 70°F. At this  
point, if the equipment has been fully powered and if a heat  
demand were present, the system would begin operating.  
DAY/TIME − Setting the Day and Time  
NOTE − Temperature scale default is Fahrenheit units but  
may be reset to show Celsius, if desired. See page 7.  
Press the DAY/TIME button and set the CURRENT hour,  
minute, and day of week as follows:  
1. ꢀAM12" will flash on the screen. Press the UP/DOWN  
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-  
spond to time of day.) Press the DAY/TIME button OR,  
if adjusting for daylight savings time, pressing the EN-  
TER button stores the single change and exits to the  
HOME screen, bypassing minutes and day of week.  
Buttons, Backlight, Timers & Settings  
Buttons are located behind the small door on the right−  
hand side of the thermostat (see figure 3).  
IMPORTANT  
Do NOT begin pressing buttons until after you read  
the following section describing each button.  
2. Minutes will flash. Use the UP/DOWN arrow button to  
display the minutes past the hour. Press the DAY/  
TIME button.  
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow  
buttons to display the current day. Day selections are  
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",  
and ꢀSU". Press the DAY/TIME button.  
A pale blue display backlight illuminates for 30 seconds  
each time any button is pressed.  
When the PROG or DAY/TIME button is pressed, a field  
begins flashing, expecting another input. Start making  
changes within 15 seconds or the HOME screen will re-  
turn.  
4. The HOME screen reappears; confirm day and time  
are correct. This completes day and time setting.  
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HEAT − Using the Heat Mode  
COOL − Using the Cool Mode  
Enabling and Disabling Cool Mode  
Use the COOL button to enable or disable cool mode as  
desired. If the thermostat is in heat or off mode, cool mode  
is enabled when the COOL button is pressed. This is indi-  
cated by COOL on the right side of the display (see figure  
Enabling and Disabling Heat Mode  
The thermostat must be in the heat mode to control the  
heating equipment. Press the HEAT button to enable or  
disable heat mode. If the thermostat is in cool or off mode,  
pressing the HEAT button enables the heat mode (indi-  
cated by HEAT in the lower right corner − see figure 4).  
Turn Cool ON/OFF  
Heat Cool  
Turn Heat ON/OFF  
Heat Cool  
Prog Auto  
i0:30  
Prog Auto  
Copy Day/Time  
5:30  
70  
Copy Day/Time  
Fan Hold  
(DOOR  
OPEN)  
75  
82  
66  
Fan Hold  
(DOOR  
OPEN)  
70  
Settings Enter  
66  
Settings Enter  
(Outside)  
(Outside)  
Figure 7  
If the thermostat is in cool mode, pressing the COOL but-  
ton disables cool mode (indicated by OFF, similar to figure  
Figure 4  
If the thermostat is in heat mode, heat mode is disabled  
when the HEAT button is pressed. This is indicated by OFF  
Cooling Demand  
Set the thermostat to cool mode to control the cooling  
equipment. Then, if the room temperature is higher than  
the temperature setpoint, as shown in figure 7, the thermo-  
stat detects a cooling demand and will activate the cooling  
equipment to satisfy the demand.  
Heat/Cool Mode Disabled  
OFF  
ROOM  
75°  
HEAT SETPOINT  
Cooling operation is indicated by flashing ꢀsnowflake"  
icons in the display. When the actual temperature drops  
below the temperature setpoint, the snowflake icons will  
disappear. This indicates that the cooling demand has  
been satisfied and that the cooling equipment has been  
turned off.  
75°  
Figure 5  
Heating Demand  
NOTE − If no buttons are pressed during a demand for  
cooling, the equipment must operate for at least 4 minutes.  
After a demand has been satisfied, cooling equipment op-  
eration is locked out for 5 minutes. If another cooling de-  
mand occurs during this 5−minute interval, ꢀCOOL" and the  
snowflakes will flash; however, the cooling equipment will  
not operate until the 5−minute delay has elapsed.  
Set the thermostat to heat mode to control the heating  
equipment. Then, if the room temperature is lower than the  
temperature setpoint, as shown in figure 6, the thermostat  
detects a heating demand and will activate the heating  
equipment to satisfy the demand.  
Heating Demand  
NOTE − The cooling setpoint cannot be lower than the  
heating setpoint. Also, the difference between the cooling  
and heating setpoint must be at least 4 degrees.  
ROOM  
72°  
HEAT SETPOINT  
HEAT  
75°  
AUTO − Using the Autochangeover Mode  
When in either heat mode or cool mode, autochangeover  
can be enabled or disabled by pressing the AUTO button. If  
enabled, AUTO appears at the lower right corner of the dis-  
play. The thermostat will then automatically change over  
from heating to cooling and vice versa, to keep the room  
temperature in between the heating and cooling setpoints.  
Autochangeover can be used in either of the thermostat  
hold modes, or when the thermostat program is running.  
These modes are described later.  
Figure 6  
Heating operation is indicated by a flame icon in the lower  
right corner. When the actual temperature rises above the  
temperature setpoint, the flame icon will disappear. This  
indicates that the heating demand has been satisfied and  
that the heating equipment has been turned off.  
NOTE − Heating equipment is activated for at least 3 min-  
utes if no buttons are pressed during the demand interval.  
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Temporary Hold Temperature Mode  
HOLD − Using Temperature Hold Modes  
Heat Cool  
When HOLD is displayed at the HOME screen, the ther-  
mostat is in a temperature hold condition. This means that  
the temperature program data is ignored and the thermo-  
stat functions much like a non−programmable thermostat.  
Prog Auto  
i0:30  
Copy Day/Time  
70  
Adjusting Temperature Setpoint in Hold Mode  
Fan Hold  
(DOOR  
OPEN)  
72  
68  
The temperature setpoint represents the desired tempera-  
ture of the space around the thermostat. The default heat  
setpoint in hold mode is 70°F; the default cool setpoint in  
hold mode is 78°F.  
Settings Enter  
(Outside)  
To adjust the setpoint, press the UP or DOWN (YB) arrow  
the right of the actual room temperature. Each button  
press adjusts the setpoint up or down by 1 degree.  
Figure 9  
PROG − Thermostat Programming  
The 51M38 thermostat can be programmed to perform a  
set of both heating and cooling events for each day of the  
week. Each day can be programmed for 4 unique events  
per day, and each day can be different from any other day.  
Hold Temperature Mode  
To Set Program Events and Temperatures  
Heat Cool  
Figure 10 gives an example of how up to 7 different pro-  
grams can be set. Programs