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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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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