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T8400C and T8401C  
Electronic Thermostats  
INSTALLATION INSTRUCTIONS  
The T8400C and T8401C Thermostats provide single-  
INSTALLATION  
stage, non-programmable temperature control for 24V  
heating-cooling systems with manual changeover from  
heat to cool. Heating cycle rate is selectable at 1, 3, 4, 5,  
When Installing this Product…  
6
, or 9 cph. Cooling cycle rate is fixed at 3 cph.  
1.  
2.  
3.  
Read these instructions carefully. Failure to follow  
them could damage the product or cause a hazard-  
ous condition.  
Check the ratings given in the instructions and on  
the product to make sure the product is suitable for  
your application.  
Temperature indication can be set for °F or °C.  
The T8400C Thermostat is powered through the heating/  
cooling system controls. The T8401C Thermostat is  
powered directly from the system transformer. Batteries  
are not required because setpoints are held permanently  
by non-volatile memory for all models.  
Installer must be a trained, experienced service  
technician.  
All TRADELINE® models include a thermostat, wallplate  
4. After installation is complete, check out product  
(for wiring and mounting thermostat) and owner’s guide.  
operation as provided in these instructions.  
The T8400C SUPER TRADELINE® model also includes  
two decorator cover plates for covering wallmarks.  
CAUTION  
Electrical Shock or Equipment Damage  
Hazard.  
Can shock individuals or short equipment  
MERCURY NOTICE  
If this control is replacing a control that contains  
mercury in a sealed tube, do not place your old  
control in the trash.  
circuitry.  
Disconnect power supply before installation.  
Contact your local waste management authority  
for instructions regarding recycling and the  
proper disposal of an old control containing  
mercury in a sealed tube.  
Location  
Install the thermostat about 5 ft (1.5m) above the floor in  
an area with good air circulation at average temperature.  
See Fig. 1. Do not install the thermostat where it can be  
affected by:  
drafts or dead spots behind doors and in corners.  
hot or cold air from ducts.  
radiant heat from the sun or appliances.  
concealed pipes and chimneys.  
unheated (uncooled) areas such as an outside wall  
behind the thermostat.  
This thermostat is a precision instrument and was  
carefully adjusted at the factory. Handle it carefully.  
®
U.S. Registered Trademark  
Copyright © 2002 Honeywell •  
• All Rights Reserved  
69- 1480- 1  
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T8400C AND T8401C ELECTRONIC THERMOSTATS  
YES  
NO  
NO  
5 FEET  
1.5 METERS]  
[
NO  
M10308  
Fig. 1. Typical location of thermostat.  
The shape of the terminals permits insertion of straight or  
wraparound wiring connections; either method is  
acceptable. See Fig. 4.  
Mounting Decorator Cover Plate and  
Wallplate to Wall  
IMPORTANT  
The T8400C Thermostat is powered through the heating/  
cooling system controls and is adaptable to most 4-wire,  
18 to 30 Vac heating-cooling systems.  
Level only for appearance. The thermostat func-  
tions normally even when not level.  
NOTE: Only SUPER TRADELINE® models have the  
cover plate.  
The T8401C Thermostat is powered directly from the  
system transformer and is adaptable to most 5-wire, 18  
to 30 Vac heating-cooling systems. Refer to Fig. 5 and 6  
for typical wiring hookups.  
Mount decorator cover plate (SUPER TRADELINE®  
model only), wallplate, T8400C or T8401C, and the  
screws provided (see Fig. 2) as follows:  
5-3/4 IN. x 5-3/4 IN.  
DECORATOR  
1
.
Place the desired decorator cover plate and the  
wallplate at the desired location on the wall.  
Pull the thermostat wire through the entrance hole  
on the decorator cover plate, then through the wall-  
plate entrance hole.  
COVER PLATE  
WALL  
2.  
1
3.  
4.  
5.  
Select the two mounting holes that best fit the  
application.  
Fasten the decorator cover plate and the wallplate  
to the wall using the anchors and screws provided.  
After wiring the wallplate, plug the hole to prevent  
drafts from affecting the thermostat; see Wiring  
section.  
2
WALL  
ANCHORS (2)  
Wiring  
IMPORTANT  
Use an 18-gauge maximum wire for wiring the  
T8400C and T8401C Thermostats.  
All wiring must comply with local electrical codes and  
ordinances. Disconnect the power supply to prevent  
electrical shock or equipment damage.  
1
2
3-7/8 IN. x 3-7/8 IN. DECORATOR COVER PLATE (SELECT MODELS).  
DRILL 3/16 INCH HOLES (IF DRYWALL) OR 7/32 (PLASTER OR WOOD)  
NOTE: To ensure proper mounting of thermostat,  
restrict all wiring to the shaded area on the left  
side of the terminals. See Fig. 3.  
WHEN USING WALL ANCHORS.  
M11041C  
Fig. 2. Mounting decorator cover plate  
and wallplate to wall.  
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T8400C AND T8401C ELECTRONIC THERMOSTATS  
KEEP WIRING IN  
SHADED AREA  
T8400C, T8401C  
W
ALTERNATE MOUNTING  
SCREW HOLE  
COOLING  
CONTACTOR  
Y
G
W
R
FAN  
RELAY  
2
C
Y
1
MOUNTING  
SCREW  
HOLE  
L1  
T
T
(HOT)  
G
1
OIL PRIMARY  
CONTROL  
L1  
HOT)  
L2  
L2  
(
24V  
R
C
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
ALTERNATE  
MOUNTING  
SCREW HOLE  
IN T8401C INSTALLATIONS, CONNECT C TERMINAL.  
M11022B  
Fig. 6. T8400C/T8401C heat-cool wiring  
diagram in oil heating/electric cooling system.  
Oil primary has its own transformer.  
WIRING ENTRANCE HOLE  
MOUNTING  
M11023  
SCREW HOLE  
Mounting Thermostat to Wallplate  
Fig. 3. Restrict wiring to shaded area.  
1.  
Engage the tabs at the top of the thermostat and  
wallplate.  
W
2.  
Swing down the thermostat and press the lower  
edge of the thermostat onto the wallplate to latch.  
See Fig. 7.  
DASHED LINES INDICATE TABS  
ON BACK OF THERMOSTAT  
Y
G
FOR WRAPAROUND  
STRIP 7/16 IN. (11 MM)  
Set  
Room  
Set  
Room  
R
FOR STRAIGHT INSERTION  
STRIP 5/16 IN. (8 MM)  
FAN  
Auto  
YSTEM  
l Off Heat  
FAN  
Auto On  
SYSTEM  
Cool Off Heat  
C
M11232  
A ENGAGE TABS AT TOP  
OF THERMOSTAT WITH  
SLOTS ON WALLPLATE.  
TERMINAL SCREW  
Fig. 4. Wiring connections.  
M18416  
B
PRESS LOWER EDGE OF  
CASE TO LATCH.  
T8400C, T8401C  
Fig. 7. Mounting thermostat wallplate.  
W
Y
COOLING  
CONTACTOR  
G
R
C
OPERATION  
2
Setting FAN and SYSTEM Switches  
FAN  
RELAY  
HEATING  
PRIMARY  
CONTROL  
Fan and system settings are controlled manually by  
using the switches located at the bottom of the  
thermostat case. See Fig. 8.  
1
L1  
HOT)  
L2  
(
24V  
FAN Switch  
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
Fan switch settings are:  
IN T8401C INSTALLATIONS, CONNECT C TERMINAL.  
On: The fan runs continuously. Use for improved air  
circulation and air quality.  
M11021B  
Auto: Normal setting for most homes. In cooling, the  
fan starts and stops with the cooling equipment. In  
heating, the fan is controlled directly by the heating  
equipment and may start a few minutes after the  
heating equipment turns on (on most systems).  
When using an electric heat thermostat, the fan  
starts and stops with the heating equipment.  
Fig. 5. T8400C/T8401C heat-cool wiring diagram in  
single transformer system with gas heat/electric  
cooling.  
3
69-1480-1  
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T8400C AND T8401C ELECTRONIC THERMOSTATS  
Slide the FAN switch in the bottom left corner of the  
thermostat to select the desired fan setting.  
3. To increase the temperature setpoint press the s  
key. Each press changes the setpoint one degree;  
press and hold to change the setpoint several  
degrees.  
SYSTEM Switch  
System switch settings control thermostat operation as  
follows:  
Cool: The thermostat controls the cooling system.  
Off: Both heating and cooling are off.  
Heat: The thermostat controls the heating system.  
M14681  
4. To decrease the temperature setpoint press the t  
key. Each press changes the setpoint one degree;  
press and hold to change the setpoint several  
degrees.  
Slide the SYSTEM switch in the bottom right corner of  
the thermostat to select the desired system setting.  
TEMPERATURE DISPLAY  
M14682  
Setting °F/°C Indication and Heat Cycle Rate  
INCREASE  
SETTING  
NOTE: To save changes to the °F/°C indication and the  
heat cycle rate, all seven steps must be com-  
pleted.  
Set  
Room  
DECREASE  
SETTING  
To set the °F/°C indication and heat cycle rate:  
1.  
If the temperature is displayed in °F, set the tem-  
perature setpoint to 52°F. If the room temperature  
is displayed in °C, set the temperature setpoint to  
11°C.  
FAN  
Auto On  
SYSTEM  
Cool Off Heat  
FAN SWITCH  
SYSTEM SWITCH  
M18417  
M14683  
Fig. 8. Digital display and system  
switches (FAN and SYSTEM).  
OPTIONAL SYSTEM CHECKOUT  
When in steps 2 and 3 only, the t key can be used to turn  
heat or cool outputs on and off. Change the SYSTEM  
switch setting to test heat or cool outputs. No action  
takes place If the system switch is in the Off position.  
Set Temperature Setpoint  
NOTE: Temperature setpoint range is 40° to 99°F (5° to  
7°C).  
3
Examples:System setting at HEAT: If heat is on, the t  
key turns it off; if heat is off, t turns it on.  
The temperature setpoint and the room temperature are  
shown separately on the digital display. The t indicator  
points to Set when the temperature setpoint is displayed  
and to Room when the room temperature is displayed  
.
M14684  
System setting at COOL: If cool is on, t key  
turns it off; if cool is off, t turns it on.  
INDICATOR  
ROOM TEMPERATURE  
Set  
Room  
SETPOINT TEMPERATURE  
M14679  
M14685  
To set temperature setpoint:  
1
.
Select Heat or Cool by sliding the SYSTEM switch  
in the lower right corner of the thermostat to the  
desired mode. See Fig. 8.  
To display the temperature setpoint on the digital  
display, press either the s or t key once. The tem-  
perature setpoint is displayed for approxi-mately  
five seconds as the indicator points to Set and  
flashes.  
2. Press the st keys simultaneously for more than one  
second to light all segments on display and to enter  
installer setup mode. When the keys are released,  
a two-digit software revision code is displayed.  
2.  
M14686  
NOTE: In installer setup mode only, each press of the s  
key momentarily displays 01. Each press of the  
t key momentarily displays 02. When the keys  
are released, these two-digit codes are no  
longer displayed.  
M14680  
69-1480-1  
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T8400C AND T8401C ELECTRONIC THERMOSTATS  
3.  
Press the s key. Factory information is displayed. A  
typical example is shown, but information dis-  
played varies by model. This information is for fac-  
tory use only.  
10. Press the s key to save all changes, exit installer  
setup mode and return to normal operation.  
NOTE: After exiting installer setup mode, change the  
setpoint to the desired room temperature.  
M14687  
CHECKOUT  
4.  
5.  
6.  
Press the s key again to display °C or °F.  
Heating  
1.  
Slide the SYSTEM switch to Heat and the FAN  
switch to Auto, if available.  
2.  
Press and hold the s key to raise the temperature  
setting several degrees above the room tempera-  
ture. After approximately ten seconds, the heating  
equipment should start. In conventional systems,  
the system turns on the fan through the use of a  
time delay relay or through a limit control. When  
using an electric heat thermostat, the fan starts  
immediately.  
M14688  
Press the t key to change the °C or °F indication.  
M14689  
Press the s key to display the heat cycle rate of 1, 3,  
, 5, 6, 9, or 12. If the desired cycle is displayed,  
3.  
Press the t key to lower the temperature setting  
below the room temperature. Heating equipment  
should stop.  
4
press the s key to exit the installer setup mode. To  
change the heat cycle rate, press the t key to scroll  
between 1, 3, 4, 5, 6, or 9. Stop scrolling when the  
desired rate is displayed. See Table 1 for the cycle  
rate options and the corresponding system equip-  
ment.  
Cooling  
CAUTION  
Low Temperature Hazard.  
Operating at too low of an outdoor  
temperature may cause compressor damage.  
Do not operate cooling if outdoor temperature is  
below 50°F (10°C). Refer to manufacturer’s  
recommendations.  
M14690  
Table 1. Heating Cycle Rates.  
System  
Cycles Per Hour  
Steam, radiant floor heat, gravity  
1
3
1. Slide the SYSTEM switch to Cool and the FAN  
switch to Auto.  
Hydronic heat, condensing gas  
furnacea  
2.  
Press the t key to lower the temperature setting  
several degrees below the room temperature. After  
approximately five minutes, the cooling equipment  
should start. The fan starts and stops with the cool-  
ing equipment.  
Gas or oil forced air  
Electric heat  
6
9
Special applicationsb  
4, 5, 12  
3. Press the s key to raise the temperature setting  
above the room temperature. Cooling system  
should shut down.  
a
High efficiency furnace.  
b
Refer to equipment manufacturer’s instructions.  
NOTE: Steps 7 and 8 are available on select models.  
NOTE: To bypass the 5 minute delay, see the Optional  
System Checkout section.  
7.  
Press the s key to display cooling algorithm config-  
uration default.  
Fan  
1
.
Slide the SYSTEM switch to Off and the FAN  
switch to On. The fan should run continuously.  
Slide the FAN switch to Auto. In heating, the fan is  
controlled directly by the heating equipment and  
may start a few minutes after the heating equipment  
turns on (on most systems). When using an electric  
heat thermostat, the fan starts and stops with the  
heating equipment. In cooling, the fan starts and  
stops with the cooling equipment.  
2.  
M18419A  
8
.
.
Press the s key again to change cooling algorithm  
to C1 or C3.  
C1 = Standard cooling algorithm.  
C3 = Aggressive cooling algorithm (can cause  
overshooting).  
Press the s key again. Current configuration (CC) is  
displayed. A typical example is shown, but CC varies  
by model. (This information is for factory use only.)  
9
Make certain all equipment responds properly to the  
thermostat.  
M18420  
5
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