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T8400C1099, T8401C1031  
Electronic Thermostats  
INSTALLATION INSTRUCTIONS  
The T8400 and T8401 Thermostats provide single-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, 6, 9, or  
CAUTION  
Electrical Shock or Equipment Damage  
Hazard.  
1
2 cph. Cooling cycle rate is fixed at 3 cph. Temperature  
indication can be set for °F or °C.  
Can shock individuals or short equipment  
circuitry.  
Disconnect power supply before installation.  
The T8400 Thermostat is powered through the heating/  
cooling system controls. The T8401 requires a common  
wire for operation. Batteries are not required because  
setpoints are held permanently by non-volatile memory.  
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:  
The T8400 and T8401 models include a thermostat,  
wallplate (for wiring and mounting thermostat) and  
owner’s guide.  
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.  
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.  
This thermostat is a precision instrument and was  
carefully adjusted at the factory. Handle it carefully.  
Contact your local waste management authority  
for instructions regarding recycling and the  
proper disposal of an old control containing  
mercury in a sealed tube.  
Mounting Wallplate to Wall  
IMPORTANT  
Level only for appearance. The thermostat func-  
tions normally even when not level.  
INSTALLATION  
Mount the T8400 and T8401 with the screws provided  
(
see Fig. 2) as follows:  
1. Place the wallplate at the desired location on the  
wall.  
2. Pull the thermostat wire through the entrance hole  
on the wallplate.  
3. Fasten the wallplate to the wall using the anchors  
and screws provided.  
4. After wiring the wallplate, plug the hole to prevent  
drafts from affecting the thermostat; see Wiring  
section.  
When Installing this Product…  
1
.
.
Read these instructions carefully. Failure to follow  
them could damage the product or cause a hazard-  
ous condition.  
2
Check the ratings given in the instructions and on  
the product to make sure the product is suitable for  
your application.  
3
.
.
Installer must be a trained, experienced service  
technician.  
After installation is complete, check out product  
operation as provided in these instructions.  
4
®
U.S. Registered Trademark  
6
9-1484-2  
Copyright © 2002 Honeywell •  
• All Rights Reserved  
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T8400C1099, T8401C1031 ELECTRONIC THERMOSTATS  
YES  
NO  
NO  
5
FEET  
[
1.5 METERS]  
NO  
M11338  
Fig. 1. Typical location of thermostat.  
Wiring  
WALL  
WALL  
ANCHORS (2)  
IMPORTANT  
1
Use an 18-gauge maximum wire to wire the  
T8400 and T8401 Thermostats.  
All wiring must comply with local electrical codes and  
ordinances. Disconnect the power supply to prevent  
electrical shock or equipment damage.  
NOTE: To ensure proper mounting of thermostat,  
restrict all wiring to the shaded area. See Fig. 4.  
The shape of the terminals permits insertion of straight or  
wraparound wiring connections; either method is  
acceptable. See Fig. 5.  
MOUNTING  
SCREWS (2)  
WALLPLATE  
1
WHEN USING WALL ANCHORS, DRILL 3/16 INCH  
HOLES FOR DRYWALL, 7/32 INCH HOLES FOR  
Refer to Fig. 6 through 10 for typical wiring hookups.  
PLASTER OR WOOD.  
M12202A  
Fig. 2. Mounting wallplate to wall.  
Setting Fan Operation (Fuel) Switch  
The fan operation (fuel) switch is preset at the factory in  
the F position. See Fig. 3. This is the correct setting for  
most systems. If this system is an electric heat system,  
set the switch to the E position. The E setting allows the  
fan to turn on immediately with the heating or cooling  
equipment in a system where the G terminal is  
connected.  
E
F
FAN OPERATION (FUEL) SWITCH  
M12580  
Fig. 3. Fan operation (fuel) switch.  
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T8400C1099, T8401C1031 ELECTRONIC THERMOSTATS  
KEEP WIRING IN  
SHADED AREA  
G
Rc  
COOLING  
CONTACTOR  
2
G
R
W
B
Y
O
Rc  
R
FAN  
RELAY  
HEATING  
PRIMARY  
CONTROL  
1
L1  
(HOT)  
24V  
L2  
W
Y
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
B
FACTORY INSTALLED JUMPER.  
M13275  
O
Fig. 6. Typical hookup of T8400C1099 in heat-cool  
system with single transformer.  
G
MOUNTING  
SCREW HOLE  
WIRING ENTRANCE  
HOLE  
MOUNTING  
SCREW HOLE  
Rc  
R
W
Y
2
4
B
O
3
3
FAN  
RELAY  
COOL  
DAMPER  
HEAT  
DAMPER  
1
1
COMPRESSOR  
CONTACTOR  
HEAT  
RELAY  
L1  
(HOT)  
L1  
HOT)  
(
L2  
L2  
M16432  
COOLING  
TRANSFORMER  
HEATING  
TRANSFORMER  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD  
PROTECTION AS REQUIRED.  
Fig. 4. Restrict wiring to shaded area  
T8400C1099 shown).  
(
2
3
REMOVE RC TO R JUMPER WHEN INSTALLED ON A TWO TRANSFORMER  
SYSTEM.  
FOR STRAIGHT INSERTION  
STRIP 5/16 IN. (8 MM)  
TERMINAL  
SCREW  
CAN BE USED FOR CHANGEOVER VALVE ON SINGLE-STAGE HEAT  
PUMP SYSTEMS.  
FOR WRAPAROUND  
STRIP 7/16 IN. (11 MM)  
4
POWER TO R TERMINAL IS REQUIRED WHEN THE SYSTEM SWITCH IS IN  
THE OFF POSITION.  
M12562A  
G
Fig. 7. Typical hookup of T8400C1099 in heat-cool  
system with two transformers.  
Rc  
R
G
Rc  
3
2
2
HEAT  
CHANGEOVER  
R
W
W
Y
B
4
COOL  
CHANGEOVER  
B
Y
O
1
FAN  
RELAY  
O
L1  
HOT)  
COMPRESSOR  
(
24V  
M13274  
L2  
TRANSFORMER  
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS  
AND OVERLOAD PROTECTION AS REQUIRED.  
Fig. 5. Wiring connections  
T8400C1099 shown).  
(
CAN BE USED FOR CHANGEOVER VALVE ON  
SINGLE-STAGE HEAT PUMP SYSTEMS.  
3
4
FACTORY INSTALLED JUMPER.  
FIELD INSTALLED JUMPER.  
M20880  
Fig. 8. Typical hookup of T8400C1099 in single-stage  
heat pump systems.  
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T8400C1099, T8401C1031 ELECTRONIC THERMOSTATS  
DASHED LINES INDICATE TABS  
ON BACK OF THERMOSTAT  
G
C
R
W
B
Y
O
1
HEAT  
COOL  
FAN  
L1  
RELAY  
RELAY  
RELAY  
(HOT)  
24V  
L2  
Auto On  
FAN  
TRANSFORMER  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS  
AND OVERLOAD PROTECTION AS REQUIRED.  
Cool O  
SYS Tf fE MH eat  
M20881  
Fig. 9. Typical hookup of T8401C1031 in heat-cool  
system.  
A
ENGAGE TABS AT TOP OF THERMOSTAT  
WITH SLOTS ON MOUNTING PLATE.  
G
C
2
HEAT  
R
CHANGEOVER  
W
B
2
3
COOL  
CHANGEOVER  
Y
O
1
FAN  
RELAY  
L1  
HOT)  
COMPRESSOR  
FAN  
uto On  
(
A
24V  
L2  
TRANSFORMER  
Cool Off Heat  
SYSTEM  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS  
AND OVERLOAD PROTECTION AS REQUIRED.  
2
3
CAN BE USED FOR CHANGEOVER VALVE ON  
SINGLE-STAGE HEAT PUMP SYSTEMS.  
FIELD INSTALLED JUMPER.  
M20882  
Fig. 10. Typical hookup of T8401C1031 in single-  
stage heat pump system.  
B
PRESS LOWER EDGE OF  
CASE TO LATCH.  
M14677  
Fig. 11. Mounting thermostat wallplate.  
Mounting Thermostat to Wallplate  
1
.
Engage the tabs at the top of the thermostat and  
wallplate.  
Swing down the thermostat and press the lower  
edge of the thermostat onto the wallplate to latch.  
See Fig. 11.  
OPERATION  
2.  
Setting FAN and SYSTEM Switches  
Fan and system settings are controlled manually by  
using the switches located at the bottom of the  
thermostat case. See Fig. 12.  
FAN Switch  
Fan switch settings are:  
On: The fan runs continuously. Use for improved air  
circulation and air quality.  
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.  
Slide the FAN switch in the bottom left corner of the  
thermostat to select the desired fan setting.  
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T8400C1099, T8401C1031 ELECTRONIC THERMOSTATS  
SYSTEM Switch  
3. To increase the temperature setpoint press the ▼  
key. Each press changes the setpoint one degree;  
press and hold to change the setpoint several  
degrees.  
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.  
Slide the SYSTEM switch in the bottom right corner of  
the thermostat to select the desired system setting.  
M14681  
4.  
To decrease the temperature setpoint press the ▼  
key. Each press changes the setpoint one degree;  
press and hold to change the setpoint several  
degrees.  
TEMPERATURE DISPLAY  
INCREASE SETTING  
DECREASE SETTING  
M14682  
Setting °F/°C Indication and Heat Cycle Rate  
NOTE: To save changes to the °F/°C indication, the  
heat cycle rate and the C3/C1 temperature  
control, tap the key until exiting the installer  
setup.  
Au  
to On  
FA  
N
To set the °F/°C indication, the heat cycle rate and the  
C3/C1 temperature control:  
SYSTEM  
Cool Off Heat  
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  
FAN SWITCH  
M14678  
SYSTEM SWITCH  
11°C.  
Fig. 12. Digital display and system  
switches (FAN and SYSTEM).  
M14683  
Set Temperature Setpoint  
OPTIONAL SYSTEM CHECKOUT  
NOTE: Temperature setpoint range is 40° to 99°F (5° to  
7°C).  
When in steps 2 and 3 only, the 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.  
3
The temperature setpoint and the room temperature are  
shown separately on the digital display. The indicator  
points to Set when the temperature setpoint is displayed  
and to Room when the room temperature is displayed.  
.
Examples:System setting at HEAT: If heat is on, the ▼  
key turns it off; if heat is off, turns it on.  
INDICATOR  
ROOM TEMPERATURE  
Set  
Room  
M14684  
SETPOINT TEMPERATURE  
M14679  
System setting at COOL: If cool is on, key  
turns it off; if cool is off, turns it on.  
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. 12.  
2.  
To display the temperature setpoint on the digital  
display, press either the or key once. The tem-  
perature setpoint is displayed for approximately  
five seconds as the indicator points to Set and  
flashes.  
M14685  
2.  
Press the ▼▼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 dis-  
played.  
M14680  
M14686  
5
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T8400C1099, T8401C1031 ELECTRONIC THERMOSTATS  
NOTE: In installer setup mode only, each press of the  
Table 1. Heating Cycle Rates.  
System Cycles Per Hour  
Steam, gravity  
key momentarily displays 01. Each press of  
the key momentarily displays 02. When the  
keys are released, these two-digit codes are no  
longer displayed.  
1
3
Hydronic heat, condensing gas  
furnacea  
3.  
Press the 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.  
Gas or oil forced air  
Electric heat  
6
9
Special applicationsb  
4, 5, 12  
a
High efficiency furnace.  
b
Refer to equipment manufacturer’s instructions.  
M14687  
4
5
6
.
.
.
Press the key again to display °C or °F.  
1
0. Press the  key to save all changes, exit installer  
setup mode and return to normal operation.  
NOTE: After exiting installer setup mode, slide the  
System switch to the desired position and  
change the setpoint to the desired room  
temperature.  
M14688  
Press the key to change the °C or °F indication.  
CHECKOUT  
M14689  
Press the key to display the heat cycle rate of 1,  
3
, 4, 5, 6, 9, or 12. To change the heat cycle rate,  
press the key to scroll between 1, 3, 4, 5, 6, 9, or  
2. Stop scrolling when the desired rate is dis-  
Heating  
1
.
Slide the SYSTEM switch to Heat and the FAN  
switch to Auto.  
1
played. See Table 1 for the cycle rate options and  
the corresponding system equipment.  
2
.
Press and hold the 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.  
M14690  
7
.
.
Press the key to display heating/cooling temper-  
ature control default..  
3
.
Press the key to lower the temperature setting  
below the room temperature. Heating equipment  
should stop.  
M18419A  
Cooling  
8
Press the key again to change heating/cooling  
temperature control to C1 or C3.  
C1 = Standard heating/cooling temperature  
control.  
C3 = Aggressive heating/cooling temperature  
control (can cause room temperature to  
overshoot the setpoint).  
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.  
9.  
Press the key again. Factory information (varies  
by model) is shown.  
For compressor protection, during power  
interruption and on initial startup, the T8400  
and T8401 Thermostats go into a five-minute  
delay. During this delay, the cooling icon (a  
snowflake ) flashes.  
M18420  
1
.
.
Slide the SYSTEM switch to Cool and the FAN  
switch to Auto.  
Press the  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.  
2
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T8400C1099, T8401C1031 ELECTRONIC THERMOSTATS  
3.  
Press the key to raise the temperature setting  
above the room temperature. Cooling system  
should shut down.  
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 equip-  
ment 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.  
NOTE: To bypass the five-minute delay, see the  
Optional System Checkout section.  
Be sure all equipment responds correctly to the  
thermostat.  
7
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T8400C1099, T8401C1031 ELECTRONIC THERMOSTATS  
Automation and Control Solutions  
Honeywell  
985 Douglas Drive North  
Golden Valley, MN 55422  
Honeywell Limited-Honeywell Limitée  
35 Dynamic Drive  
Scarborough, Ontario  
M1V 4Z9  
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69-1484-2 G.H. Rev. 11-02  
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