Honeywell T8424C Thermostat User Manual

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T8400C, T8401C and T8424C,D  
Electronic Thermostats  
PRODUCT DATA  
FEATURES  
T8400Cand  
T8401C  
Attractive styling complements any decor to the  
homeowner’s delight.  
Ease-of-use means fewer homeowner questions and  
increased homeowner satisfaction.  
Large display for quick easy readability.  
Conveniently sized 3-1/2 x 3-1/2 thermostat with  
optional decorator cover plates (3-7/8 in. x 3-7/8 in. and  
5
-3/4 in. x 5-3/4 in.) makes it easy to install in a variety  
T8424C,D  
of locations.  
Keys are located for easy access.  
Fan and system switches are located on the lower  
edge to eliminate accidental setting changes.  
Easy installation, setup and self-test saves time and  
increases installer productivity.  
Manual changeover from heat to cool eliminates  
unexpected system operation.  
°F or °C temperature display for added model  
flexibility.  
Selectable heating cycle rate (1, 3, 4, 5, 6 or 9, 12 cph)  
for a variety of applications which reduces the amount  
of inventory.  
APPLICATION  
The T8400C and T8401C Standard Electronic Thermostats  
provide single-stage, non-programmable temperature control  
for 24V heating-cooling systems with manual changeover  
from heat to cool.  
Cooling cycle rate is fixed (3 cph), the standard setting  
for compressors, for speedy installation.  
Setpoints are permanently held in memory (no  
batteries used) and retained during power outages for  
increased installer and homeowner convenience.  
Power stealing and hard wired models available for  
virtually all equipment and application needs including  
gravity or radiating systems, hydronic heat,  
condensing gas furnaces, gas or oil forced air and  
electric heat.  
The T8424C Electronic Multistage Thermostat provides two-  
stage heat and one-stage cool control.  
The T8424D Electronic Multistage Thermostat provides two-  
stage heat and two-stage cool control.  
Contents  
Application ........................................................................  
Features ...........................................................................  
Specifications ...................................................................  
Ordering Information ........................................................  
Installation ........................................................................  
Settings and Adjustments .................................................  
1
1
2
2
3
8
Operation .......................................................................... 10  
Checkout .......................................................................... 12  
Troubleshooting ................................................................ 13  
®
U.S. Registered Trademark  
Copyright © 2001 Honeywell • All Rights Reserved  
68- 0180- 3  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
SPECIFICATIONS  
IMPORTANT  
Ambient Temperature Range: 40°F (4°C) to 110°F (43°C).  
The specifications given in this publication do not  
include normal manufacturing tolerances; therefore,  
an individual unit might not exactly match the listed  
specifications. Also, this product is tested and cali-  
brated under closely controlled conditions, and some  
minor differences in performance can be expected if  
those conditions are changed.  
Shipping Temperature Range: -20°F (-29°C) to 120°F  
(49°C).  
Operating Relative Humidity: 5% to 90% RH, non-  
condensing.  
Cycle Rates (at 50% Load):  
T8400C, T8401C, T8424C,D TRADELINE® models include a  
thermostat, wallplate for wiring and mounting thermostat and  
owner’s guide.  
Heating: Selectable at 1, 3, 4, 5, 6, 9 or 12 cph. See Table  
1 for cycle rate options and corresponding system  
equipment.  
Cooling: Fixed at 3 cph.  
T8400C, T8401C SUPER TRADELINE® models include a  
thermostat, wallplate (for mounting and wiring thermostat),  
two decorator cover plates for covering wall marks (3-7/8 in. x  
Table 1. Heating Cycle Rates.  
System  
Steam, gravity  
Cycles Per Hour  
3
-7/8 in. and 5-3/4 in x 5-3/4 in.), and owner’s guide.  
1
3
Power:  
Hydronic heat, condensing  
T8400C: 24 Vac nominal, 18 to 30 Vac, 50/60 Hz.  
a
gas furnaces  
T8401C: 24 Vac nominal, 18 to 30 Vac, 50/60 Hz.  
T8424C,D: 24 Vac nominal, 18 to 30 Vac, 50/60 Hz.  
Special applicationsb  
4, 5  
Gas or oil forced air (default)  
Electric heat  
Special applicationsb  
6
9
Electrical Ratings:  
Heating: .02 to 1.5A run; 3.5A inrush.  
Cooling: .02 to 1.5A run; 7.5A inrush.  
Fan: .02 to 0.5A run; 2.5A inrush.  
12  
a
High efficiency furnace.  
Refer to equipment manufacturer’s instructions.  
Current Draw:  
b
T8400C Power Stealing Thermostat requires less than  
0
.25 mA current draw when the thermostat calls for heat  
Finish:  
and is compatible with most 24V microprocessor systems.  
The T8400C Power Stealing Thermostat can be used in  
any non-millivolt application where the T8401C and  
T8424C,D Hard Wired Thermostats cannot be used. For  
further detail, see Power Stealing in the Operation section.  
T8400C, T8401C and T8424C,D TRADELINE® AND SUPER  
TRADELINE®: Taupe finish.  
T8400C, T8401C and T8424C,D STANDARD MODELS:  
Premier White® finish and only available in builder packs of  
2
0.  
Temperature Adjustment:  
Setpoint temperature is adjusted by using the  or ▲  
keys. One press changes the setpoint one degree; press-  
ing and holding changes the setpoint several degrees.  
Accessory: Decorator cover plates (for covering wall marks):  
5
-3/4 in. x 5-3/4 in. and 3-7/8 in. x 3-7/8 in.  
Dimensions: See Fig. 1 or 2.  
Temperature Setting Range: 40°F (4°C) to 99°F (37°C).  
ORDERING INFORMATION  
When purchasing replacement and modernization products from your TRADELINE® wholesaler or distributor, refer to the  
TRADELINE® Catalog or price sheets for complete ordering number.  
If you have additional questions, need further information, or would like to comment on our products or services, please write or  
phone:  
1
.
.
Your local Home and Building Control Sales Office (check white pages of your phone directory).  
Home and Building Control Customer Relations  
2
Honeywell, 1985 Douglas Drive North  
Golden Valley, Minnesota 55422 (800) 468-1502  
In Canada—Honeywell Limited/Honeywell Limitée, 35 Dynamic Drive, Scarborough, Ontario M1V 4Z9.  
International Sales and Service Offices in all principal cities of the world. Manufacturing in Australia, Canada, Finland, France,  
Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A.  
6
8-0180-3  
2
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
THERMOSTAT  
WALLPLATE  
THERMOSTAT  
AND WALLPLATE  
2-7/8 (78)  
3-1/2 (89)  
1-3/16  
(55)  
2-1/2 (64)  
UP  
W
1
(
-1/2  
37)  
Y
2
(
-7/8  
78)  
3-1/2  
(89)  
G
Set  
Room  
R
C
FAN  
Auto On  
SYSTEM  
Cool Off Heat  
M10283A  
Fig. 1. T8400/T8401 Thermostat dimensions in in. (mm).  
INSTALLATION  
WALLPLATE  
-3/8 (111)  
4
3-5/16 (84)  
When Installing this Product…  
1
.
Read these instructions carefully. Failure to follow them  
could damage the product or cause a hazardous condi-  
tion.  
2
(
-3/8  
70)  
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 techni-  
cian.  
4
.
After installation is complete, check out product opera-  
tion as provided in these instructions.  
THERMOSTAT  
AND WALLPLATE  
THERMOSTAT  
-1/2 (140)  
5
1-3/16  
(55)  
CAUTION  
Disconnect power supply before beginning wiring  
to prevent electrical shock or equipment damage.  
3
(
-1/2  
89)  
Set  
Room  
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. Dispose of it properly.  
FAN  
Auto On  
SYSTEM  
Cool Off Heat Em. Ht.  
M18585  
Contact your local waste management authority for  
instructions regarding recycling and the proper  
disposal of the control. If you have questions, call the  
Honeywell Customer Response Center at  
Fig. 2. T8424 Thermostat dimensions in in. (mm).  
1
-800-468-1502.  
3
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
hot or cold air from ducts.  
Location  
radiant heat from the sun or appliances.  
concealed pipes and chimneys.  
unheated (uncooled) areas such as an outside wall behind  
the thermostat.  
Install the thermostat about 5 ft (1.5m ) above the floor in an  
area with good air circulation at average temperature. See  
Fig. 3. Do not install the thermostat where it can be affected  
by:  
drafts or dead spots behind doors and in corners.  
YES  
NO  
NO  
5
FEET  
[
1.5 METERS]  
NO  
M10308  
Fig. 3. Typical location of thermostat.  
Mounting Decorator Cover Plate and  
Wallplate to Wall  
5
-3/4 IN. x 5-3/4 IN.  
DECORATOR  
COVER PLATE  
WALL  
IMPORTANT  
Level for appearance only. The thermostat functions  
normally even when not mounted level.  
1
2
NOTE: The TRADELINE® models include a thermostat and  
wallplate (for wiring and mounting thermostat).  
T8400C, T8401C SUPER TRADELINE® models  
include a thermostat, wallplate (for wiring and mount-  
ing thermostat) and two decorator cover plates (for  
covering wall marks).  
WALL  
ANCHORS (2)  
Mount decorator cover plate (SUPER TRADELINE® model  
only), wallplate, T8400C, T8401C or T8424C,D, and the  
screws provided (see Fig. 4 or 5) as follows:  
1
2
.
.
Place the desired decorator cover plate and the wall-  
plate at the desired location on the wall.  
Pull the thermostat wire through the entrance hole on  
the decorator cover plate, then through the wallplate  
entrance hole.  
3
4
5
.
.
.
Select the two mounting holes that best fit the applica-  
tion.  
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)  
Fasten the decorator cover plate and the wallplate to  
the wall using the screws provided.  
WHEN USING WALL ANCHORS.  
M11041C  
After wiring the wallplate, plug the hole to prevent drafts  
from affecting the thermostat; see Wiring section.  
Fig. 4. Mounting the T8400 and T8401 decorator  
cover plate and wallplate to wall.  
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8-0180-3  
4
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
WALL  
KEEP WIRING IN  
SHADED AREA  
WALL  
ANCHORS (2)  
1
G
C
W2  
R
W1  
Y2  
MOUNTING  
SCREWS (2)  
WALLPLATE  
B
1
WHEN USING WALL ANCHORS, DRILL 3/16 INCH  
HOLES FOR DRYWALL, 7/32 INCH HOLES FOR  
Y
O
PLASTER OR WOOD.  
M12202A  
Fig. 5. Mounting the T8424 wallplate to wall.  
MOUNTING  
SCREW HOLE  
WIRING ENTRANCE  
HOLE  
MOUNTING  
SCREW HOLE  
Wiring  
IMPORTANT  
Use an 18-gauge maximum wire for wiring the ther-  
mostats.  
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 on the left side of the termi-  
nals. See Fig. 6 or 7.  
M20211  
KEEP WIRING IN  
SHADED AREA  
ALTERNATE MOUNTING  
SCREW HOLE  
Fig. 7. Restrict T8424 wiring to shaded area.  
The shape of the terminals permits insertion of straight or  
wraparound wiring connections; either method is acceptable.  
See Fig. 8 or 9.  
W
Y
W
MOUNTING  
SCREW  
HOLE  
G
R
C
Y
ALTERNATE  
MOUNTING  
SCREW HOLE  
G
FOR WRAPAROUND  
STRIP 7/16 IN. (11 MM)  
WIRING ENTRANCE HOLE  
MOUNTING  
M11023  
SCREW HOLE  
R
FOR STRAIGHT INSERTION  
STRIP 5/16 IN. (8 MM)  
Fig. 6. Restrict T8400 and T8401 wiring to shaded area.  
C
M11232  
TERMINAL SCREW  
Fig. 8. T8400 and T8401 wiring connections.  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
T8400C, T8401C  
TERMINAL FOR STRAIGHT  
SCREW  
INSERTION STRIP  
/16 IN. (8 MM)  
FOR WRAPAROUND  
STRIP 7/16 IN. (11 MM)  
W
Y
5
COOLING  
CONTACTOR  
G
R
G
FAN  
RELAY  
2
C
C
1
L1  
HOT)  
W2  
Y2  
T
T
(
1
OIL PRIMARY  
CONTROL  
R
L1  
L2  
(HOT)  
24V  
L2  
W1  
Y
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
B
IN T8401C INSTALLATIONS, CONNECT C TERMINAL.  
M11022B  
O
Fig. 11. T8400C/T8401C heat-cool wiring diagram  
in oil heating/electric cooling system. Oil  
primary has its own transformer.  
M20212  
Fig. 9. T8424 wiring connections.  
The T8400C Thermostats steal power through the heating  
and cooling system controls and are adaptable to most 4-wire,  
18 to 30 Vac heating-cooling systems.  
T8424  
STAGE-ONE  
HEATING  
W1  
W2  
1
ST STAGE  
The T8401C and T8424C,D Thermostats are powered directly  
from the system transformer and are adaptable to most  
Y
Y2  
G
COOLING  
5
-wire, 18 to 30 Vac heating-cooling systems. Refer to Fig. 10  
2
ND STAGE  
through 14 for typical wiring hookups.  
COOLING  
R
C
T8400C, T8401C  
W
Y
FAN  
RELAY  
STAGE-TWO  
HEATING  
COOLING  
CONTACTOR  
1
L1  
G
R
(HOT)  
24V  
L2  
2
C
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
1
M20213  
FAN  
RELAY  
HEATING  
PRIMARY  
CONTROL  
1
Fig. 12. T8424C,D two-stage heat and two-stage cool  
system with manual changeover wiring diagram.  
L1  
HOT)  
(
24V  
L2  
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
IN T8401C INSTALLATIONS, CONNECT C TERMINAL.  
M11021B  
Fig. 10. T8400C/T8401C heat-cool wiring diagram  
in single transformer system with  
gas heat/electric cooling.  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
ZONE 1  
ZONE 2  
ZONE 3  
ZONE 4  
T8400C, T8401C  
T8400C, T8401C  
T8400C, T8401C  
T8400C, T8401C  
4
C
W
R
4
C
W
R
4
C
W
R
4
C
W
R
2
R8222B1067  
R8239A1052  
R8222B1067  
R8222B1067  
C
Y
W
R
G
L2  
L1  
1
3
3
3
ZONE 2  
ZONE 3  
ZONE 4  
(
HOT)  
VALVE OR  
DAMPER  
MOTOR  
VALVE OR  
DAMPER  
MOTOR  
VALVE OR  
DAMPER  
MOTOR  
3
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS  
AND OVERLOAD PROTECTION AS REQUIRED.  
3
4
USE FOR 3-WIRE ZONE VALVE OR DAMPER.  
ZONE 1  
VALVE OR  
DAMPER  
MOTOR  
IN T8401C INSTALLATIONS, CONNECT C TERMINAL.  
M10285A  
USE ONE R8239 FOR EVERY FOUR ZONES.  
Fig. 13. T8400C/T8401C heat only wiring diagram for controlling incompatible or 3-wire zone valves or dampers. Heating  
equipment is operated by end switch on the zone valve or motor, or by thermostat in master zone.  
T8400C, T8401C  
W
Y
G
R
6
C
D892/M892  
4
4
197516A  
CABLE  
L8148E1265  
4
TV  
W
Z
T
K
1
3
2
HIGH LIMIT  
CONTROLLER  
2
3
FUSE  
R
1K2  
3
B
2
1
5
1
C1  
L1  
C2  
L2  
1K1  
POLARIZED 4-PRONG  
MALE MOLEX PLUG AND  
-PRONG FEMALE  
MATING PLUG ON CABLE  
6-PRONG FEMALE MOLEX  
RECEPTABLE IN L8148E  
AND 5-PRONG MALE  
B1  
B2  
4
5
CIRCULATOR  
MOLEX PLUG ON CABLE  
L1  
(HOT)  
1
L2  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD  
PROTECTION AS REQUIRED.  
S8610  
2
REMOVE PLUG-IN THAT JUMPERS PINS 2 AND 3 WHEN USING VENT  
DAMPER.  
2
4V GND  
3
4
AFTER VENT DAMPER IS PLUGGED INTO L8148E1265, FUSE BLOWS  
WHEN THERMOSTAT FIRST CLOSES. AFTER FUSE BLOWS, L8148E1265  
OPERATES ONLY WHEN VENT DAMPER IS CONNECTED.  
2
4V  
SEE VENT DAMPER PRODUCT SPECIFICATIONS FOR INTERNAL  
SCHEMATICS.  
5
B1 IS 1/4 IN. TAB TERMINAL.  
M10286  
6
IN T8401C INSTALLATIONS, CONNECT C TERMINAL.  
Fig. 14. T8400C/T8401C heat only wiring diagram with L8148E1265 (with internal plug) wired  
directly to vent damper in hydronic intermittent pilot system.  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
Mounting Thermostat to Wallplate  
SETTINGS AND ADJUSTMENTS  
1
2
.
.
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. 10.  
Setting Fan and System Switches  
Fan and system settings are controlled manually using the  
switches located at the bottom of the thermostat case. See  
Fig. 17 or 18.  
DASHED LINES INDICATE TABS  
ON BACK OF THERMOSTAT  
TEMPERATURE DISPLAY  
Set  
Room  
Set  
Room  
INCREASE  
SETTING  
FAN  
Auto  
SYSTEM  
ol Off Heat  
FAN  
Auto On  
SYSTEM  
Cool Off Heat  
Set  
Room  
A ENGAGE TABS AT TOP  
OF THERMOSTAT WITH  
SLOTS ON WALLPLATE.  
DECREASE  
SETTING  
M11025B  
B PRESS LOWER EDGE OF  
CASE TO LATCH.  
Fig. 15. Mounting T8400 or T8401 thermostat to wallplate.  
FAN  
Auto On  
SYSTEM  
Cool Off Heat  
DASHED LINES INDICATE TABS  
ON BACK OF THERMOSTAT  
FAN SWITCH  
SYSTEM SWITCH  
M11020A  
Fig. 17. T8400 and T8401 temperature  
display and system switches.  
TEMPERATURE DISPLAY  
INCREASE SETTING  
DECREASE SETTING  
FAN  
Au  
to  
O
n
SYSTEM  
Cool Off Heat  
A
ENGAGE TABS AT TOP OF THERMOSTAT  
WITH SLOTS ON MOUNTING PLATE.  
FA  
Au  
to On  
N
SYSTEM  
Cool Off Heat  
FAN SWITCH  
M14678  
SYSTEM SWITCH  
Fig. 18. T8424 temperature display and system switches.  
FAN  
Auto On  
Fan Switch  
SYSTEM  
Cool  
Fan switch settings are:  
O
ff  
Heat  
ON: The fan runs continuously. Use for  
improved air circulation.  
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  
B
PRESS LOWER EDGE OF  
CASE TO LATCH.  
M14677  
Fig. 16. Mounting T8424 thermostat to wallplate.  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
most systems). When using an electric  
heat thermostat, the fan starts and stops  
with the heating equipment.  
To set the °F/°C indication and heat cycle rate:  
1. If the room temperature is displayed  
in °F, set the temperature setpoint to  
Slide the switch in the bottom left corner of the thermostat to  
select the desired fan setting.  
52°F. If the room temperature is dis-  
M14683  
played in °C, set the temperature  
setpoint to 11°C.  
2
.
Press the and keys simulta-  
neously 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 code is displayed.  
System Switch  
System switch settings control thermostat operation as  
follows:  
M14686  
COOL: The thermostat controls the  
cooling system.  
HEAT: The thermostat controls the heating  
system.  
Optional System Checkout  
OFF: Both heating and cooling are off.  
Slide the switch in the bottom right corner of the thermostat to  
select the desired system setting.  
When in steps B and C 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 is in the Off position.  
Set Temperature Setpoint  
EXAMPLES:  
System setting at HEAT: If  
heat is on, the key turns it  
off; if heat is off, turns it on.  
NOTE: Temperature setpoint range is 40° to 99°F (4° to  
37°C).  
M14684  
M14685  
The temperature setpoint and the room temperature are  
shown separately on the display. The indicator points to Set  
when the temperature setpoint is displayed and to Room  
when the room temperature is displayed.  
System setting at COOL: If  
cool is on, key turns it off; if  
cool is off, turns it on.  
NOTE: In installer setup mode only, each  
press of the key momentarily displays 01 and  
INDICATOR  
each press of the  key momentarily displays 02.  
When the keys are released, these two-digit codes  
are no longer displayed.  
ROOM TEMPERATURE  
Set  
Room  
SETPOINT TEMPERATURE  
M11247  
3. Press the  key. Factory information is dis-  
played. A typical example is shown, but infor-  
mation displayed varies by model. This  
To set the 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. 17 or 18.  
To display the selected temperature setpoint  
on the digital display, press either the  or ▲  
key once. The temperature setpoint is dis-  
played for approximately five seconds as the  
indicator points to Set and flashes.  
To increase the temperature setpoint press  
the key. Each press changes the setpoint  
one degree; press and hold to change the set-  
point several degrees.  
To decrease the setpoint temperature press  
the key. Each press changes the setpoint  
one degree; press and hold to change the  
setpoint several degrees.  
M14687  
information is for factory use only.  
4
.
Press the key again to display °C or °F.  
2
.
M14688  
M14689  
5
.
Press the key to change the °C or °F indi-  
cation.  
M14680  
NOTE: Steps 6 through 9 apply only to T8424  
Thermostats.  
3
4
.
.
6
7
8
9
.
.
.
.
Press key to display number of cooling  
M14681  
M14682  
stages.  
M20208  
M20207  
Press key to change the number of cooling  
stages.  
Setting °F/°C Indication and Heat Cycle Rate  
NOTE: To save changes to the °F/°C indication and the heat  
cycle rate, all seven steps must be completed.  
Press key to display the number of heating  
stages.  
M20209  
M20210  
Press key to change the number of heating  
stages.  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
1
0. Press the  key to display the heat cycle  
rate of 1, 3, 4, 5, 6, 9 or 12. When set at two  
stages of heat (T8024 only) , both stages  
operate at selected cycle rate. If the desired  
cycle is displayed, press the key to exit  
OPERATION  
The T8400C and T8401C Standard Electronic Thermostats  
provide single-stage temperature control using proportional  
plus integral (P+I) control to eliminate droop. The T8424C  
provides two-stage heat and two-stage cool temperature  
control with P+I control. The T8424D Electronic Multistage  
Thermostat provides two-stage heat and two-stage cool  
temperature control and also uses P+I control.  
M20230  
the installer setup mode. To change the heat cycle rate,  
press the key to scroll between cycle rates. Stop  
scrolling when the desired rate is displayed. See Table 2  
for the cycle rate options and the corresponding system  
equipment.  
Table 2. Heating Cycle Rates.  
Zero Droop Performance  
System  
Cycles Per Hour  
The thermostats control temperature closer to setpoint than  
electromechanical thermostats. This is because proportional  
plus integral control maintains the same indoor temperature  
independent of the outdoor temperature. Thus the T8400C  
and T8401C Thermostats provide tight temperature control  
with virtually no noticeable swings in temperature setpoint, a  
condition referred to as droop. This zero droop performance  
provides improved occupancy comfort. Occupants do not  
need to continually adjust thermostat settings to be  
comfortable, even during severe weather. Electromechanical  
thermostats have little control over droop, thus resulting in  
room temperature changes of 3°F to 5°F as outdoor  
temperature varies.  
Steam, gravity  
1
3
Hydronic heat, condensing  
gas furnacesa  
Special applicationsb  
4, 5  
Gas or oil forced air (default)  
Electric heat  
Special applicationsb  
6
9
12  
a
High efficiency furnace.  
Refer to equipment manufacturer’s instructions.  
b
Power Stealing  
1
1. Press the  key to display algorithm configu-  
ration.  
Unlike hard wired thermostats which are powered directly  
from the system transformer, the T8400C Thermostat steals  
power through the heating and cooling system controls and  
can be used in applications where existing wiring does not  
permit using hard wired thermostats. See Fig. 19 and 20.  
12. Press the  key again to change the  
algorithm to C1 or C3.  
M18419  
C1 = standard algorithm.  
C3 = aggressive algorithm (can cause  
overshooting).  
13. Press the  key again. Current algorithm  
CC) is displayed. A typical example is  
Previous Honeywell power stealing thermostats required 80  
mA of load current when the heating and cooling was on. The  
T8400C Thermostat requires only 2.5 mA of load current (1/32  
of previous models) when the heating or cooling is on. When  
the heating or cooling is off, the thermostat operates at 0.1 mA  
(1/100 of previous models). Other Honeywell power stealing  
thermostats are available, but they have different powering  
requirements than the T8400C. This low current draw makes  
the T8400C applicable to most 24V heating and cooling  
systems.  
(
shown, but CC varies by model. This informa-  
tion is for factory use only.  
M18420  
14. Press the  key to save all changes, exit  
installer setup mode and return to normal operation.  
NOTE: After exiting installer setup mode, change the set-  
point to the desired room temperature.  
The T8400C, T8401C and T8424C,D Thermostats only  
operate when mounted on the wallplate with 24V applied.  
Batteries are not required for operation because temperature  
setpoints and configuration settings are retained permanently  
in memory.  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
L1  
L2  
(HOT)  
1
R
G
FAN SWITCH  
ON  
FAN RELAY  
THERMISTOR  
SENSOR  
AUTO  
THERMOSTAT  
LOGIC  
HEAT RELAY  
COOL RELAY  
SYSTEM SWITCH  
HEAT  
W
Y
OFF  
COOL  
POWER  
SUPPLY  
MOSFET  
LOGIC AND  
CONTROL  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
MOSFET  
M10360  
Fig. 19. T8400C power 0stealing internal schematic.  
R
G
FAN SWITCH  
ON  
FAN RELAY  
AUTO  
1
L1  
RELAY  
(
HOT)  
THERMISTOR  
SENSOR  
L2  
THERMOSTAT  
LOGIC  
HEAT RELAY  
COOL RELAY  
SYSTEM SWITCH  
HEAT  
W
OFF  
COOL  
Y
C
POWER  
SUPPLY  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.  
M10292  
Fig. 20. T8401C hardwired internal schematic.  
energy saving periods you choose and your energy savings  
temperature settings. Refer to the maps in the Operation  
section for typical energy savings.  
Energy Savings  
You can save energy with the T8400C, T8401C and  
T8424C,D Thermostats in the heating mode by lowering the  
temperature setting 5°F (3°C) to 10°F (5°C) at night and  
during the day. You can also save energy in the cooling mode  
by raising the temperature setting during the day before you  
leave for work or school. Savings can be as high as 7% to  
Sequence of Operations  
The Fan, Heat and Cool outputs are relay or mosfet  
controlled. A loss of ac power or a change in the SYSTEM  
switch setting can cause the outputs to turn off. The truth table  
shows the sequence of operations for the FAN settings and  
SYSTEM modes. See Table 3 Sequence of Operations.  
33% depending on your home, where you live, the number of  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
Table 3. Sequence of Operations.  
FAN Switch Settings  
SYSTEM Switch Settings  
Call for Action  
None  
Energize Terminals  
Display Icons  
None  
Auto  
On  
Off  
Any  
Cool  
None  
G
Any  
None  
Auto  
Cooling  
G,Y  
Auto  
Heat  
Heating  
Wa,b  
a
b
In electric heat models, W and G are energized with calls for heat.  
W1 and W2 terminals are energized in the T8424C,D models.  
3
.
Press the key to lower the temperature setting below  
the room temperature. Heating equipment should stop.  
Fan Control  
The FAN switch settings are On and Auto.  
ON:The fan runs continuously.  
AUTO:In cooling, the fan starts and stops  
with the cooling equipment. In heating, the  
fan starts a few minutes after the heating  
equipment turns on. When using an  
electric heat thermostat, the fan starts and  
stops with the heating equipment.  
Cooling  
CAUTION  
Low Temperature Hazard.  
Operating at too low of an outdoor temperature  
can cause compressor damage.  
Heat Mode  
Do not operate cooling if outdoor temperature is below  
With the SYSTEM switch at Heat, the thermostat controls the  
heating system. On a call for heat, the W terminal is energized  
and a flame is shown on the digital display.  
5
0°F (10°C). Refer to manufacturer recommendations.  
To avoid compressor damage, allow the compressor to  
remain off for five minutes before restarting.  
1
.
Slide the system switch to Cool and the fan switch to  
Auto.  
Cool Mode  
With the SYSTEM switch at Cool, the thermostat controls the  
cooling system. On a call for cool, the G and Y terminals are  
energized and a snowflake is shown on the digital display.  
2
.
Press the key to lower the temperature setting sev-  
eral degrees below the room temperature. After approx-  
imately five minutes the G and Y terminals are  
energized and the cooling equipment starts.  
Press the key to raise the temperature setting above  
the room temperature. Cooling system should shut  
down.  
3
.
Minimum Off-Timer  
A minimum off-timer in the T8400C, T8401C and T8424C,D  
assures that the cooling compressor does not come on again  
for at least five minutes after it turns off. The minimum-off  
timer is triggered when the compressor turns off and when the  
system switch is changed. If the compressor turns off when  
the setpoint is changed, then the minimum-off timer is  
triggered. Power interruption and power restoration also  
trigger the minimum off-timer. The minimum-off timer operates  
only during the cooling cycle on the thermostat.  
Fan  
1
2
.
.
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 con-  
trolled 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 equip-  
ment.  
CHECKOUT  
Heating  
Make certain all equipment responds properly to the  
thermostat.  
1
2
.
.
Slide the system switch to Heat and the fan switch to  
Auto.  
Press and hold the key to raise the temperature set-  
ting several degrees above the room temperature. After  
approximately ten seconds the W terminal is energized  
and the heating equipment starts.  
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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS  
TROUBLESHOOTING  
Refer to Fig. 21 for the proper troubleshooting procedure.  
SYMPTOM  
POSSIBLE CAUSE  
ACTION  
HEATING OR COOLING EQUIPMENT  
WON'T OPERATE.  
NO AC POWER TO THERMOSTAT.  
CHECK THAT C TERMINAL IS CONNECTED TO  
THE SYSTEM TRANSFORMER.  
CHECK POWER TO HEATING AND COOLING EQUIPMENT  
ON-OFF SWITCH  
FUSE OR CIRCUIT BREAKER  
LOOSE 24V CONNECTION  
AT THERMOSTAT  
AT FURNACE/AIR CONDITIONER  
INCORRECT WIRING (SEE WIRING DIAGRAM)  
THERMOSTAT INOPERATIVE.  
CONDUCT SELF-TEST; SEE CHECKOUT SECTION.  
PRESENT SETTING TOO LOW/HIGH.  
ADJUST TEMPERATURE BY PRESSING  
RESET THERMOSTAT SYSTEM SWITCH.  
OR  
KEYS.  
SYSTEM SWITCH ON THERMOSTAT  
IN WRONG POSITION.  
MINIMUM-OFF TIMES IN THERMOSTAT  
IN OPERATION ON COOLING OR  
FIRST STAGE HEATING.  
WAIT FIVE MINUTES OR FOLLOW STEPS IN  
CHECKOUT SECTION.  
INCORRECT WIRING.  
CHECK WIRING DIAGRAM.  
HEATING OR COOLING EQUIPMENT  
INOPERATIVE.  
CONSULT EQUIPMENT MANUFACTURER INSTRUCTIONS.  
REPLACE THERMOSTAT.  
PARTIAL DISPLAY  
INOPERATIVE THERMOSTAT.  
STILL IN INSTALLER SETUP MODE.  
SEE THE SETTINGS AND ADJUSTMENTS SECTION FOR  
INSTRUCTIONS ON INSTALLER SETUP MODE.  
NO DISPLAY  
NO POWER TO THERMOSTAT.  
CHECK THAT C TERMINAL IS CONNECTED TO  
THE SYSTEM TRANSFORMER.  
CHECK POWER TO HEATING AND COOLING EQUIPMENT  
ON-OFF SWITCH  
FUSE OR CIRCUIT BREAKER  
LOOSE 24V CONNECTION  
AT THERMOSTAT  
AT FURNACE/AIR CONDITIONER  
INCORRECT WIRING (SEE WIRING DIAGRAM)  
THERMOSTAT MOUNTED  
INCORRECTLY ON WALLPLATE.  
SEE INSTALLATION INSTRUCTIONS FOR  
CORRECT MOUNTING.  
TEMPERATURE DISPLAY IS  
INCORRECT.  
THERMOSTAT IS CONFIGURED FOR  
F OR C DISPLAY.  
RECONFIGURE THE DISPLAY. SEE SETTINGS  
AND ADJUSTMENTS SECTION.  
TEMPERATURE SETTINGS WILL NOT  
CHANGE. (EX: CANNOT SET THE  
HEATING HIGHER OR THE  
COOLING LOWER).  
THE UPPER OR LOWER  
TEMPERATURE LIMITS WERE  
REACHED.  
CHECK THE TEMPERATURE SETPOINTS; TEMPERATURE  
SETTING RANGE IS 40 F (4 C) TO 99 F (37 C).  
FAN DOES NOT OPERATE PROPERLY  
IN HEAT OR COOL MODES.  
INCORRECT WIRING.  
CHECK WIRING DIAGRAM.  
HEATING OR COOLING EQUIPMENT  
INOPERATIVE.  
CONSULT EQUIPMENT MANUFACTURER INSTRUCTIONS.  
M10291B  
Fig. 21. Troubleshooting procedure.  
1
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ꢜꢒꢇꢓꢄꢂꢖꢊꢉꢝꢇꢓꢘꢉꢙꢃ  
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