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T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS

68-0180-3

10

10.

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 

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.

a

High efficiency furnace.

b

Refer to equipment manufacturer’s instructions.

11.

Press the 

 key to display algorithm configu-

ration.

12.

Press the 

 key again to change the         

algorithm to C1 or C3.

C1 = standard algorithm.

C3 = aggressive algorithm (can cause 

overshooting).

13.

Press the 

 key again. Current algorithm 

(CC) is displayed. A typical example is 

shown, but CC varies by model. This informa-
tion is for factory use only.

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.

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.

Zero Droop Performance

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.

Power Stealing

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.

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.

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.

System

Cycles Per Hour

Steam, gravity

1

Hydronic heat, condensing 
gas furnaces

a

3

Special applications

b

4, 5

Gas or oil forced air (default)

6

Electric heat

9

Special applications

b

12

M20230

M18419

M18420

Summary of Contents for T8401C

Page 1: ...ns 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 fo...

Page 2: ...ner s guide Power T8400C 24 Vac nominal 18 to 30 Vac 50 60 Hz T8401C 24 Vac nominal 18 to 30 Vac 50 60 Hz T8424C D 24 Vac nominal 18 to 30 Vac 50 60 Hz 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 Current Draw T8400C Power Stealing Thermostat requires less than 0 25 mA current draw when the thermostat calls for heat and is ...

Page 3: ...TION Disconnect power supply before beginning wiring to prevent electrical shock or equipment damage 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 Contact your local waste management authority for instructions regarding recycling and the proper disposal of the control If you have questi...

Page 4: ...nd mount ing thermostat and two decorator cover plates for covering wall marks 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 Place the desired decorator cover plate and the wall plate at the desired location on the wall 2 Pull the thermostat wire through the entrance hole on the decorator cover plate t...

Page 5: ...ng 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 Fig 8 T8400 and T8401 wiring connections M12202A WALL WALL ANCHORS 2 WALLPLATE WHEN USING WALL ANCHORS DRILL 3 16 INCH HOLES FOR DRYWALL 7 32 INCH HOLES FOR PLASTER OR WOOD MOUNTING SCREWS 2 1 1 M11023 ALTERNATE MOUNTING SCREW HOLE MOUNTING SCREW HO...

Page 6: ...mer Fig 12 T8424C D two stage heat and two stage cool system with manual changeover wiring diagram TERMINAL SCREW M20212 G R C Y W1 B O FOR STRAIGHT INSERTION STRIP 5 16 IN 8 MM FOR WRAPAROUND STRIP 7 16 IN 11 MM Y2 W2 L1 HOT L2 1 2 1 2 W Y G R C COOLING CONTACTOR FAN RELAY HEATING PRIMARY CONTROL 24V POWER SUPPLY PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED IN T8401C INSTALLATIONS...

Page 7: ...TALLATIONS CONNECT C TERMINAL 4 4 4 4 4 ZONE 1 VALVE OR DAMPER MOTOR ZONE 2 VALVE OR DAMPER MOTOR ZONE 3 VALVE OR DAMPER MOTOR ZONE 4 VALVE OR DAMPER MOTOR M10285A C C C 24V GND 24V S8610 L1 HOT L2 1 1 2 3 4 5 L8148E1265 4 2 3 3 FUSE 5 1 2 5 6 PRONG FEMALE MOLEX RECEPTABLE IN L8148E AND 5 PRONG MALE MOLEX PLUG ON CABLE 1K1 B1 B2 L1 C1 C2 L2 CIRCULATOR POLARIZED 4 PRONG MALE MOLEX PLUG AND 4 PRONG ...

Page 8: ...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 M11025B FAN SYSTEM Auto On Cool Off Heat DASHED LINES INDICATE TABS ON BACK OF THERMOSTAT FAN SYSTEM Auto Cool Off Heat ENGAGE TABS AT TOP OF THERMOSTA...

Page 9: ...OTE To save changes to the F C indication and the heat cycle rate all seven steps must be completed To set the F C indication and heat cycle rate 1 If the room temperature is displayed in F set the temperature setpoint to 52 F If the room temperature is dis 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 displa...

Page 10: ... 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 ...

Page 11: ...ps in the Operation section for typical energy savings 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 L1 HOT L2 1 1 POWER SUPPLY PROVIDE DISCONNECT...

Page 12: ...at and the fan switch to Auto 2 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 3 Press the key to lower the temperature setting below the room temperature Heating equipment should stop Cooling CAUTION Low Temperature Hazard Operating at too low of an out...

Page 13: ...NS HEATING OR COOLING EQUIPMENT INOPERATIVE REPLACE THERMOSTAT PARTIAL DISPLAY INOPERATIVE THERMOSTAT SEE THE SETTINGS AND ADJUSTMENTS SECTION FOR INSTRUCTIONS ON INSTALLER SETUP MODE STILL IN INSTALLER SETUP MODE RECONFIGURE THE DISPLAY SEE SETTINGS AND ADJUSTMENTS SECTION TEMPERATURE DISPLAY IS INCORRECT THERMOSTAT IS CONFIGURED FOR F OR C DISPLAY CHECK THAT C TERMINAL IS CONNECTED TO THE SYSTEM...

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