T874G,R MULTISTAGE HEAT PUMP THERMOSTATS AND Q674F,J,L SUBBASES
MOUNTING SLOTS (2)
MOUNTING POST (2)
Do not operate cooling if outdoor temperature is
below 50°F (10°C). Refer to manufacturer
Move the system switch on the subbase to COOL. Move
the setpoint lever (cooling lever on T874G) on the thermo-
stat about 10°F (6°C) below the room temperature.
Cooling and fan should start. Move the lever (cooling lever
on T874G) about 10°F (6°C) above the room temperature.
Cooling and fan should stop.
60 70 80
Move the system switch on the subbase to EM. HT. Move
the setpoint lever (heating lever on T874G) about 10°F
Fig. 14. Mounting thermostat on subbase.
(6°C) above the room temperature. The electric strip
heater(s) should come on. Move the lever (heating lever
on T874G) about 10°F (6°C) below the room temperature.
The electric strip heater should de-energize.
The T874R has one setpoint lever that controls both
heating and cooling. The T874G has one lever that
controls heating and another lever that controls cooling.
Move the setpoint lever(s) to the desired control
Move the subbase system switch to OFF, and the fan
switch to ON. The fan should run continuously. When the
fan switch is in the AUTO position, fan operation is
controlled by the heating or cooling system.
Outdoor Reset Thermistor
Some systems are supplied with a Honeywell C815A1005
Thermistor for outdoor mounting. The thermistor must be
used; if not, thermostat performance will deviate radically
from proper operation.
System switching positions control thermostat operation as
EM. HT.: Emergency heating system is energized. Cooling
system is off. Compressor is de-energized.
HEAT: Heating system is controlled by the thermostat.
Cooling system is off.
OFF: Both the heating and cooling systems are off.
COOL: Cooling system is controlled by the thermostat.
Heating system is off.
The proper thermistor operation must be verified to assure
the correct operation of the thermostat. Check thermistor
operation as follows:
Disconnect the T wire on the subbase.
Use an ohmmeter to measure resistance between
the T wire and the transformer secondary common
or C terminal.
AUTO: Thermostat automatically changes between heat
and cool modes depending on indoor temperature.
Take outdoor temperature at thermistor location
and find the correct thermistor resistance on chart
in Fig. 15.
If the resistance measured in step 2 and the
calculated resistance in step 3 vary by more than
Fan switching positions control fan operation as follows:
ON: Fan operates continuously.
AUTO: Fan operates with equipment as controlled by the
5 percent, the thermistor requires replacement.
To switch positions, use your thumb and index finger to
slide the lever to the desired position. The switch lever
must be set directly over the desired function indicator
mark for proper circuit operation.
Contact the installation dealer for an outdoor
These thermostats are accurately calibrated at the factory.
They do not have provision for field calibration.
Move the system switch on the subbase to HEAT and the
fan switch to AUTO. Move the setpoint lever (heating lever
on T874G) on the thermostat about 10°F (6°C) above the
room temperature. Heating and fan should start if there is
no time delay or outdoor temperature limiting system.
Move the lever (heating lever on T874G) about 10°F (6°C)
below the room temperature. Heating and fan should shut
The thermometer in your thermostat has been accurately
calibrated at the factory. The thermometer should only
need adjustment if it has been dropped or jarred.