T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES
C815A THERMISTOR RESISTANCE
R = 400 ohms ± 10% AT 77°F (25°C)
Setting the Heat Anticipator (T874Q Only)
Set each heat anticipator to match its primary control
current draw. If the primary control nameplate has no
rating or if further adjustment is necessary, use the
following procedure to determine the current draw of each
stage. (Measure the current draw of each heating stage
with the thermostat removed and the power on.)
Connect an ac ammeter of appropriate range
between the heating terminals of the subbase as
Stage 1: between W1 and RH or R.
Stage 2: between W2 and R.
Move the system switch to AUTO (if applicable).
After one minute, read the ammeter and record the
TEMPERATURE OF THERMISTOR (°F)
After mounting the thermostat, set the adjustable
heat anticipator(s) to match the respective
reading(s) measured in step ꢂ.
Fig. 10 Thermistor resistance chart.
The proper thermistor operation must be verified to assure
the correct operation of the thermostat. Check the
thermistor operation as follows:
Move the heating and cooling levers to the desired
positions. The minimum differential between the heating
and cooling setpoints is 3°F (1.7°C), which means the
setting levers cannot be set closer together than 3°F
Disconnect the T wire on the subbase.
Use an ohmmeter to measure resistance between
the T wire and the transformer common.
Take the outdoor temperature at thermistor location
and find the correct thermistor resistance on the
chart in Fig. 10.
(1.7°C) (5° F [2.8°C] for T874W).
If the resistance measured in step ꢁ and the
calculated resistance in step ꢂ vary by more than
15 percent, the thermistor requires replacement.
Contact the installing dealer for the packaged
replacement outdoor thermistor.
System switching positions control thermostat operation as
OFF: Both the heating and cooling systems are off. If
the fan switch is in the AUTO position, the cooling
fan is also off.
AUTO: Thermostat automatically changes between the
heat and cool modes, depending on the indoor
HEAT: Heating system is controlled by the thermostat.
Cooling system is off.
EM. HT.: Emergency heat relay is energized. The
cooling system is off. When the compressor has
failed and the heat pump is not operating, switch to
the EM. HT. position.
Move the system switch on the Q674 Subbase to HEAT or
AUTO. Move the heating lever on the T874 about 10°F
(6°C) above room temperature. Both stages of heating
start and the fan runs after a short delay. Move the heating
lever about 10°F (6°C) below room temperature. The
heating equipment shuts off, and the fan runs for a short
time, and then shuts off.
Fan switching positions control fan operation as follows
Q674B,C, or L):
ON: Fan operates continuously.
AUTO: Fan operates with cooling equipment as
controlled by the thermostat or with the heating
equipment as controlled by the plenum switch.
Move the system switch on the Q674 Subbase to COOL
or AUTO. Move the cooling lever on the T874 Thermostat
about 10°F (6°C) below room temperature. The cooling
equipment and fan start. Move the cooling lever about
To switch positions, use thumb and index finger to slide
the lever to the desired position. The switch lever must
stop in detent over the desired function indicator mark for
proper circuit operation.
10°F (6°C) above room temperature. The cooling equip-
ment and fan stop.
C815 Outdoor Reset Thermistor (T874W only)
The T874W is used with a C815 Thermistor mounted in
the outdoor section. The thermistor must be used; if not,
thermostat performance will deviate radically from proper
Move the system switch to EM. HT. The EM. HT. LED
comes on. Move the heating lever about 10°F (6°C) above
room temperature. The electric strip heater(s) comes on.
Move the heating lever about 10°F (6°C) below room
temperature. The electric strip heater de-energizes. The
EM. HT. LED remains on until the system switch is moved
to another postion.