Calibration; Understanding Circuits; Fan/Outdoor Reset Thermistor; Thermometer - Honeywell T874 Product Data

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Move the system switch on the Q674 Subbase to COOL or
AUTO. Move the cool setting lever on the T874 Multistage
Thermostat about 10°F (6°C) below room temperature. See
Fig. 12. The cooling equipment and fan should start. If the
system has two stages of cooling, both stages should start.
Move the cool lever about 10°F (6°C) above room
temperature. The cooling equipment and fan should stop.
Fan
Move the system switch to COOL, OFF, or AUTO. If
necessary, position both temperature setting levers so that the
heating and cooling equipment are off. Move the fan switch to
ON or CONT. The fan should run continuously. When the fan
switch is in AUTO, LO, MED, or HI position, fan operation is
controlled by the heating or cooling system.
Outdoor Reset Thermistor (Where
Applicable)
If the system is supplied with a thermistor, it must be used; if
not used, thermostat performance deviates radically from
proper operation.
The proper thermistor operation must be verified to ensure the
correct operation of the thermostat. Check thermistor
operations as follows:
1. Disconnect the T wire on the subbase.
2. Use an ohmmeter to measure resistance between the
T wire and the A subbase terminal.
3. Take outdoor temperature at thermistor location and find
the correct thermistor resistance on the Fig. 14 chart.
4. If the resistance measured in step 2 and the calculated
resistance in step 3 vary by more than 15 percent, the
thermistor requires replacement. Contact Honeywell or
installing dealer for replacement packaged outdoor ther-
mistor, part no. C815A1005.
4600
4400
4200
4000
C815A THERMISTOR RESISTANCE
3800
R = 400 ohms
3600
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
-20 0 20
TEMPERATURE OF THERMISTOR ( F)
Fig. 14. Thermistor resistance chart.

CALIBRATION

Thermostat
T874 Thermostats are accurately calibrated at the factory.
They do not have provision for field calibration.
60-2485—8
10% AT 77 F (25 C)
40 60 80 100 120 140
M1590A

Thermometer

The thermometer in your thermostat has been accurately
calibrated at the factory. The thermometer should only need
adjustment if it has been dropped or shifted due to mishandling.
If the setpoint lever and the thermometer reading do not
agree, use the following procedure:
1. Remove the thermostat cover by pulling up from the
bottom edge of the cover away from the base until it
snaps free of the cover clip.
2. Set the thermostat cover on a table near an accurate
thermometer.
3. Allow ten minutes for cover thermometer to sense area
temperature; compare the readings. Be careful not to
touch thermometer or breathe on it.
4. If the readings are the same, replace cover and put the
system into operation.
5. If the readings are different, insert a small screwdriver in
the thermometer slot and turn it until the thermometers
have the same reading. See Fig. 15.
6. Replace thermostat cover and put the system into oper-
ation.
Fig. 15. Thermometer calibration.

UNDERSTANDING CIRCUITS

To understand wiring diagrams, it is important to know what all
the symbols mean and how to trace the path of the circuits
from the transformer. See Fig. 16 through 25.
Circuit descriptions and terminology are defined as follows:
For standard heating-cooling circuits:
Auto changeover—refers to the presence of an AUTO
position in the system switching (EX: Q674E with OFF-
HEAT-AUTO-COOL switching). The thermostat auto-
matically changes between heat and cool modes as
indoor temperature changes.
Manual changeover—requires a system switch movement
to change mode (EX: Q674B with HEAT-OFF-COOL
switching). T874D Multistage Thermostats with 2 heat
or 2 cool switches are shown on most standard circuits.
Most standard or TRADELINE® subbases (Q674A-E,G)
can be used with T874A-F standard or TRADELINE
14
M5070