Honeywell T874 Benutzerhandbuch

60-2485—8

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Due to calibration techniques used for T874 

Thermostats with outdoor reset, the C815A Thermistor 

must be wired into the system at all times. Failure to 

do so will result in serious degradation of performance.

To check and verify thermistor operation, perform the 

following steps:

1. Disconnect wire from T terminal on subbase.

2. Measure resistance with ohmmeter across the T sub-

base wire and X subbase terminal.

3. Take outdoor temperature measurement at thermistor 

location and find correct thermistor resistance on the 

chart in Fig. 29.

4. Compare resistance on the chart with measured resis-

tance. Replace C815A if resistance varies more than 15 

percent. Contact installing dealer for packaged replace-

ment outdoor thermistor. 

Two of the features of the T874/Q674 include LED indicators 

and restricted setpoint.

The light-emitting diodes (LED) indicators on the subbase 

light on command when something specific happens in the 

system. See Fig. 30.

Up to four different LEDs are available. The thermostat has a 

clear lens window for viewing each LED. On TRADELINE 

models, a small insert is used so the LED function desired can 

be selected. This must be done during installation.

A blank insert is factory-installed in some T874 models. To 

remove it, push both temperature setting levers to the far ends 

of the thermostat. Use index fingernail to gently pull out the 

scaleplate a fraction of an inch. Turn thermostat upside-down, 

and the blank insert falls out.

A strip of four inserts is included with TRADELINE T874. Drop 

a strip into the recessed area behind the scaleplate so 

selected LEDs show. Make sure insert is completely seated in 

recessed area. Let scaleplate pop back; then set levers to 

desired position.

• FILTER LED lights when the filter is clogged and needs 

replacement.

• CHECK LED lights when something needs to be checked 

or done to maintain efficient operation of system. See 

heating system instructions for CHECK LED meaning.

• EM. HT. LED lights when the emergency heat is operating.

• SUPL. HT. LED lights when the supplemental heat is 

operating.

• LOCKOUT LED lights when the system is shut down and 

needs maintenance.

• AUXILIARY HEAT LED lights when the auxiliary heat is 

operating.

• SERVICE or MALFUNCTION LED can have several 

meanings. Consult heating system instructions.

LEDs cannot be replaced or added in the field.

The Department of Defense (DoD) models are equipped with 

a restricted setpoint feature for fuel efficiency.

Fixed stops are factory-set so setpoint levers cannot be set 

above 72°F (22°C) on heating, or below 78°F (26°C) on cooling.

The T874/Q674 can be applied to standard residential 

systems for automatic or manual changeover, to commercial 

rooftop applications, or to heat pump applications.

In a standard residential heating-cooling circuit, changeover 

between heating and cooling can be done either automatically 

or manually.

Automatic changeover is done by an AUTO position on the 

subbase system switch. See Fig. 31. When the switch is in the 

AUTO position, the thermostat automatically changes between 

heat and cool modes, depending on the indoor temperature.

THERMISTOR RESISTANCE (ohms)

C815A THERMISTOR RESISTANCE
R = 400 ohms 

±

 10% AT 77

°

F (25

°

C)

4600
4400
4200
4000
3800
3600
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000

800
600
400
200

0

-20         0         20        40        60        80        100      120       140

TEMPERATURE OF THERMISTOR (

°

F)

M1590A

80

50 60 70 80

COOL

EM.
HEAT FILTER SERV.

AUX
HEAT

M5830

LEDS