Honeywell T874H,Q,W Multistage Thermostats and Q674B,C,G,L Subbases Manual

Honeywell T874H,Q,W Multistage Thermostats and Q674B,C,G,L Subbases Manual

Honeywell T874H,Q,W Multistage Thermostats Application

The T874H,Q,W Multistage Thermostats provide 24 to 30 Vac control for heating and cooling systems as listed in Table 1.

Table 1. Heating and Cooling Stages.

 

T874 Model

Stage

Ha

Q

W

Heating Stages

1

1

3

Cooling Stages

1

2

a Plus a changeover stage that operates with cooling.

The Q674B,C,G,L Subbases provide wiring terminals, system and fan switching, and mounting bases for T874 Multistage Thermostats as listed in Table 2.

Table 2. Switching Positions. 

Q674

Switch Positions

Model

System

Fan

B

Heat-Off-Cool

Auto-On

C

Off-Auto

Auto-On

G

Off-Auto

None

L

Em Ht-Heat-Off-Cool

Auto-On

 

Honeywell T874H,Q,W Multistage Thermostats OPERATION

On a two-stage or three-stage heat thermostat, the stages of heat make sequentially as the temperature drops. Make refers to the mercury switch initiating a call for heat or cool. There are about 2°F (1°C) between the stages so that the second stage makes only when the first stage can not handle the load. About one degree later, the third stage makes (T874W only).

This degree difference is called the interstage differential. Select models include LEDs (light-emitting diodes). The LED lights to indicate a problem with the system; refer to the system manufacturer instructions. 

When an LED lights, refer to this list for the meaning:
FILTER: clogged filter needs replacement.
EM. HT.: emergency heat is operating. The compressor has failed, and the heat pump is not operating. The LED lights when the homeowner places the system switch in the EM. HT. position.
AUX. HT.: auxiliary heat is operating because the weather is so cold that the heat pump alone cannot handle the load.
SERVICE or CHECK: system needs to be checked. See heating system instructions for specific meaning.

LEDs are not field addable or replaceable.

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Honeywell T874H,Q,W Multistage Thermostats