T874G,R Multistage Heat Pump Thermostats
The T874G,R Thermostats with Q674F,J,L Subbases
provide 24 to 30 Vac control of a two-stage heating/one-
On a two-heat thermostat, the two stages of heat
sequentially as the temperature drops.
mercury switch initiating a call for heat or cool.
There are about 2°F (1°C) between stages so that the
second stage makes only when the first stage cannot
handle the load. This is the
LEDs (light emitting diodes) are included on your subbase.
Refer to the list below for specific meaning.
The CHECK LED lights when something needs to be
checked or done to maintain efficient operation of the
system. See your heating system instructions for the
The SYSTEM LED lights when the system is calling for
heating or cooling.
The AUX. HT. LED lights when the auxiliary heat (second
stage) is operating. Auxiliary heat operates when the heat
pump alone cannot handle the load.
The EM. HT. LED lights when the system switch is placed
in the EM. HT. position. Emergency heat is operating; in
most systems, the compressor has failed and the heat
pump is not operating.
LEDs are not field replaceable or addable.
Copyright © 1995 Honeywell Inc. •
Table 1. Thermostat/Subbase Specifications.
refers to the
• All Rights Reserved
and Q674F,J,L Subbases
stage cooling heat pump system. See Table 1 for
This control contains mercury in a sealed tube. Do
not place the control in the trash at the end of its
If this control is replacing a control that contains
mercury in a sealed tube, do
control in the trash.
Contact your local waste management authority for
instructions regarding recycling and the proper
disposal of this control, or of an old control
containing mercury in a sealed tube.
When Installing this Product...
1. Read these instructions carefully. Failure to follow
them could damage the product or cause a
2. Check the ratings given in the instructions and on
the product to make sure the product is suitable for
3. Installer must be a trained, experienced service
4. After installation is complete, check out product
operation as provided in these instructions.
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