T874H,Q,W Multistage Thermostats and
The T874H,Q,W Multistage Thermostats provide 24 to 30
Vac control for heating and cooling systems as listed in
Table 1. Heating and Cooling Stages.
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.
On a two-stage or three-stage heat thermostat, the stages
sequentially as the temperature drops.
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
Select models include LEDs (light-emitting diodes). The
LED lights to indicate a problem with the system; refer to
the system manufacturer instructions.
Copyright © 1996 Honeywell Inc. •
• All Rights Reserved
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
LEDs are not field addable or replaceable.
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 the recycling and the proper
disposal of this control, or of an old control
containing mercury in a sealed tube.
When Installing this Product...
Read these instructions carefully. Failure to follow
them could damage the product or cause a hazard-
Check the ratings given in the instructions and on
the product to make sure the product is suitable for
Installer must be a trained, experienced service
After installation is complete, check out product
operation as provided in these instructions.
place your old