T874A-F Multistage Thermostats
The T874A-F Thermostats provide 24 to 30 Vac control
for heating and/or cooling systems. See Tables 1 and 2.
TABLE 1—HEATING AND/OR COOLING STAGES.
The Q674A-E,G Subbases provide wiring terminals,
system and fan switching, and mounting bases for T874
TABLE 2—SWITCHING POSITIONS.
On a 2-heat thermostat, the two 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 stages so that the
second stage makes only when the first stage can not
handle the load. This is the interstage differential.
The LED indicators are light emitting diodes on the
subbase that light when something specific happens with-
in the system.
Up to four of the LEDs listed below may be included on
your subbase. When an LED lights, refer to the following
list for the meaning:
FILTER: Clogged filter needs replacement.
with Restricted Set Point
Installation Instructions for the Trained Service Technician.
S. M. • Rev. 6-93 •
EM. HT.: Emergency heat is operating. The compres-
sor has failed, and the heat pump is not operating.
LED lights when system switch is placed in the EM.
AUX. HT.: Auxiliary heat is operating, which means the
weather is so cold that the heat pump can not handle
the load alone.
LOCKOUT: System is shut down and needs maintenance.
CHECK: System needs to be checked. See heating sys-
tem instructions for specific meaning.
SERVICE: See heating system instructions for specific
LEDs are not field replaceable.
This control contains mercury in a sealed tube. Do not
place control in the trash at the end of its useful life.
If this control is replacing a control that contains mercury
in a sealed tube, do not place your old 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
WHEN INSTALLING THIS PRODUCT...
1. Read these instructions carefully. Failure to fol-
low them could damage the product or cause a hazardous
2. Check the ratings given in the instructions and on
the product to make sure the product is suitable for your
3. Installer must be a trained, experienced service
4. After installation is complete, check out the product
operation as provided in these instructions.
• ©Honeywell Inc. 1993 • Form Number 60-1227—1