On a 2-stage heating 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 the
sec-ond stage makes only when the first stage cannot
handle the load. This 2° F [1° C] is referred to as the
The LED indicators are light emitting diodes on the
subbase that light up when something specific happens
within the system. When an LED lights up, refer to the
following list for the meaning:
EM.HT.: Emergency heat is operating, which means
the system switch is at EM. HT.
AUX.HT.: Auxiliary heat is operating, which means
the weather is so cold that the heat pump can not
handle the load alone.
CHECK: System needs to be checked. See Heating/
Cooling System Instructions for specific meaning.
LEDs are not field replaceable or addable.
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 sealed tube.
WHEN INSTALLING THIS PRODUCT...
1. Read these instructions carefully. Failure to
follow them could cause a hazardous condition.
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 product
operation as provided in these instructions.
1. Disconnect power supply to prevent electri-
cal shock or equipment damage.
2. To prevent interference with the thermostat
linkage, keep wire length to a minimum and
run wires as close as possible to the subbase.
3. Do not overtighten thermostat captive mount-
ing screws because damage to subbase threads
4. Do not short across coil terminals on relay. This
may burn out the thermostat heat anticipator.
5. Never install more than one wire per termi-
nal unless factory-supplied jumper with spade
terminal is used.
IMPORTANT: Thermostats are calibrated at the factory by
using subbases mounted at true level. Inaccurate sub-
base leveling will cause thermostat control deviation.
Install the thermostat about 5 ft [1.5 m] above the
floor in an area with good air circulation at average
Do not install the thermostat where it can be affected
• drafts, or dead spots behind doors and in cor-
• hot or cold air from ducts.
• radiant heat from sun or appliances.
• concealed pipes and chimneys.
• unheated (uncooled) areas such as an outside
wall behind the thermostat.
MOUNTING THE SUBBASE
The thermostat subbase can be mounted on a vertical
outlet box, horizontal outlet box or directly on the wall.
1. If you must mount the subbase on a vertical outlet
box, order 193121A (beige) or 202689A (gray) Adapter
Assembly. See Fig. 1. The assembly includes an adapter ring,
two screws and a cover plate to cover marks on the wall. Install
the ring and cover plate on the vertical outlet box.
For a wall installation, hold subbase in position and
mark holes for anchors. See Fig. 2. Obtain wall anchors
locally. Set subbase aside. Drill four 3/16 in. [5 mm] holes
and gently tap anchors into the holes until flush with the wall.
2. Run wires to the thermostat location. See wiring
diagrams, Figs. 5-11.
Fig. 1—Installation of Q674 Subbase on outlet
1 NOT INCLUDED WITH UNIT.
2 ACCESSORY PARTS AVAILABLE (193121A).