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Emergency Heat Switch (Defrost Thermostat) Continuity Check
Electric Heat Switch Operation
(Heat Pumps Only)
The electric heat switch is a dual function control and is
shown on the wiring diagram as a defrost thermostat.
It may be adjusted using a screwdriver. As the control
shaft is rotated counter clockwise a detent will be
encountered. Turning the control past the detent will lock
out the compressor and acts as an emergency heat switch.
Turning the control shaft clockwise will lower the change
over point for compressor operation. The control it self is
a double throw, single pole switch operated by a bellows
and a gas fi lled capillary tube. The capillary tube senses
a combination of outdoor coil temperature and outdoor air
temperature. As the combined temperatures reach a point
that the outdoor coil is iced, where heat pump operation
is no longer effi cient, the control shuts off the compressor
and turns on the electric heat. At its lowest setting the cut
off point is approximately 25 degrees, the highest setting
is 52 degrees, with a 10 degree differential. It is possible,
under certain conditions, for the unit to cycle between
compressor and electric heat operation.
Electric Heat Switch Check Out
The switch may be checked out with an ohmmeter.
Remove and label the three wires from the switch.
Terminal 2 is common and the contacts make to Terminal
3 on temperature rise and to Terminal 1 on temperature
fall. With the control set in the emergency heat position
continuity should be read between Terminal 2 and Terminal
1 regardless of coil temperature. As the control shaft is
rotated clockwise, through the adjustment range, continuity
will be read between Terminal 2 and Terminal 3, providing
the temperature of the capillary tube is above 25º (± 5%). If
the temperature at the capillary tube is above approximately
52 degrees it may be necessary to place the end of the
capillary tube in ice water to determine if the control is
sensing temperature changes. Should the control lose the
gas charge in the capillary tube it will fail to the electric heat
position and the compressor will not operate.
SWITCH POSITION TEMPERATURE AT CAPILLARY CONTINUITY READ
EMERGENCY HEAT N/A 1 and 2 = Electric Heat
ANYWHERE IN
ADJUSTMENT RANGE
ABOVE SET POINT 2 and 3 = Compressor
ANYWHERE IN
ADJUSTMENT RANGE
BELOW SET POINT 1 and 2 = Electric Heat