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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 filled 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 efficient, 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.
(Some thermostats may have the terminals identified by the letters C, H and L)
L
C
H
SWITCH
POSITION
TEMPERATURE
AT CAPILLARY
CONTINUITY READ
EMERGENCY HEAT
N/A
1(H) and 2(C) = Electric Heat
ANYWHERE IN
ADJUSTMENT RANGE
ABOVE
SET POINT
2(C) and 3(L) = Compressor
ANYWHERE IN
ADJUSTMENT RANGE
BELOW
SET POINT
1(H) and 2(C) = Electric Heat
