9
This is accomplished by reversing the heat pump to the cooling
cycle. At the same time, the outdoor fan stops to hasten the
temperature rise of the outdoor coil and lessen the time required
for defrosting. The indoor blower continues to run, and the
supplementary heaters are energized.
Cooling Cycle
Heating Cycle
Defrost Cycle
If the outdoor ambient conditions are such that frost forms on the
outdoor coil, the defrost control monitors a defrost cycle. It then
runs the defrost cycle as ambient temperatures require.
The defrost control is time/temperature initiated and temperature
terminated with a maximum defrost time (time-out) of 10 minutes.
The time between defrost cycles is preset at 60-minute intervals
at the factory, but can be field adjusted between 30, 60, or
90 minutes.
The defrost control will initiate a defrost cycle when the selected
time period has elapsed and the defrost sensor sees a
temperature below freezing. At the start of a defrost cycle, the
defrost control will energize the reversing valve solenoid, shifting
the reversing valve and de-energizing the outdoor fan. The
defrost relay will also close, energizing temporary heat for
increased comfort during defrost (if the indoor unit is so
equipped). The heat pump will remain in defrost until the defrost
sensor has determined that the frost has been removed from the
coil or a 10-minute period has elapsed, whichever comes first.
Defrost Control
During operation, the power to the circuit board is controlled by a
temperature sensor, which is clamped to a feeder tube entering
the outdoor coil. Defrost timing periods of 30, 60 and 90 minutes
may be selected by connecting the circuit board jumper to 30,
60 and 90, respectively. Accumulation of time for the timing
period selected starts when the sensor closes (approximately
34°F [1ºC]), and when the wall thermostat calls for heat. At the
end of the timing period, the unit’s defrost cycle will be initiated
provided the sensor remains closed. When the sensor opens
(approximately 60°F [16ºC]), the defrost cycle is terminated and
the timing period is reset. If the defrost cycle is not terminated
due to the sensor temperature, a 10-minute override interrupts
the unit’s defrost period.
Circuit Board
Suggested Field Testing/Troubleshooting
1.
Run the unit in the heating mode (room thermostat calling for
heat).
2.
Check the unit for proper charge.
NOTE:
Bands of frost on the condenser coil indicate low
refrigerant charge.
3.
Turn off power to the unit.
4.
Disconnect the outdoor fan by removing the purple lead from
“DF2” on the defrost control.
5.
Restart the unit and allow frost to accumulate.
6.
After a few minutes of operation, the defrost thermostat
should close. To verify this, check for 24 volts between “DFT”
and “C” on the control board. If the temperature at the
thermostat is less than 28°F (-2ºC) and the thermostat is
open, replace the defrost thermostat, since it is defective.
7.
When the defrost thermostat has closed, short the test pins
on the circuit board until the reversing valve shifts, indicating
defrost. This should take up to 21 seconds depending on
what timing period the control is set.
NOTE:
After defrost initiation, the short must instantly be
removed, or the unit’s defrost period will last only 2.3 seconds.
8.
After the defrost thermostat has terminated, check the
defrost thermostat for 24 volts between “DFT” and “C.” The
reading should indicate 0 volts (open sensor).
9.
Turn off power to the unit.
10.
Replace the outdoor fan motor lead to terminal “DF2” on the
circuit board and turn on power.
A. Evaporator
B. Service valves
C. Service ports
D. Reversing valve
E. Condenser
F. Accumulator
G. Compressor
H. Distributors
I. Indoor coil
J. Check valve orifices
K. Outdoor coil
A. Condenser
B. Service valves
C. Service ports
D. Reversing valve
E. Evaporator
F. Accumulator
G. Compressor
H. Distributors
I. Indoor coil
J. Check valve orifices
K. Outdoor coil
G
H
J
B
I
E
F
D
C
J
K
A
C
C
B
H
G
H
J
B
I
E
F
D
C
J
K
A
C
B
H
C Y
W2
R
R DFT
TEST
DF1
DF2
Jumper Wire
90
60
30
A
