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Manual 2100-549G
Page 33 of 59
DEFROST CYCLE
The defrost cycle is controlled by temperature and time
on the solid state heat pump control. See Figure 20.
When the outdoor temperature is in the lower 40°F
temperature range or colder, the outdoor coil temperature
is 32°F or below. This coil temperature is sensed by the
coil temperature sensor mounted near the bottom of the
outdoor coil. Once coil temperature reaches 30°F or
below, the coil temperature sensor sends a signal to the
control logic of the heat pump control and the defrost
timer will start accumulating run time.
After 30, 60 or 90 minutes of heat pump operation at
30°F or below, the heat pump control will place the
system in the defrost mode. Factory default setting is 90
minutes.
During the defrost mode, the refrigerant cycle switches
back to the cooling cycle, the outdoor motor stops,
electric heaters are energized, and hot gas passing
through the outdoor coil melts any accumulated frost.
When the temperature rises to approximately 57°F, the
coil temperature sensor will send a signal to the heat
pump control which will return the system to heating
operations automatically.
If some abnormal or temporary condition such as a high
wind causes the heat pump to have a prolonged defrost
cycle, the heat pump control will restore the system to
heating operation automatically after 8 minutes.
The heat pump defrost control board has an option of 30,
60 or 90-minute setting. All models are shipped from the
factory on the 90-minute pin. If special circumstances
require a change to another time, remove the wire from
the 90-minute terminal and reconnect to the desired
terminal.
The manufacturer’s recommendation is for 90-minute
defrost cycles. Refer to Figure 20.
There is a cycle speed up jumper on the control. This can
be used for testing purposes to reduce the time between
defrost cycle operation without waiting for time to elapse.
Use a small screwdriver or other metallic object, or
another ¼ inch QC, to short between the SPEEDUP
terminals to accelerate the HPC timer and initiate defrost.
Be careful not to touch any other terminals with the
instrument used to short the SPEEDUP terminals. It may
take up to 15 seconds with the SPEEDUP terminals
shorted for the speedup to be completed and the defrost
cycle to start.
As soon as the defrost cycle kicks in remove the
shorting instrument from the SPEEDUP terminals.
Otherwise the timing will remain accelerated and run
through the 1-minute minimum defrost length sequence in
a matter of seconds and will automatically terminate the
defrost sequence.
There is an initiate defrost jumper (sen jump) on the
control that can be used at any outdoor ambient during
the heating cycle to simulate a 0° coil temperature.
This can be used to check defrost operation of the unit
without waiting for the outdoor ambient to fall into the
defrost region.
By placing a jumper across the SEN JMP terminals (a
¼ inch QC terminal works best) the defrost sensor
mounted on the outdoor coil is shunted out & will
activate the timing circuit. This permits the defrost cycle
to be checked out in warmer weather conditions without
the outdoor temperature having to fall into the defrost
region.
In order to terminate the defrost test the SEN JMP
jumper must be removed. If left in place too long, the
compressor could stop due to the high pressure control
opening because of high pressure condition created by
operating in the cooling mode with outdoor fan off.
Pressure will rise fairly fast as there is likely no actual
frost on the outdoor coil in this artificial test condition.
There is also a 5-minute compressor time delay function
built into the HPC. This is to protect the compressor from
short cycling conditions. The board’s LED will have a fast
blink rate when in the compressor time delay. In some
instances, it is helpful to the service technician to override
or speed up this timing period, and shorting out the SPEEDUP
terminals for a few seconds can do this.
FIGURE 20
DEFROST CYCLE
120*
SW
1
SW
2 TIME (SEC)
OFF
OFF
ON
ON
OFF
ON
OFF
ON
30
60
180
MIS-2668 A
OFF
LOW PRESSURE BYPASS TIMER SWITCH
*(FACTORY SETTING 120 SECONDS)
ACCUMULATED DEFROST TIME TIMER
(FACTORY SETTING 60 MIN.)
ON
(FACTORY SETTING 90 MIN.)