Manual
2100-468E
Page
19 of 27
MIS-1191
DEFROST CYCLE
The defrost cycle is controlled by temperature and time on
the solid state heat pump control. See Figure 9.
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
defrost sensor mounted near the bottom of the outdoor coil.
Once the Heat Pump Control board sees the resistance of
the defrost sensor has been below the resistance of 34545
(30°F) for 60 minutes of accumulated run time. The Heat
Pump Control Board will start the defrost cycle by de-
energizing the reversing valve and condenser fan. It will
also send a signal to W2 to energize the electric heat if
equipped. When the Heat Pump Control Board reads the
resistance of the defrost sensor has risen to 16547 (57°F) or
it has been in defrost for 10 minutes the defrost cycle will
terminate.
After 30 minutes at 30°F or below, the heat pump control
will place the system in the defrost mode.
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 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 10 minutes.
There are three settings on the heat pump control – 30
minute, 60 minute and 90 minute. Models are shipped
wired on the 60 minute setting for greatest operating
FIGURE 9
HEAT PUMP CONTROL BOARD
economy. If special circumstances require a change to
another time, remove wire connected to terminal 60 and
reconnect to desired terminal. Refer to Figure 9. The
manufacturer’s recommendation is for 60 minute defrost
cycles.
There is a cycle speed up jumper on the control. This can
be used to reduce the time between defrost cycle operation
without waiting for time to elapse.
Use a small screwdriver or other metallic object, or another
1/4 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 instrument
used to short the
SPEEDUP
terminals. It may take up to 10
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 maximum 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 1/4
inch QC terminal works best) the defrost sensor mounted
on the outdoor coils is shunted out and 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 in 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 the 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
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.
