Manual 2100-354
Page
20
FIGURE 12
HEAT PUMP CONTROL BOARD
MIS-1191
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.
DEFROST CYCLE
The defrost cycle is controlled by temperature and time on
the solid state heat pump control. See Figure 12.
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 sensor mounted near the bottom of the outdoor coil.
Once coil temperature reaches 30° F or below, the coil
sends a signal to the control logic of the heat pump control
and the defrost timer will start.
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
economy. If special circumstances require a change to
another time, remove wire connected to terminal 60 and
reconnect to desired terminal. Refer to Figure 12. 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 defront 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
