Filter
Top Products
Manual 2100-373
Page 14
PRESSURE SERVICE PORTS
High and low pressure service ports are installed on all
units so that the system operating pressures can be
observed. Pressure curves can be found later in the
manual covering all models on both cooling and heating
cycles. It is imperative to match the correct pressure
curve to the unit by model number.
DEFROST CYCLE
The defrost cycle is controlled by temperature and time
on the solid state heat pump control. See Figure 10.
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 30 minute setting for greatest operating
economy. If special circumstances require a change to
another time, remove wire connected to terminal 30 and
reconnect to desired terminal. Refer to Figure 10. The
manufucturer’s recommendation is for 3 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 screwdr4iver or other metallic object, or
another 1/4 inch QC, to short between the SPEEDUP
terminals to accedlerate 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 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 coil 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
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. 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 10
DEFROST CONTROL BOARD
MIS-1174