Manual 2100-578L
Page
28 of 42
DEFROST CYCLE
The defrost cycle is controlled by temperature and time
on the solid state heat pump control. See Figure 14.
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 14.
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" 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 JMP
) 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" 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
condition created by operating in the cooling mode with
outdoor fan off. Pressure will rise rapidly 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 Heat Pump Control to protect the compressor
from short cycling conditions. The board’s LED will have a
fast blink rate when in the compressor time delay function.
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 14
Heat Pump Control
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.)
