Operation - Magic-pac MHP4 Series Installation And Maintenance Instructions Manual

OPERATION

Sequence of Operation
Upon initial "power up" to unit, there is a 3-minute time delay
to the compressor contactor (R to Y). Any 24V interrupt (R,
C) to the defrost control will initiate the
3-minute delay to the contactor.
Cooling
When the thermostat is in the cooling mode, the O circuit is
powered which energizes the reversing valve. Upon cooling
demand, the thermostat closes circuit R to Y and G. Closing R
to Y closes the unit contactor, starting the compressor and
outdoor fan. The thermostat automatically closes R to G circuit
which also brings on the indoor blower at the same time. Upon
satisfying cooling demand, the thermostat will open the above
circuits and open the main contactor, stopping the compressor
and outdoor fan. The unit is equipped with a time delay; the
blower will continue to operate for 90 seconds which improves
system efficiency.
Heating
Upon heating demand, the thermostat closes circuit R to Y,
which closes the unit contactor, starting the compressor and
outdoor fan. The reversing valve is not energized in the heating
mode. The thermostat again automatically brings on the indoor
fan at the same time. The second stage of the thermostat
closes circuit R to W, which closes the unit sequencers, bringing
the auxiliary electric heat on. Upon satisfying heating demand,
the thermostat opens the above circuits and stops unit
operation.
NOTE: The 7.2 and 10 kW heats strips offer a W1 and W2
thermostat connection. Taking advantage of the two heat strip
circuits requires a 3-stage thermostat (Y for heat pump, W1 for
electric heat stage 1, and W2 for electric heat stage 2). If a 2-
stage thermostat is used, "pigtail" W1 and W2 wires together
when connecting the thermostat wires at the unit.
Defrost Cycle
If the outdoor ambient conditions are such that frost forms on
the outdoor coil, the defrost control monitors the need for,
initiates, and terminates defrost cycles as necessary to
maintain system performance. The defrost control is time/
temperature initiated and temperature terminated with a
maximum defrost time (time-out) of 10 minutes. Time between
defrost cycles is preset at 60-minute intervals at the factory,
but can be field adjusted between 30, 60, or 90 minutes. To
field adjust time between defrost cycles, place defrost time
plug in the proper position (see Figure 8).
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Defrost control will initiate a defrost cycle if the time period has
elapsed and the defrost sensor sees a temperature below
freezing. At the start of a defrost cycle, the defrost control will
energize the reversing valve solenoid, shifting the reversing
valve and de-energizing the outdoor fan. The defrost relay
will also close, energizing auxiliary heat for increased comfort
during defrost. The unit will remain in defrost until the defrost
sensor has determined that the frost has been removed from
the coil or a 10-minute time period has elapsed.
The defrost control is also equipped with a set of pins to aid
in the troubleshooting of the defrost system (see Figure 8).
Issue 0941
Defrost Control
Figure 7
Defrost Jumpers
Figure 8
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