Heat Pump Operation; Cooling Cycle; Heating Cycle; Defrost Control - Goodman M Series Installation Instructions Manual

HEAT PUMP OPERATION

Cooling Cycle

When the heat pump is in the cooling cycle, it operates
exactly as an Air Conditioner Unit. See Figure 9: HEAT
PUMP COOLING SCHEMATIC
Figure 9: Heat Pump Cooling Schematic

Heating Cycle

The heat pump operates in the heating cycle by redirecting
refrigerant flow through the refrigerant circuit external to the
compressor. This is accomplished with the reversing valve.
Hot discharge vapor from the compressor is directed to
the indoor coil (evaporator on the cooling cycle) where
the heat is removed, and the vapor condenses to liquid.
It then goes through the expansion device to the outdoor
coil (condenser on the cooling cycle) where the liquid is
evaporated, and the vapor goes to the compressor.
When the solenoid valve coil is operated either from
heating to cooling or vice versa, the piston in the reversing
valve to the low pressure (high pressure) reverse positions
in the reversing valve. The following figure, FIGURE 10:
HEAT PUMP HEATING SCHEMATIC, shows a schematic
of the heat pump in the heating cycle.
Figure 10: Heat Pump Heating Schematic
For 5-Ton Heat Pump unit, the expansion devices are
Thermal Expansion Devices (TXV) and perform the same
function on the heating cycle as on the cooling cycle. The
TXVs also act as check valves to allow for the reverse of
refrigerant flow.
When the heat pump is on the heating cycle, the outdoor
coil is functioning as an evaporator. The temperature
of the refrigerant in the outdoor coil must be below the
temperature of the outdoor air in order to extract heat from
the air. Thus, the greater the difference in the outdoor
temperature and the outdoor coil temperature, the greater
the heating capacity of the heat pump. This phenomenon
is a characteristic of a heat pump. It is a good practice to
provide supplementary heat for all heat pump installations
in areas where the temperature drops below 45° F. It is
also a good practice to provide sufficient supplementary
heat to handle the entire heating requirement should
there be a component failure of the heat pump, such as a
compressor, or refrigerant leak, etc.
Since the temperature of the liquid refrigerant in the
outdoor coil on the heating cycle is generally below
freezing point, frost forms on the surfaces of the outdoor
coil under certain weather conditions of temperature and
relative humidity. Therefore, it is necessary to reverse the
flow of the refrigerant to provide hot gas in the outdoor coil
to melt the frost accumulation. This is accomplished by
reversing the heat pump to the cooling cycle. At the same
time, the outdoor fan stops to hasten the temperature
rise of the outdoor coil and lessen the time required for
defrosting. The indoor blower continues to run, and the
supplementary heaters are energized.

Defrost Control

During operation the power to the circuit board is controlled
by a temperature sensor, which is clamped to a feeder tube
entering the outdoor coil. Defrost timing periods of 30,60
and 90 minutes may be selected by setting the circuit
board jumper to 30, 60 and 90 respectively. Accumulation
of time for the timing period selected starts when the
sensor closes (approximately 30 ± 5°F), and when the wall
thermostat calls for heat. At the end of the timing period,
the unit's defrost cycle will be initiated provided the sensor
remains closed. When the sensor opens (approximately
60°± 5°F), the defrost cycle is terminated and the timing
period is reset. If the defrost cycle is not terminated due to
the sensor temperature, a twelve-minute override interrupts
the unit's defrost period.
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Figure 11