SYSTEM OPERATION
16
up heat and vaporizes as it proceeds through the coil, cooling the
indoor coil down to about 48°F.
Heat is continually being transferred to the cool fins and tubes of
the indoor evaporator coil by the warm system air. This warming
process causes the refrigerant to boil. The heat removed from the
air is carried off by the vapor.
As the vapor passes through the last tubes of the coil, it becomes
superheated. That is, it absorbs more heat than is necessary to
vaporize it. This is assurance that only dry gas will reach the
compressor. Liquid reaching the compressor can weaken or break
compressor valves.
The compressor increases the pressure of the gas, thus adding
more heat, and discharges hot, high pressure superheated gas
into the outdoor condenser coil.
In the condenser coil, the hot refrigerant gas, being warmer than
the outdoor air, first loses its superheat by heat transferred from
the gas through the tubes and fins of the coil. The refrigerant now
becomes saturated, part liquid, part vapor and then continues to
give up heat until it condenses to a liquid alone. Once the vapor
is fully liquefied, it continues to give up heat which subcools the
liquid, and it is ready to repeat the cycle.
The inverter system can stop the compressor or outdoor fan to
protect the unit. The inverter system can run higher compressor
speed than required from thermostat to recover compressor oil
that flows.
HEATING
The heating portion of the refrigeration cycle is similar to the
cooling cycle. By de-energizing the reversing valve solenoid coil,
the flow of the refrigerant is reversed. The indoor coil now be-
comes the heat pump condenser coil, and the outdoor coil be-
comes the evaporator coil. The check valve at the outdoor coil
will be forced closed by the refrigerant flow, thereby utilizing the
outdoor expansion device. An electronic expansion valve meters
the condensed refrigerant to the outdoor coil.
DEFROST CYCLE
The defrosting of the outdoor coil is controlled by the PCB and the
outdoor coil temperature thermistor and defrost sensor. The out-
door coil temperature thermistor (T
m
) sensor is clamped to a
return bend entering the outdoor coil and the defrost sensor at
bottom flowrator leg at outdoor coil outlet. Defrost timing peri-
ods of 30, 60, 90 or 120 minutes may be selected via the thermo-
stat setting. PCB will initiate time defrost at the interval selected
from the thermostat. During operation, the microprocessor on
the PCB checks the two coil and defrost temperature (T
m
and T
b
)
via the sensor every 5 seconds in heating mode. When the PCB
detects the coil temperature to be high enough (approximately 54
°F) and defrost sensor more than 43 °F for 30 seconds, the defrost
cycle is terminated and the timing period is reset. The field ser-
vice personnel can also advance a heat pump to the defrost cycle
by selecting “force defrost” option from thermostat.
