10
The following figures show a schematic of a heat pump on
the cooling cycle and the heating cycle
Heat Pump Refrigerant Circuit
In
In addition to a reversing valve, a heat pump is equipped with
an expansion device and check valve for the inside coil, and
similar equipment for the outside coil. It is also provided with
a defrost control system.
The expansion device performs the same function on the
heating cycle as on the cooling cycle. The check valves are
required due to the reverse flow of refrigerant when chang-
ing from cooling to heating or vice versa.
When the heat pump is on the heating cycle, at which time
the outdoor coil is functioning as an evaporator, the tempera-
ture of the refrigerant in the outdoor coil must be below the
temperature of the outdoor air in order for the refrigerant in
the outdoor coil to extract heat from the air. Thus, the greater
the difference in outdoor temperature and outdoor coil
temperature, the greater the heating capacity of the heat
pump. Since this is characteristic of heat pumps, it is good
practice to provide supplementary heat for all heat pump
installations in areas where the temperature drops below
7.2°C. It is also good practice to provide sufficient supple-
mentary heat to handle the entire heating requirements if
there should be a failure of heat pump, such as a compressor
failure, or refrigerant leak, etc.
Since the temperature of the liquid refrigerant in the outdoor coil
on the heating cycle is generally below the 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 refriger-
ant 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 ener-
gized.
SERVICE:
The following information is for use by qualified service
agency only; others should service this equipment.
Common Causes of Unsatisfactory Operation of Heat
Pumps on the Heating Cycle.
A. Dirty Filters or inadequate air volume through indoor coil.
When the heat pump is on the heating cycle, the indoor
coil is functioning as a condenser; therefore, the filters
must always be clean, and sufficient air volume must
pass through the indoor coil to prevent excessive dis-
charge pressure and high pressure cut-out.
B. Outside Air into Return Duct: Cold outside air should
not be introduced in the return duct of a heat pump in-
stallation on the heating cycle close enough to the in-
door coil to reduce temperature of the air entering the
coil below 18.3°C. Air below this temperature will cause
low discharge pressure, thus low suction pressure and
excessive defrost cycling with resultant low heating out-
put. It may also cause false defrosting.
C. Undercharge: Undercharge on the heating cycle will
cause low discharge pressure resulting in low suction
pressure and frost accumulation on the lower part of
the outdoor coil.
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