13
HEAT PUMP OPERATION
Cooling Cycle
When the heat pump is in the cooling cycle, it operates
exactly as a Summer Air Conditioner unit. In this mode,
all the charts and data for service that apply to summer
air conditioning apply to the heat pump. Most apply on
the heating cycle except the “condenser” becomes the
“evaporator”, “evaporator” becomes “condenser”, “cooling”
becomes “heating”.
Heating Cycle
The heat pump switches from cooling cycle by redirecting
refrigerant flow through the refrigerant circuit external to
the compressor. This is accomplished by 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.
The following figures show a schematic of a heat pump on
the cooling cycle and the heating cycle. The heat pump
is equipped with thermal expansion valves for the indoor
and outdoor coils. It is also provided with a defrost control
system.
Heating
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 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 Defrost signal 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 connecting the circuit board jumper to 30, 60 and
90 respectively. Accumulation of time for the timing
period selected starts when the sensor contact closes
(approximately 31°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 contact remains
closed. When the sensor contact opens (approximately 75°
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 ten minute override interrupts the
unit’s defrost period.
Summary of Contents for DP16H M Series
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