11
Some room thermostats do not energize the motor
during electric heat. This relay ensures blower
operation when the room thermostat energizes
heat. This relay is energized by the electric heat kit
sequencer.
HEAT PUMP OPERATION
Cooling Cycle
When the heat pump is in the cooling cycle, it operates
exactly as a Air Conditioner unit. See Figure 9 - Heat Pump
Cooling Schematic.
Indoor
Coil
Accumulator
Outdoor
Coil
Reversing Valve
(Energized)
Typical Heat Pump System in Cooling
Heat Pump Cooling Schematic
Figure 9
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, show a schematic of a
heat pump in the heating cycle.
Indoor
Coil
Accumulator
Outdoor
Coil
Reversing Valve
(De-Energized)
TYPICAL HEAT PUMP SYSTEM IN HEATING
Heat Pump Heating Schematic
Figure 10
For Heat Pump units, 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.
The expansion devices are flowrator distributors and
perform the same function on the heating cycle as on the
cooling cycle. The flowrator distributors 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 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.
