Installation manual

11
Explanation and Guidance—Heat Pump
When the heat pump is in the cooling cycle, the heat pump
operates exactly as an air conditioning unit.
The heat pump operates in the heating cycle by redirecting
refrigerant flow through the refrigerant circuit external to the
compressor. This is accomplished through 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 a 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 cooling to heating, the piston in the reversing valve
moves to the low pressure (high pressure) reverse position.
The following illustrations show schematics of a heat pump on
the cooling cycle and the heating cycle. In addition to a reversing
valve, a heat pump is equipped with an expansion device and
check valve for the indoor coil, and similar equipment for the
outdoor coil. It is also provided with a defrost control system.
Cooling Cycle
Heating Cycle
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
(7ºC). 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, refrigerant leak, etc.
Since the temperature of the 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.
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.
A. Evaporator
B. Service valves
C. Service ports
D. Reversing valve
E. Condenser
F. Accumulator
G. Compressor
H. Distributors
I. Expansion device
J. Indoor coil
K. Check-valve orifices
L. Outdoor coil
G
H
J
B
I
E
F
D
C
J
K
A
C
C
B
H
A. Condenser
B. Service valves
C. Service ports
D. Reversing valve
E. Evaporator
F. Accumulator
G. Compressor
H. Distributors
I. Indoor coil
J. Check-valve orifices
K. Outdoor coil
G
H
J
B
I
E
F
D
C
J
K
A
C
B
H