85
Heating, Cooling, Defrost, and Reheat
Figure 38 - Heat Pump with Reheat Option
NOTE: Figure 38 is one example of many configurations offered.
Air velocity should be maintained between 200 and 550 fpm through the indoor coil.
Heating cycle (heat pump)
• In heating mode, the outdoor coil acts as the evaporator coil. When the thermostat calls for a heating
sequence, the reversing valve is automatically powered. The compressor and outdoor fan start. The
heating system is now in operation. Once the thermostat is satisfied, the system will shut down.
• The compressor pumps out high pressure refrigerant vapor. The vapor leaves the compressor, and
then through the energized reversing valve.
• The refrigerant then flows through the indoor coil. Supply air removes heat from the refrigerant vapor,
warming the indoor air and heating the building. When enough heat is removed, the vapor condenses
into a high pressure liquid. The liquid temperature is slightly warmer than indoor air temperature. The
liquid refrigerant then passes through an Electronic Expansion Valve (EEV), reducing its pressure and
temperature, then passes through a filter/drier. The filter/drier adsorbs water and filters system
contaminants.
• As the cool, low pressure liquid refrigerant enters the outdoor coil, it expands and absorbs heat from
the outdoor air passing over the finned surface. Heat from the outdoor air causes the low pressure
liquid to evaporate into a cool vapor.
• The cold refrigerant vapor passes through the outdoor vapor line to the reversing valve. The reversing
valve directs refrigerant into the accumulator. The accumulator holds a liquid refrigerant and oil
mixture, and controls flow back to the compressor. The liquid refrigerant and oil mixture are metered
back to the compressor through a small orifice near the bottom of the accumulator.
• The refrigerant vapor passes through the suction line to the intake of the compressor. The cycle then
repeats.
Outdoor Fan(s)
Filter/Drier
Outdoor Coil
Indoor Coil
Reheat Coil
Electronic
Expansion Valve
Reversing
Valve
Accumulator
Compressor
Supply Fan
Suction
Line
Discharge Line
High
Refrigeration
Pressure
Switch
Hot Gas Reheat Valve (HG-1)
Low Refrigeration
Pressure Switch
Discharge
Pressure
Sensor
Liquid Line Pressure
Sensor
