Heat Pump Systems
Reversing valve operation, defrost cycles, auxiliary heat, accumulator function, and heat pump diagnostics.
- Explain how a heat pump reverses the refrigeration cycle for heating
- Describe the function and operation of the reversing valve
- Explain defrost initiation and termination methods
- Identify when auxiliary and emergency heat are used
- Diagnose common heat pump problems including stuck reversing valves and defrost failures
Lesson 1
How Heat Pumps Work - Reversing the Cycle
The Heat Pump Concept
A heat pump is an air conditioning system that can reverse the refrigeration cycle to provide both cooling and heating. In cooling mode, it works exactly like a standard air conditioner - absorbing heat from indoors and rejecting it outdoors. In heating mode, it absorbs heat from outdoor air and delivers it indoors.
Cooling Mode
Indoor coil = Evaporator (absorbs heat)
Outdoor coil = Condenser (rejects heat)
Heat moves from indoors to outdoors
Same as standard air conditioning
Heating Mode
Indoor coil = Condenser (rejects heat)
Outdoor coil = Evaporator (absorbs heat)
Heat moves from outdoors to indoors
Reversing valve switches flow direction
The Reversing Valve
The reversing valve (four-way valve) is the component that switches the refrigerant flow direction between cooling and heating modes. It directs hot discharge gas from the compressor to either the indoor coil (heating) or the outdoor coil (cooling).
The reversing valve is controlled by a solenoid coil, typically energized by 24 VAC from the thermostat. Industry convention varies by manufacturer:
- Most brands: solenoid energized in cooling mode
- Rheem/Ruud: solenoid energized in heating mode
Why Energize in Cooling?
Most manufacturers energize the reversing valve solenoid in cooling mode. If the solenoid fails or loses power, the system defaults to heating mode. This is considered the safer failure mode because most heat pump problems occur in winter, and you want heating available even with a failed solenoid.
Heat Pump Components
In addition to the standard four refrigeration components, heat pumps include:
- Reversing valve - switches between heating and cooling
- Accumulator - protects compressor from liquid floodback (critical in heating mode)
- Two metering devices - one for each direction of flow (with check valves)
- Defrost board - controls the defrost cycle
- Auxiliary heat - backup heating for cold weather
In heating mode, a heat pump absorbs heat from outdoor air through the outdoor coil (now acting as evaporator) and rejects heat indoors through the indoor coil (now acting as condenser). The reversing valve directs refrigerant flow to swap the roles of the indoor and outdoor coils.