Advanced & Specialty Systems
Heat pumps, cascade systems, compound systems, ammonia systems, CO2 transcritical, and Canadian regulations.
- Explain heat pump operation and the reversing valve
- Compare cascade, compound, and two-stage refrigeration systems
- Describe ammonia (R-717) system safety requirements
- Explain CO2 (R-744) transcritical system operation
- Identify Canadian ODS regulations and certification requirements
Lección 1
Heat Pumps & Reversing Valves
Heat Pump Operation
A heat pump is a refrigeration system that can move heat in either direction - providing cooling in summer and heating in winter. The key component that enables this is the reversing valve (also called a four-way valve). The reversing valve switches the roles of the indoor and outdoor coils.
Cooling Mode
Indoor coil: Evaporator (absorbs heat)
Outdoor coil: Condenser (rejects heat)
Metering device: Indoor TXV or check valve active
Reversing valve: De-energized (most systems)
Heating Mode
Indoor coil: Condenser (rejects heat)
Outdoor coil: Evaporator (absorbs heat)
Metering device: Outdoor TXV or check valve active
Reversing valve: Energized (most systems)
Reversing Valve Operation
The reversing valve is a slide-type valve operated by a solenoid pilot valve. The solenoid shifts a small pilot valve that uses system pressure to slide the main valve. Most manufacturers configure the valve to be de-energized in heating mode so that if the solenoid coil fails, the system defaults to heating (the more critical function in cold climates).
Exam Tip - Default Mode
Most heat pumps default to heating mode when the reversing valve is de-energized. However, some manufacturers (Rheem) default to cooling. The exam may specify the brand or state "energized in cooling" - read carefully.
Defrost in Heat Pump Mode
During heating mode, the outdoor coil acts as the evaporator and can accumulate frost. To defrost, the system temporarily switches to cooling mode (energizes the reversing valve) while an auxiliary electric heater warms the indoor air to prevent cold air delivery. Defrost is initiated by a timer/temperature combination and terminated by a temperature sensor on the outdoor coil.
A heat pump uses a reversing valve to switch between heating and cooling. Most systems default to heating mode when de-energized. During defrost, the system switches to cooling mode with auxiliary heat to prevent cold air delivery indoors.