Heat Pumps, Reversing Valves and Defrost

In this episode of HVAC School, Bryan covers the basics of heat pumps.

Heat pumps are common technologies in Florida. They reverse the sequence of the typical refrigerant circuit: the indoor coil can become the condenser, and the outdoor coil can become the evaporator. Heat pumps can achieve that transition via a reversing valve, which changes the directions of the suction and discharge lines. They also have two metering devices.

Reversing valves contain a solenoid (typically 24v) that rediverts the suction and discharge lines via shifting the slider with a pressure differential. Pilot tubes shift gas from one side of the slider to the other, which shifts it and triggers heat mode or cooling mode. Reversing valves are typically energized in cool mode (except for Ruud/Rheem reversing valves; they energize in heat mode).

Defrosting is rarely necessary for us in Florida, but it can be a scary occurrence when we do need it. The outdoor coil can freeze over entirely when it gets cold enough due to Florida's high humidity. Hot gas goes through the coils during defrosting, and it may make alarming noises. Many Floridian heat pumps also use auxiliary heat strips to provide heat while the system defrosts. Many defrosts rely on set times and sensors to determine when to initiate and terminate defrost. (That is true of heat pumps AND most refrigeration systems.) Thermistors are common sensing technologies used in defrost.

Join Bryan on this informative monologue about:

• Reversing valves
• Aux heat
• W and W2
• Heat Pumps
• Defrost
• Checking refrigerant charge in heat mode
• Heat mode expansion valves
• Common heat pump considerations

For a more detailed written explanation of heat pump reversing valves with pictures, check out this article.

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