Which thermostat terminal is most commonly used to energize the reversing valve in a heat pump?

• A. Y
• B. A
• C. C
• D. O

Answer: (D)

The reversing valve in a heat pump is controlled by a dedicated thermostat terminal that supplies 24 VAC to energize the valve. On most systems, that terminal is the O terminal. When cooling is requested, applying power to O energizes the valve and shifts refrigerant flow to the cooling position. In some brands, the valve is energized in heating using the B terminal, but that setup is less common. The C terminal is just the common power return for the thermostat, not used to energize the valve, and Y only controls the compressor contactor. So the terminal most commonly used to energize the reversing valve is O.