The B terminal powers the reversing valve to call for heat on some brands, while the O terminal powers the reversing valve to call for cool on most other brands.

On a heat pump the reversing valve directs refrigerant flow to switch between heating and cooling, and the B and O terminals are simply control outputs from the thermostat to energize that valve. If your thermostat has a B terminal and the system requires B you wire the control conductor from the B terminal to the valve coil and set the thermostat to energize B for heat; for an O terminal you wire from O and set the thermostat to energize O for cool. Many residential systems follow one of these conventions: Ruud and Rheem often use B to energize for heat and most other manufacturers use O to energize for cool. Always confirm with the equipment documentation and the valve coil label since some valves are energized in the opposite state or have built in polarity requirements. When replacing a thermostat enable the heat pump mode and set the changeover valve or valve orientation setting to match the terminal being used so the system does not run in the wrong mode.

The three common mistakes are wiring the reversing valve to the wrong terminal, expecting one convention to apply to every brand, and assuming the valve coil does not require a common. Wiring to the wrong terminal makes the system run in the opposite mode so swap the conductor to the correct terminal and test in both heat and cool. Assuming one convention everywhere causes repeated callbacks so verify the valve manufacturer and the outdoor unit wiring diagram before finalizing wiring. Forgetting the common or using a non compatible thermostat can leave the valve without a complete circuit so ensure the thermostat provides the correct voltage and returns to the transformer common or use a separate relay if required.

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