Use the O terminal on a heat pump thermostat when the reversing valve needs to energize in cooling mode; use B when it must energize in heating mode.

Thermostats control the reversing valve by sending a dedicated signal through either the O or B terminal; O is conventional for most manufacturers where the valve is energized for cooling and de-energized for heating. Begin by checking the outdoor unit wiring diagram or the unit label to confirm whether the reversing valve is energized on cooling or heating. When replacing a thermostat, match the thermostat wiring configuration to the outdoor unit function and set the thermostat installer options so the reversing valve output is assigned correctly to O or B. If you are unsure, power off the system and trace the compressor contactor and reversing valve wires to identify which terminal on the outdoor unit is the reversing valve lead. For smart or programmable thermostats, use the setup menu to choose whether the reversing valve is O or B and then test by calling for cooling and observing the valve or listening for the outdoor changeover click. Always verify with a functional test and note the terminal assignment on your maintenance record so future technicians can wire consistently.

Common mistakes include wiring both O and B simultaneously which causes conflicting signals and can damage the thermostat or outdoor valve; correct that by connecting only the single correct terminal and removing the other wire from the thermostat. Another frequent error is assuming all brands use O when some models from specific manufacturers energize in the opposite state; avoid this by checking the outdoor unit documentation or contacting the manufacturer if documentation is unclear. People often skip the final system test after changing the setting; save time later by running a brief cooling and heating cycle and confirming the outdoor unit changes mode as expected.

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