Confirm the correct reversing valve terminal by checking the heat pump’s wiring diagram or manufacturer label before you make any connections.

Start by locating the outdoor unit nameplate or the wiring diagram inside the control compartment and read the terminal designation for the reversing valve. If the label shows a single letter, it will usually be B or O and that is the terminal you should use for the thermostat wire. For Ruud and Rheem units the industry pattern is to use B energized in heat so connect to B when replacing a thermostat unless the label states otherwise. For most other manufacturers the common practice is O energized in cool so connect to O, but always verify because there are exceptions and some models allow either configuration via dip switches or a built in board setting. When the manufacturer uses a changeover valve that is energized on heat versus cool the thermostat configuration must match, so set your thermostat to control the reversing valve accordingly after wiring. If the outdoor unit has a separate wiring diagram that lists valve operation like energize in heat or energize in cool write that down and confirm thermostat behavior with a multimeter or by observing compressor state during a controlled call for heating or cooling.

Common mistakes include assuming all brands follow the same convention which can cause the system to run in reverse; prevent this by always reading the unit label first. Another frequent error is swapping wires while the system is powered which risks compressor short cycling; always turn power off at the outdoor disconnect and verify power is removed before touching wiring. A third issue is failing to set the thermostat’s reversing valve option after wiring so even with correct wiring the valve will operate opposite the desired mode; correct this by programming the thermostat to match the unit’s energize in heat or energize in cool behavior and test a heating and cooling call to confirm.

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