Wire letter guide
O/B Thermostat Wire: Heat Pump Reversing Valve (O vs B Setting)
A heat pump heats or cools by flipping a reversing valve. Most brands (Carrier, Trane, Goodman, Lennox, etc.) energize the valve in cooling, labeled O. Rheem, Ruud and a few others energize it in heating, labeled B.
Smart thermostats have one O/B terminal and a setting to pick O or B. If heat comes on when you ask for cooling (or the reverse), the setting is wrong. Nothing is damaged; just switch it in the app.
How to find the right O or B setting
- Check the outdoor unit’s brand and installation manual: it states whether the reversing valve is energized in cooling (O) or heating (B).
- If you can’t find it, try it: set cooling a few degrees below room temperature and wait five minutes. Warm air at the vents means the setting is reversed.
- Change the O/B setting in the thermostat app, not the wiring.
On thermostats without an app the setting is in the installer menu; on a Honeywell T6 Pro it is installer setting 218, covered in our Honeywell T6 Pro heat pump wiring guide.
A heat pump in heating mode can take a few minutes to blow warm air, and during defrost it may briefly blow cool air. Give it time before deciding the setting is wrong.
Old thermostat with both an O and a B terminal
Some older heat pump thermostats have separate O and B terminals. If both have wires, the wire on O is the reversing valve and the wire on B is almost always the common side of the transformer. On a new thermostat, O goes on O/B (set to O) and the B wire goes on C. Putting that B wire on O/B would short the transformer. See the B wire page for how to confirm.
Why the valve setting doesn’t damage anything
Choosing the wrong O or B setting just makes the heat pump run in the opposite mode to the one you asked for. Nothing is shorted, so you can safely test and switch it. What does cause damage is a wire on the wrong terminal, especially a common wire on O/B.