The O/B wire only exists on heat pump systems, and it does one job: energize the reversing valve that decides whether the system is heating or cooling. Get the O-vs-B convention backward during setup, and the heat pump runs exactly opposite of what the thermostat says: heat mode blows cold air, cool mode blows warm air.
For what the reversing valve physically does with that signal, see what is a reversing valve; for the rest of the terminal map, see the complete thermostat wiring guide.
O or B: it’s one wire, two conventions
There’s only one physical wire (commonly orange), but two different conventions for what it does, and manufacturers are not consistent:
| Convention | Energized to cool | Energized to heat | Common on |
|---|---|---|---|
| O (most common) | Yes: valve energizes in cooling | No: valve is de-energized in heating | Most major brands: Trane, Carrier, Lennox, Goodman |
| B (reverse convention) | No: valve is de-energized in cooling | Yes: valve energizes in heating | Rheem and Ruud commonly use this convention |
This isn’t a labeling quirk you can ignore. A thermostat set to the wrong convention for your actual equipment will run the reversing valve backward relative to what you asked for.
How to get it right
- Check the outdoor unit’s nameplate or install manual for the O/B convention that specific model uses. This is the authoritative source, not a general rule of thumb.
- Most smart thermostats ask directly during setup. Nest, ecobee, and Honeywell heat pump installation flows all include an O/B selection step; answer it from the equipment documentation, not a guess.
- If heating and cooling seem swapped after install, that’s the single most common heat-pump setup mistake, and it’s almost always this setting, not a hardware fault. Check the O/B convention in the thermostat’s installer menu before assuming the reversing valve itself has failed.
This circuit runs on the same 24VAC Class 2 power-limited wiring as the rest of the thermostat (NEC Article 725). The mistake here is almost never electrical, it’s a software/convention setting.
What happens if it’s set wrong
- Set to heat, the system blows cold air.
- Set to cool, the system blows warm air.
- The compressor and blower are both running normally; the reversing valve is just receiving the signal at the wrong time relative to the mode you asked for.
This is worth ruling out before assuming a failed reversing valve, since the fix (flipping one setting) takes under a minute, while a mechanical reversing valve replacement is a full refrigerant-system repair.
Frequently Asked Questions
Q: How do I know if my heat pump uses O or B?
A: Check the outdoor unit’s nameplate or the manufacturer’s installation manual. As a rough pattern, most major brands use O; Rheem and Ruud commonly use B. Don’t guess: the two conventions are opposite of each other.
Q: My system heats when I set it to cool. Is the reversing valve broken?
A: Check the thermostat’s O/B convention setting first. This exact symptom is the signature of a wrong O/B setting, not a failed valve, in the large majority of cases. A genuinely stuck reversing valve is a less common cause of the same symptom.
Q: Does a conventional (non-heat-pump) furnace have an O/B wire?
A: No. O/B only exists on heat pump systems because only heat pumps have a reversing valve to control. A gas furnace and separate AC condenser has no such terminal.
Q: Is O/B a line-voltage or low-voltage wire?
A: Low voltage: 24VAC, the same NEC Class 2 circuit as every other thermostat wire, not the higher voltage that runs the compressor itself.
Related: What is a reversing valve? · Complete thermostat wiring guide · What is the C-wire? · What is the R-wire? · Best thermostat for a heat pump
