Heat Pump Heats in Cool Mode (or Cools in Heat Mode): The O/B Setting and Other Causes
This call usually comes right after a thermostat was replaced. The system ran fine, someone installed a new thermostat, and now it blows warm air in cooling. Nothing is broken. The thermostat is energizing the reversing valve in the wrong mode.
Safety: Turn off power before moving thermostat wires. A shorted R wire blows the low-voltage fuse or the transformer.
O versus B
The reversing valve has one relaxed position and one energized position. Which mode is which depends on the manufacturer.
| Thermostat setting | Valve is energized in | Typical of |
|---|---|---|
| O | Cooling | Most brands |
| B | Heating | Rheem and Ruud, and some others |
Always go by the wiring diagram on the outdoor unit, not by brand memory. The diagram shows which terminal feeds the reversing valve solenoid.
Backwards in both modes
If heat gives cooling and cool gives heating:
- Find the reversing valve setting in the thermostat's installer setup. It may be called O/B, changeover valve or reversing valve.
- Compare it to the outdoor unit's wiring diagram.
- Change it and test both modes. Give the system a few minutes in each; most thermostats hold the compressor off for a short delay after a mode change.
On older mechanical thermostats there is no setting. The valve wire goes on the O terminal or the B terminal, and moving it to the other one is the fix.
One wiring trap: on some systems B is used as the 24 V common, not as a reversing valve terminal. If a blue wire was on B at the old thermostat, check where it lands at the air handler before putting it on O/B at the new one.
Stuck in one mode
If the unit only heats, or only cools, whatever the thermostat is set to, the valve isn't shifting. Work from the thermostat out:
- Is the thermostat calling? Check for 24 V between O/B and C at the thermostat sub-base in the mode that should energize the valve.
- Is the signal reaching the outdoor unit? Check at the outdoor low-voltage connections. A broken conductor in the thermostat cable is common.
- Is the coil getting 24 V? Check at the solenoid. On many units the valve is switched through the defrost board, so voltage at the board's input but not at the coil points at the board.
- Is the coil good? With power off, measure its resistance. An open coil doesn't pull the pilot valve.
- Does the valve shift? With 24 V on a good coil and the compressor running, the valve should move. If it doesn't, see reversing valve troubleshooting.
It's not always the valve
- Cool air in heat mode for a few minutes, then warm again is defrost. See the heat pump defrost cycle.
- Lukewarm air in both modes with a hot compressor can be a valve stuck partway, or low charge. See heat pump blowing cold air in heat mode.
- Heat works only on auxiliary means the outdoor unit isn't running at all. See auxiliary and emergency heat.
Common mistakes
- Changing the outdoor wiring to fix a thermostat setting.
- Putting the common wire on O/B because it was blue or was on B.
- Replacing the reversing valve before checking for 24 V at the coil.
- Testing too fast. The thermostat's compressor delay makes it look like nothing changed.
FAQ
Should my heat pump be set to O or B?
Whichever the outdoor unit's wiring diagram shows. Most are O (energized in cooling). Rheem and Ruud are the best-known B units (energized in heating).
Can the wrong O/B setting damage the system?
It runs the system in the wrong mode, which is uncomfortable and wastes energy, and auxiliary heat may fight the cooling. Correct it right away, but it isn't known for causing damage by itself.
Why did it work before the new thermostat?
The old thermostat was set or wired for the right valve type. The new one came with a default that doesn't match the equipment.
Sources
- Carrier 25HNB6 / 25HNB9 Installation Instructions (wiring diagram and reversing valve operation)
- See the thermostat manufacturer's installer setup guide for the name and location of the reversing valve setting.
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Field reference for trained technicians. Always follow the equipment manufacturer's instructions, local code and safe work practices. See our editorial standards, or report a correction.