A reversing valve is the part that makes a heat pump a heat pump instead of just an air conditioner. It’s a single valve, controlled electrically, that swaps which coil is absorbing heat and which is releasing it: the same refrigerant loop, the same compressor, running in two directions depending on the season.
For the thermostat wire that controls it, see what is the O/B wire; for the terminal map around it, see the complete thermostat wiring guide.
How it actually works
Every vapor-compression system, air conditioner or heat pump, moves heat by compressing and expanding refrigerant between two coils: one that releases heat (the condenser) and one that absorbs it (the evaporator). A standard air conditioner only ever runs one direction, so its condenser is always outside and its evaporator always inside.
A heat pump adds the reversing valve between the compressor and the two coils. Energized one way, refrigerant flows so the outdoor coil condenses (releases heat outside) and the indoor coil evaporates (absorbs heat inside). That’s cooling mode, identical to a standard AC. Energized the other way, the flow reverses: the indoor coil condenses (releases heat inside) and the outdoor coil evaporates (absorbs heat from the outside air, even when it’s cold). That’s heating mode.
| Mode | Outdoor coil | Indoor coil |
|---|---|---|
| Cooling | Condenser (rejects heat outside) | Evaporator (absorbs heat inside) |
| Heating | Evaporator (absorbs heat outside) | Condenser (rejects heat inside) |
The valve itself is switched by the thermostat’s O/B signal. See what is the O/B wire for the two competing conventions manufacturers use for that terminal.
What goes wrong
Stuck in one position. The most common failure: the valve mechanically sticks in either heating or cooling mode regardless of what the thermostat requests. Symptom: the system blows cold air in heat mode, or warm air in cool mode, and the O/B setting checks out correctly at the thermostat, which rules out the wiring-convention mistake covered on the O/B page and points at the valve itself.
Refrigerant leak at the valve body. Reversing valves have multiple brazed joints, which are potential leak points over the system’s life. A leak here presents like general low refrigerant symptoms (reduced capacity, longer run times) rather than a mode-reversal failure specifically.
Solenoid coil failure. The electrical coil that shifts the valve can fail independently of the valve body itself: the valve doesn’t move because it isn’t getting the signal, not because it’s mechanically stuck.
Reversing valve vs. a wiring mistake: how to tell them apart
Before assuming a failed valve, rule out the far cheaper explanation: an incorrect O/B convention setting at the thermostat, covered in detail on the O/B wire page. The symptom is identical (system runs the wrong mode). The difference: a wiring/convention mistake is a one-minute thermostat setting change; a genuinely stuck valve or failed solenoid is a refrigerant-system repair requiring a licensed technician to recover refrigerant, replace the part, and recharge the system.
What replacement costs
Reversing valve replacement is refrigerant-system work: it requires recovering the system’s refrigerant charge, brazing the new valve into the line set, evacuating the system, and recharging to spec. Installed cost typically runs $400–$1,200, varying with refrigerant type, accessibility, and local labor rates. This is not a DIY repair. Anything requiring refrigerant recovery and recharge needs EPA Section 608 certification to handle legally.
Frequently Asked Questions
Q: How do I know if my reversing valve is bad?
A: The system runs the opposite mode from what’s set (cold air in heat mode, or the reverse), and the thermostat’s O/B convention setting is confirmed correct. If the O/B setting is the problem, that’s a much cheaper fix; always check it first.
Q: Can a reversing valve be repaired instead of replaced?
A: In most cases no. A stuck valve or failed solenoid is typically replaced as a unit rather than repaired internally, since the valve body is a sealed, brazed component.
Q: Does every heat pump have a reversing valve?
A: Yes. It’s the defining component that separates a heat pump from a standard air conditioner. A conventional AC-plus-furnace system has no reversing valve because it never needs to run its refrigerant loop backward.
Q: Can I test the reversing valve myself?
A: A technician typically confirms it by listening for the valve shifting (an audible click) when the thermostat calls for a mode change, combined with temperature readings at the coils. Diagnosing it correctly requires refrigerant-system gauges and experience distinguishing it from a low-charge or wiring issue, not a DIY test.
Related: What is the O/B wire? · Complete thermostat wiring guide · Best thermostat for a heat pump · Heat pump balance point temperature
