When a heat pump blows cold air in heating mode — or won’t switch to cooling even when that’s what the thermostat is calling for — the reversing valve is the component to suspect. It’s the mechanical heart of what makes a heat pump different from a straight air conditioner: a 4-way valve that routes refrigerant in one direction for heating, another for cooling.
The reversing valve is switched by the O/B wire — see how it fits the rest of your terminals in the thermostat wiring guide.
Reversing valve diagnosis is mostly DIY-accessible with a multimeter. Replacement is not — it requires refrigerant recovery, brazing, and recharge, all requiring EPA 608 certification and professional equipment.
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What the reversing valve does
A heat pump is fundamentally an air conditioner that can run its refrigerant cycle in reverse. In cooling mode, it absorbs heat from indoors and rejects it outdoors. In heating mode, it absorbs heat from outdoor air and moves it indoors. The refrigerant cycle doesn’t change — the direction it flows does.
The reversing valve makes this possible. It’s a 4-way valve with one port connected to the compressor discharge (high-pressure hot gas), one to the compressor suction (low-pressure), one to the indoor coil, and one to the outdoor coil. By switching the valve position, the high-pressure discharge gas is routed to either the indoor coil (heating) or the outdoor coil (cooling).
The valve is controlled by a solenoid — a coil of wire that, when energized, creates a magnetic field that moves a small pilot valve, which then uses refrigerant pressure differential to slide the main valve piston.
The O/B terminal on your thermostat controls this solenoid. Most heat pump thermostats have an O or B terminal for this purpose:
- O terminal (most brands): solenoid energized in cooling mode — Carrier, Trane, Lennox, Goodman, Rheem, most others.
- B terminal (some brands): solenoid energized in heating mode — older Rheem, Ruud, Amana, Aire-Flo; some others.
This is why reversing valve diagnosis starts at the thermostat and works outward.
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Symptoms of a stuck or failed reversing valve
Heat pump blows cool/cold air in heating mode. The most common presentation. The heat pump runs, the indoor unit blows air, but the air is cold or only slightly warm — the system is stuck in cooling mode even with the thermostat calling for heat.
Heat pump blows warm air in cooling mode. Less common but same mechanism, opposite direction — stuck in heating mode.
Normal in one mode, failed in the other. Some valve failures are partial: the system cools well but won’t heat effectively (or vice versa). This can indicate a partial bypass leak in the valve body rather than a full failure to shift.
Hissing sound from the outdoor unit. A leaking reversing valve can produce a hissing sound as refrigerant bypasses the slide internally. Note that a defrost cycle also makes hissing sounds when it activates — this is normal.
System switches correctly in some conditions, fails in others. Temperature-dependent failures are common: the valve moves correctly when the system is warm but sticks when refrigerant pressures are extreme (very hot day in cooling, very cold day in heating). This is an early-failure sign.
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How to diagnose a reversing valve problem
Start at the thermostat and work toward the refrigerant system. The closer you can get to confirming the valve as the fault, the more useful the diagnosis is when you call a tech.
Step 1: Confirm the thermostat O/B setting
A mis-configured O/B terminal is misdiagnosed as a reversing valve failure more often than you’d expect. Before touching anything, confirm:
- In your thermostat settings, find the “O/B configuration” or “reversing valve” setting.
- Confirm whether it’s set to O (energized in cooling) or B (energized in heating).
- Match this to your heat pump brand’s standard. If it doesn’t match, change it and test.
On a Nest, ecobee, or other smart thermostat: look under Equipment → Heat Pump → Reversing Valve → O or B. An incorrectly set thermostat can cause exactly the “heat pump blows cold in heat mode” symptom — and costs nothing to fix.
Step 2: Check for 24V at the O/B terminal during a heat call
With the thermostat calling for heat:
- At the air handler or furnace, check voltage on the O/B wire (the wire connected to the O or B terminal on the control board).
- If your thermostat uses O-terminal convention (energize in cooling): there should be no voltage on O during a heat call.
- If your thermostat uses B-terminal convention (energize in heating): there should be 24V AC on B during a heat call.
Correct voltage behavior at the O/B wire means the thermostat and wiring are working correctly. Incorrect voltage means the thermostat, wiring, or terminal setting is the problem.
Step 3: Check voltage at the reversing valve solenoid
At the outdoor unit, locate the reversing valve — a copper tube assembly with a small cylindrical solenoid attached via two wires. The solenoid is usually on top of or adjacent to the valve body.
With a multimeter set to 24V AC, probe the two solenoid wires during a heating call (or cooling call, depending on your convention). Compare to expected behavior:
- O-terminal system, calling for heat: solenoid should be de-energized (0V). If you measure 24V here during a heat call, the wiring has an issue.
- O-terminal system, calling for cooling: solenoid should be energized (24V). If you measure 0V here during a cooling call, trace the wiring back to the board.
If voltage at the solenoid is correct and the valve still doesn’t shift positions: the valve itself is stuck. This is the mechanical failure that requires a tech.
Step 4: Test the solenoid coil
With the solenoid wires disconnected (power off):
- Measure resistance across the two solenoid terminals.
- A working solenoid typically reads 10–60 ohms (varies by manufacturer).
- An open reading (∞ / OL) = the coil is burned out and isn’t generating a magnetic field.
A burned solenoid with a mechanically free valve: some technicians will replace only the solenoid coil (slide it off the valve stem and replace). This is cheaper than full valve replacement. But this option depends on the valve body being mechanically free — if the slide is stuck, coil replacement alone won’t fix it.
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Stuck valve vs. thermostat/wiring problem: the quick distinguisher
| Condition | Heat pump in heat mode | Heat pump in cool mode |
|---|---|---|
| Thermostat/wiring issue | Voltage is wrong at O/B terminal | Voltage is wrong at O/B terminal |
| Solenoid failure | 24V absent at solenoid (correct call), no shift | Valve won’t shift to cooling |
| Stuck valve (mechanical) | Solenoid energizes correctly, valve doesn’t move | Same — valve stays in wrong position |
| Partial bypass | Heating capacity reduced but not absent | Cooling capacity reduced but not absent |
A technician with manifold gauges can confirm a partial bypass by measuring suction and discharge pressures in both modes — the pressure differential across the valve shows whether refrigerant is bypassing the slide.
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Reversing valve replacement: what’s involved and what it costs
Reversing valve replacement is a refrigerant-system repair and cannot legally be done without EPA 608 certification (the refrigerant must be recovered before the valve is removed). The repair sequence:
- Recover refrigerant from the system.
- De-braze the old valve from the refrigerant lines using a torch.
- Braze in the new valve with nitrogen flowing through the lines to prevent oxide formation inside the tubing.
- Pressure test, pull vacuum (evacuation) to remove moisture and non-condensables.
- Recharge refrigerant to manufacturer spec (weighed in).
Cost:
| Component | Typical range |
|---|---|
| Reversing valve (part) | $80–250 depending on tonnage/brand |
| Labor (recovery, replacement, evacuation) | $200–450 |
| Refrigerant (recovery + recharge) | $100–200 |
| Total installed | $400–900 |
These are regional averages — costs vary significantly by market. On an older heat pump (10+ years), get a quote for the valve repair vs. full system replacement before committing. A $700 valve repair on a 14-year-old system with declining efficiency may not be the best value.
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When to call a pro (immediately)
This component has no DIY repair path once the valve or solenoid is confirmed faulty. Call a licensed HVAC technician if:
- Solenoid tests show correct voltage but the system doesn’t shift modes.
- Solenoid coil tests open (burned out).
- A tech’s gauge readings confirm partial bypass.
- The system is getting worse — partial failure often becomes full failure.
The diagnosis steps above (thermostat setting, O/B voltage, solenoid voltage and resistance) are DIY-accessible and can save a service call or at least give a more precise diagnosis when you do call.
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FAQ
My heat pump heats fine in mild weather but fails when it gets really cold. Is that the reversing valve? Possibly — some valve failures are pressure-dependent and show up at extreme operating conditions. But it’s also possible you’re simply past the heat pump’s balance point and the system is relying on auxiliary heat you don’t have or that isn’t working. First check: does your auxiliary heat work? Does the system switch to emergency heat (if available)?
Can I switch to emergency heat to stay warm while waiting for repair? Yes. Emergency heat mode bypasses the heat pump entirely and runs only the auxiliary electric resistance heat or gas backup. It’s less efficient and more expensive, but it will heat the house while the reversing valve is diagnosed and replaced. Switch to emergency heat from your thermostat settings.
My technician says the reversing valve is “intermittently stuck” and recommends replacing the whole system. Is that right? Not necessarily. An intermittent reversing valve failure on an otherwise healthy system in its first 10 years is worth repairing. On a 15+ year old system with other aging components, it may be the right call. Get a second opinion from another HVAC contractor before committing to a full replacement.
What’s the difference between O and B on a heat pump thermostat? Both control the reversing valve solenoid, but the convention is inverted. O = the solenoid energizes in cooling mode (the vast majority of brands). B = the solenoid energizes in heating mode (some older Rheem, Ruud, Amana). This setting must match your heat pump’s design — getting it wrong is the most common heat pump thermostat installation mistake. See Heat Pump Thermostat Wiring Guide for the full wiring reference.
For the full cost picture on repair versus system replacement, see the HVAC repair and replacement cost index.
