Quick answer
A heat pump stuck in cooling mode has a reversing valve that is not shifting. The four usual causes are a thermostat with the O/B setting backwards, a broken O/B wire, a failed solenoid coil or board output, or a valve jammed mid-position. If it still blows cold after 15 minutes in Heat, switch to emergency heat and book a repair; only the first cause is a settings fix.
When a heat pump is stuck in cooling mode, the thermostat says Heat, the outdoor unit runs, and the air at the registers comes out cool. One part decides which way a heat pump moves heat: the reversing valve, a four-way valve in the outdoor unit that swaps the hot and cold coils. If it does not shift, or the signal telling it to shift never arrives, the system keeps air-conditioning the house in January.
How to tell a stuck reversing valve from a normal defrost
Wait 15 minutes before you assume a fault. Every heat pump switches into cooling on purpose during defrost: the outdoor fan stops, steam rises off the coil, and the air indoors goes cool for 5 to 15 minutes. If the cool air continues past that with the thermostat set to Heat, the unit is stuck.
The second check is at the outdoor unit. In heating mode the larger of the two copper lines, the insulated one, is warm to hot. If that line is cold and sweating while the thermostat calls for heat, the system is running in cooling. Air that is only lukewarm is a different fault, covered in why a heat pump blows cold air in heating mode.
Check this yourself first
- Thermostat mode: set it to Heat, not Auto or Cool, and raise the setpoint 2 °C above room temperature.
- Recent thermostat change: if the problem started the day a new thermostat went in, the O/B setting is the first suspect. Look for “O/B”, “changeover valve” or “reversing valve” in the installer menu.
- Power reset: turn the thermostat to Off, switch the outdoor breaker off for five minutes, then restore both. A valve that was sticking sometimes shifts on a clean restart, which tells the technician the valve is on its way out.
- Backup heat: if the house is cooling down, switch the thermostat to Emergency Heat, or let the furnace run on a hybrid system, so the heat pump stops working against you.
The O/B thermostat setting, and why the brand matters
The thermostat shifts the valve through one wire, landed on the terminal marked O/B. Most brands, including Carrier, Trane, Lennox, Goodman and York, energise the valve in cooling (O). Rheem and Ruud energise it in heating (B). Set a thermostat to the wrong one and the system does the opposite of what you ask: cooling on a call for heat, heating on a call for cool.
That backwards behaviour in both modes is the signature of a configuration error, and it costs nothing in parts to fix. It shows up after a do-it-yourself smart thermostat swap where the app guessed the wrong valve type. If the unit cools no matter which mode you pick, the cause is past the thermostat.
What fails when the thermostat setting is right
- Broken or loose O/B wire: the thin thermostat cable runs outdoors to the unit, where sun, a weed trimmer or rodents damage it. On a B-type system, energised in heat, a broken wire leaves the valve resting in cooling.
- Failed solenoid coil: the 24-volt coil that clips onto the valve burns open and the valve drops to its resting position. On a system that energises in heat, the resting position is cooling.
- Failed board output: on many units the O/B signal passes through the defrost or control board. A relay stuck in the defrost position, or a sensor telling the board the coil is always frozen, holds the valve in cooling.
- Valve stuck mechanically: the slide inside jams with debris or wear, sometimes halfway. A valve stuck mid-position leaks hot gas straight back to the suction side, so the system heats and cools poorly in both modes and can be mistaken for a failing compressor.
How a technician tests a reversing valve
Two measurements settle it. First, a meter across the solenoid coil: with the thermostat calling for the mode that energises the valve, there should be 24 volts at the coil. No voltage sends the technician back along the wire to the board and the thermostat. Voltage present with no shift means the coil or the valve itself.
Second, temperature across the valve body. The suction tube entering the valve and the suction tube leaving it for the compressor should read nearly the same. A gap of more than a few degrees means hot gas is leaking through inside, and that valve has to be replaced.
Replacement is refrigerant work. The charge is recovered, the old valve is cut out and a new one brazed in with its body kept cool, then the system is pressure-tested, evacuated and recharged by weight. In Ontario that requires a technician holding an ODP card, and it takes most of a day.
What it costs to fix a heat pump that only cools
- Diagnostic visit: $90–$150, credited to the repair when you go ahead.
- Thermostat O/B correction or a repaired wire: usually finished during the diagnostic visit.
- Solenoid coil or control board: inside the $150–$800 band most repairs fall in, with boards at $400–$800.
- Reversing valve replacement: priced by written quote, because recovery, brazing and refrigerant vary with the unit. On a heat pump past 12 years old, ask for the replacement price beside it.
Is it safe to leave a stuck heat pump until morning?
Yes, if you have backup heat. A stuck valve is not a safety hazard, but a heat pump cooling the house in winter fights your auxiliary heat and runs the hydro bill up, so take it out of the loop with Emergency Heat. Call the same day if there is no backup heat and the indoor temperature is falling, or if the breaker trips when the unit starts.
ZK Mechanical diagnoses reversing valves, O/B wiring and defrost boards on the brands we service, including Lennox, Carrier, Trane, Goodman, Rheem and Daikin. See heat pump repair for what a visit covers, browse the other heat pump questions, or call (647) 801-1252 and book a diagnostic.