Heat Pump Not Switching From Heat to Cool

Quick Answer

When a heat pump won't switch from heating to cooling, the most common cause is a stuck or failed reversing valve -- the component that changes the direction of refrigerant flow. A thermostat set incorrectly, a faulty thermostat wire (the O/B wire), or a defective control board can also prevent the switch. Start by checking your thermostat settings, then move on to the reversing valve and wiring.

How a Heat Pump Switches Modes

A heat pump does not generate heat or cold the way a furnace or standalone air conditioner does. It moves heat from one place to another using refrigerant. In winter, it pulls heat from outdoor air and moves it inside. In summer, it reverses the process, pulling heat from your home and dumping it outside.

The key to this reversal is the reversing valve, a four-way valve in the outdoor unit that redirects refrigerant flow. When your thermostat calls for cooling instead of heating, it sends a signal -- typically through the O wire (for Rheem, Ruud, and most brands) or the B wire (for some Carrier and Trane systems) -- that energizes or de-energizes a solenoid on the reversing valve, sliding it into the opposite position.

If any link in that chain fails, the system stays stuck in whatever mode it was last in.

Check the Thermostat First

Before you start worrying about mechanical failures, make sure the thermostat is actually calling for the right mode. This sounds obvious, but it catches people more often than you might expect.

Set your thermostat to "Cool" mode explicitly. If it is set to "Auto," verify that the setpoint is below the current room temperature so the system has a reason to switch. Some thermostats have a changeover delay of 5 to 10 minutes to prevent short cycling -- wait at least 10 minutes before deciding it is not working.

If you have a programmable or smart thermostat, check that the heat pump settings are configured correctly. Many smart thermostats have an O/B wire setting that determines whether the reversing valve is energized in cooling mode (O) or heating mode (B). If this is set wrong, the thermostat will send the signal at the wrong time, and the system will appear stuck.

The Reversing Valve

If the thermostat checks out, the reversing valve is the next suspect. This valve lives in the outdoor unit and can fail in a few ways.

Stuck mechanically. The slide inside the valve can get stuck partway, especially if the system has not switched modes in months. You may hear the outdoor unit make a brief hissing or clunking sound when the thermostat calls for a mode change -- that is the valve attempting to shift. If it does not complete the shift, refrigerant continues flowing in the old direction.

Solenoid failure. The solenoid is a small electromagnetic coil mounted on top of the reversing valve. When it receives an electrical signal, it moves a pilot valve that shifts the main valve. If the solenoid burns out, the valve gets no signal and stays put. An HVAC technician can test the solenoid with a multimeter -- it should read somewhere between 5 and 25 ohms, depending on the model.

Internal leak. The reversing valve has internal seals. If these wear out, refrigerant leaks past the slide and the system partially heats and partially cools at the same time, delivering lukewarm air in both modes. This is harder to spot, but a clear sign is the system running constantly without reaching the setpoint in either mode.

Replacing a reversing valve is not a DIY job. It requires recovering the refrigerant, brazing copper lines, and recharging the system -- all of which require EPA Section 608 certification and specialized equipment. Expect $500 to $1,500 for parts and labor, depending on the unit.

Thermostat Wiring Problems

The signal from your thermostat to the reversing valve travels through a low-voltage wire, typically the O or B wire in the thermostat cable bundle. If this wire is broken, corroded, or not connected properly at either end, the reversing valve never gets the signal to switch.

If you recently installed a new thermostat, double-check that the O/B wire is connected to the correct terminal. Different thermostat brands label terminals differently, and a wire in the wrong spot can leave the reversing valve permanently energized or permanently de-energized.

At the outdoor unit, the wiring terminates at the control board or a relay. Corrosion, loose connections, or rodent damage to the wire at any point between the thermostat and the outdoor unit can break the circuit. If you are comfortable removing the access panel on the outdoor unit, visually inspect the wiring for obvious damage. Otherwise, this is a straightforward diagnostic for an HVAC technician.

The Control Board

The outdoor unit's control board interprets signals from the thermostat and directs the compressor, fan, and reversing valve. If the board is faulty, it may run the compressor and fan correctly but fail to activate the reversing valve relay.

Look for visible signs of damage on the board -- burn marks, swollen capacitors, or corroded solder joints. Control board replacement runs $150 to $600 depending on the brand and model.

A Quick Diagnostic Approach

Here is a logical sequence to narrow down the cause:

Set the thermostat to "Cool" with the setpoint well below room temperature. Wait 10 minutes.
Go outside and listen to the outdoor unit. If the compressor and fan are running, the thermostat is communicating. If nothing happens, the problem may be upstream of the reversing valve.
Check whether the air coming from vents is warm (still heating) or ambient temperature (compressor not running in cooling mode). Warm air with the system set to cool strongly suggests a reversing valve issue.
If you have a multimeter, check for 24 volts AC at the reversing valve solenoid when the thermostat is set to cool. Voltage present but no click means the solenoid or valve is faulty. No voltage means the signal is not getting there -- check wiring and control board.

When It Is Not Worth Repairing

If your heat pump is over 12 to 15 years old and the reversing valve has failed, consider the age of the entire system before sinking $1,000+ into a repair. Compressors and other components are likely nearing the end of their lifespan too. A new heat pump in the 2 to 3 ton range typically costs $4,000 to $8,000 installed, and newer models are significantly more efficient. If your system is in that age range, get a quote for replacement alongside the repair quote.

If the room temperature does not match what your thermostat reads, the switching issue might be masking a calibration problem worth addressing at the same time.