A reversing valve is the part of a heat pump that switches the system between heating and cooling mode. When it fails, your heat pump may blow cold air in winter, refuse to cool in summer, or make a strange hissing sound. Testing the valve yourself is possible with a multimeter and a few basic tools. The most reliable test is checking the solenoid coil for electrical continuity, followed by verifying that the valve spool moves freely when power is applied. If the coil tests fine, the problem is usually mechanical, meaning the valve body needs replacement.
What Does a Reversing Valve Actually Do?
The reversing valve is a four-way valve located in the outdoor unit of your heat pump. It changes the direction of refrigerant flow through the system. In heating mode, the valve directs hot refrigerant gas to the indoor coil. In cooling mode, it sends that hot gas to the outdoor coil instead.
Inside the valve is a sliding spool that moves back and forth. A small solenoid coil on the outside controls this movement. When the thermostat calls for heating or cooling, the solenoid receives 24 volts of power and shifts the spool. If either the coil or the spool fails, the valve stops working.
What Are the Signs of a Bad Reversing Valve?
Before you test anything, know what a failing valve looks like. Common symptoms include:
- The heat pump blows cold air during heating mode
- The system never switches between heating and cooling
- A loud hissing or whooshing sound comes from the outdoor unit
- The compressor runs but the indoor temperature never changes
These symptoms can also come from other problems, like a stuck contactor or a faulty thermostat. Testing the reversing valve rules it in or out.
How To Test A Reversing Valve On A Heat Pump: Step-by-Step
Start by turning off power to the unit at the breaker or disconnect switch. Heat pumps run on high voltage, and the reversing valve sits right next to live electrical components. Never work on the unit with power on.
Remove the service panel from the outdoor unit. Locate the reversing valve. It is the large brass or steel valve with several refrigerant lines connected to it. The solenoid coil is the cylindrical component attached to the top or side of the valve body, with two wires running to it.
You need a multimeter set to measure resistance (ohms). Disconnect the two wires from the solenoid coil terminals. Place one multimeter lead on each terminal.
A healthy coil typically reads between 20 and 60 ohms. Check your heat pump’s service manual for the exact specification. If the meter shows infinite resistance (OL on most meters), the coil is open and needs replacement. If it shows near zero ohms, the coil is shorted and also needs replacement.
Next, test that the coil receives power. This requires the system to be running. Reconnect the wires, turn the power back on, and set the thermostat to call for the opposite mode. For example, if the unit is in heating, set it to cooling. Place the multimeter leads on the solenoid terminals while the system runs. You should see approximately 24 volts AC. If you see no voltage, the problem is upstream, in the thermostat or control board, not the valve itself.
How to Test the Valve Spool Mechanically
An electrical test only tells you about the coil. The spool inside the valve can stick even when the coil works perfectly. Testing the spool requires care because the system must be running under refrigerant pressure.
With the system running, place a strong magnet against the solenoid and move it back and forth. If the spool is free, you may hear a slight change in refrigerant flow. Some technicians tap the valve body gently with a screwdriver handle while the system runs. This can free a lightly stuck spool, but it is not a reliable fix.
A more direct test involves comparing the temperatures of the refrigerant lines. In cooling mode, the large suction line should be cold and the smaller liquid line should be warm. If both lines are the same temperature, the valve is not shifting properly. This temperature test is often more useful than the magnet method.
What If the Solenoid Coil Tests Fine?
If the coil has continuity and receives 24 volts, but the valve does not shift, the spool is mechanically stuck or the valve body has internal damage. These valves are not serviceable. You cannot take them apart and clean them. The entire valve must be replaced.
Replacing a reversing valve is not a DIY job for most homeowners. It requires recovering the refrigerant, brazing new connections, and evacuating the system. This demands specialized tools and certification to handle refrigerant legally. Most jurisdictions require a licensed HVAC technician for this work.
Can a Stuck Reversing Valve Be Fixed Without Replacement?
Sometimes a spool sticks due to debris or sludge in the refrigerant system. Tapping the valve while the system runs can occasionally free it. Some technicians report success with this method, but it is temporary at best. The valve will likely stick again.
There is no chemical additive proven to clean a reversing valve. Some products claim to flush the system, but no clinical evidence or industry standard supports their effectiveness for this specific failure. If the valve is stuck, plan for replacement.
Is a Reversing Valve Failure Common?
Reversing valves are reliable components. Most failures happen after many years of service or after another part fails first. A compressor that runs hot can send degraded refrigerant oil through the system, which can clog the valve spool. A dirty air filter or low refrigerant charge can cause conditions that stress the valve.
If your valve fails, it is worth asking the technician to check the rest of the system. Replacing a valve without addressing the underlying cause means the new valve may fail the same way.
When to Call a Professional
Testing the solenoid coil is safe for a homeowner with a multimeter and basic electrical knowledge. The temperature test is also within reach. Beyond that, the risks increase quickly.
Refrigerant is under high pressure and can cause severe frostbite or injury if released. Electrical components carry lethal voltage. If you are not confident in your ability to work safely, call a licensed HVAC technician. The cost of a service call is far less than the cost of an injury or a damaged system.
Frequently Asked Questions
What is the most common cause of a bad reversing valve?
A stuck spool caused by debris or sludge in the refrigerant system is the most common mechanical failure. The solenoid coil can also burn out electrically, which is less common but easier to test.
Can I test a reversing valve without a multimeter?
Yes, you can compare the temperatures of the refrigerant lines while the system runs. If both lines feel the same temperature, the valve is likely not shifting. This test is less precise than an electrical test.
How much does it cost to replace a reversing valve?
Replacement costs vary widely by region and system size, but expect a significant expense because the job requires refrigerant recovery and recharging. Get a written quote from a licensed technician before authorizing the work.
Will a heat pump run with a bad reversing valve?
The system will run, but it will not heat or cool properly. Running the system in this condition can strain the compressor and lead to more expensive damage over time.
