The reversing valve controls the direction the refrigerant flows in a heat pump. A reversing valve is controlled by means of a pilot solenoid. The pilot solenoid creates a pressure difference in the reversing valve through pilot tubes. This pressure differential across the reversing valve body allows an internal slide to redirect refrigerant flow. The reversing valve requires a pressure differential of 30 to 70 pounds to shift. This will differ depending on the valve.
Do not condemn a reversing valve if the compressor is not operational or a pressure differential between the suction and head pressures is not enough. If there is the required pressure differential and the valve is not shifting, confirm there is control voltage going to the solenoid. Low control voltage can cause a reversing valve not to shift. This is a very common issue on commercial units where there is 208v and the transformer is not configured properly. If control voltage is present, is the solenoid creating a magnetic field? One of the easiest ways to test this is by removing the solenoid from the reversing valve body, apply control voltage, and inserting a screwdriver. If the screwdriver is held in place, the solenoid is good. If the screwdriver falls, the solenoid needs to be replaced.
If the valve is not shifting, switch back and forth between cooling and heating. This will sometimes free the valve. Creating a greater pressure differential greater than the 30 to 70lbs. required may force the internal side to shift. Last resort is lightly tapping on the body of the reversing valve. Take care not to hit the pilot tubes. These tubes can easily be damaged or broken.
It can be very difficult to tell the difference between a compressor that has bad valves or warped scrolls and a reversing valve that is mixing refrigerant due to not shifting completely. Low head pressure and high suction pressure readings are common for both.
One way to test a reversing valve is to record the temperatures of the lines approximately 6 inches away from the body and see if there is a heat gain or loss. In the cooling mode take temperature readings at the discharge line (A) and the line going to the outdoor coil (B). Take another set of temperature readings at the suction line (C) and the line going to the indoor coil (D).
The temperature difference between readings at the discharge line (A) and the line going to the outdoor coil (B) should be 4 degrees or lower. The temperature difference between readings at the suction line (C) and the line going to the indoor coil (D) should be 4 degrees or lower. The temperature difference is due to the heat transfer as refrigerant passes through the reversing valve. Under extremely high outdoor ambient temperatures, differences can be as high as 7 degrees and there is nothing wrong with the reversing valve.