Heat Pump Reversing Valves, what are they, and how do they work?
The heat pump reversing valve; this simple device causes almost as much confusion as defrost controls. Based on appearance alone the confusion is understandable, if not outright scary...
The purpose of the reversing valve is to swap the refrigerant lines going to the compressor's suction line and discharge line; making the indoor coil the condenser, and the outdoor coil the evaporator.
The valve has four tubing connections; three outlets on the top, and one inlet on the bottom. The bottom is always connected to the compressors discharge port, and the top, center port is always connected to the compressors suction port. One of the remaining two top outlet ports connects to the suction tube going to the indoor coil, and the other connects to the hot gas discharge tube going to the outdoor coil.
Inside the body of the reversing valve is a sliding piston that has ports in it. Sliding the piston to one side or the other connects the hot gas and suction ports from the compressor to either the indoor or outdoor coils allowing the coils to swap functions.
How does the piston move? A small capillary tube runs from the compressor hot gas port (bottom port) to the solenoid valve attached to the reversing valve. Hot gas is directed to one side or the other of the piston, this sides the piston back and forth. The direction the piston moves depends on whether the solenoid is energized or not.
This is where the "O" and "B" terminals on a heat pump come into play. Equipment that uses an "O" thermostat energizes the solenoid for cooling, while equipment that uses "B" energizes the valve for heating operation. The only difference is the way the valve is piped in.
Now that we know what a reversing valve is and how it works, let's look at how it can fail. Keep in mind the only serviceable part on the reversing valve is the solenoid coil; all other failures require replacing the whole valve. Typical reversing valve failures are:
- · Failed solenoid coil
- · Refrigerant leak
- · Sticking in one position (heat or cool)
- · Sticking mid travel
- · Internal seals leaking by
#1- Determining if the solenoid coil is bad is easy; first the compressor must be running and there must be a pressure difference between the high and low side of the system (the valve shifts due to pressure differential) if the valve doesn't shift when power is applied or removed to the coil, ohm out the coil. If it's open, replace the coil, if the coil ohms out ok, replace the entire valve.
#2- A leaking valve is easy to, if the valve is leaking, replace it. Don't try to repair a leaking valve.
#3- The check out procedure for a stuck valve is the same as the check out for the solenoid coil.
#4- Diagnosing a valve that's stuck mid-travel is a little trickier; the systems high and low side pressures will be close to the same, the refrigerant lines at the reversing valve and at the compressor will be warm to hot, and the compressor usually makes a loud, growling sound; like a scroll running in the wrong direction.
Be careful; a valve that's stuck mid stroke can be mistaken for a failed compressor, or a metering device (bypassing psiton, or TXV) that's stuck open. If the metering device is stuck open the compressor's suction line will be cold, and its discharge line will be warm. An easy check is to close the liquid line service valve to see if the system will pump down.
#5- A reversing valve that's leaking by internally will still function, but the suction pressure will be slightly high, and the system will loose capacity. This is a relatively easy problem to diagnose: Start the system and let it run about 15 minutes (it doesnt matter if its heating or cooling mode) then measure the temperature of the two suction ports on the valve, there should be less than 5 degrees difference.
Never hit a reversing valve with a wrench, hammer, or anything else. If you do, and wasn't bad before, it probably is now.
If you need to replace a reversing valve, remove the old valve by cutting the lines a few inches away from the valve, in an area that you can acceess with a torch. (Use a tubing cutter, never use a hack saw). Mark how the tubes are inserted in the valve and un-sweat them. Wrap the new valve with a damp cloth and braze the tubes into the valves stubs. Use slip couplings to install the valve in the unit. This helps prevent overheating the valve.