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.
Helpful Tips-
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.
Comments
Glenn, we appreciate the honest feedback; it's what we need to make sure the information here is useful to everyone.
If anyone has a sugestion on how to further improve this, speak up. (that includes you Glenn!)
If the system pumps down it means the compressor is good (maybe not 100% but it's doing something) and the reversing valve isn't hung-up mid stroke; it's probably the metering device is stuck open or is bypassing.
As far as making the content printer friendly, I'll check with Kevin, he's Zen's site guru.
And we don't mind your questions at all, keep them coming.
Thank you,
Larry
The most common style has a 5/16" hex head screw holding the coil on the valve stem.
Some models have a cap or 'button' retainer that snaps onto the stem.
Some older ones had a spring wire clip.
After removing whatever is retaining the coil, it should slide off the valve stem.
Patrick
A slip coupling is a copper sweat coupling that does not have a dimple or shoulder formed in the middle. This enable the coupling to slide all the way over a pipe. In short, it makes it easier to position the new valve in place without bending the piping.
We appreciate the comment.
Patrick
Quiet shift shuts the compressor off as the reversing valve shifts into the defrost position; this lessens the loud WOOoooSH noise and the compressor growl noise.
30seconds after the valve shifts the compressor is restarted.
If the starting and stopping bothers you the feature can be disabled via a dip switch on the defrost board, or if it's an Infinity system in the advanced setup menu's.
Have a good thanksgiving!
Patrick
I have a split unit heat pump and when I call for cool in the house it pulls warmish and the outside unit is cooling, seems a reversing valve problem which made me look at your site. Now at the same time the outside fan does not come on so I am wondering:
If the valve or solenoid is broken would this also cause the fan not to work or
Is this maybe a thermostat problem or
Just coincidently seperate problems or?
I am really struggling with this and the only advice I get seems to be based on guesswork and replacing bits on a guess. I'd really appreciate some advice based upon analysis. Thanks
neither would a T-Stat problem.
It almost sounds like a failed outdoor fan motor/capacitor or failed
defrost board relay.
First, lets determine if the unit is actually in cooling mode. If the
thermostat uses a 'O' terminal there should be 24 volts on 'Y' and 'O'.
If it uses a 'B' terminal there should only be 24 volts on 'Y'.
Let me know and we can take it from there. Also, what brand unit is it?
Patrick.
Is it possible we have a "B" system and the rain was causing the solenoid to energize? I did not know about reversing valves this morning.
Patrick
Heat pumps have an outdoor metering device for heating operation- either a txv with a bypass check valve installed or a sliding piston meters in the heat mode and is bypassed in the cool mode.
I'm guessing that the bypass check valve (or sliding piston) is stuck open (the cooling position).
If it runs ok in the cooing mode, this is likely the culprit.
Let me know what you find.
Patrick.
Patrick
Thank
In a nutshell; without more information I can't say what's wrong.
I don't see them fail to often. Usually they stick due to plugged ports and capillaries from burn out related gunk and debris, or their coils fail.
I've also run across a few with broken slide rings, but I cant say why they broke.
If anyone out there has an answer let us know.
Some valves use a fourth capillary tube that connects to the true suction port. They use this to bleed off the high pressure gas inside the valve body so the piston can slide. Some valves bleed off the high pressure using a bleed hole in the slide valve piston, those types only have three capillary tubes, both styles still basically work the same.
Patrick
Because when the unit is in heat pump mode the outdoor coil is the evaporator, so shutting off the outdoor fan reduces the heat load, which in turn reduces the head pressure. I'm not sure if Payne had this feature or not. The other cause could be the defrost control or the fan motor itself. I would start by checking the voltage to the motor; if it's constant, its a motor or capacitor issue. If the power cycles it could be a defrost control issue.
Patrick
If the unit uses an "O" terminal, the valve should not be powered during a heat call unless it is in defrost. If the valve is energized it's likely either the defrost control is stuck in defrost mode or the thermostat energizing the reversing valve.
If the system uses a "B" terminal the valve should be energized in heat mode. If it's not, the thermostat or the defrost control is the culprit.
Being that it only happens occasionally, I'm guessing it's either pitted contacts on the defrost board ("B" terminal system) or a failing "B" output on the thermostat (again "B" terminal system) that is not allowing voltage to the valve.
Let me know what you find. If you write back, include the equipment brand.
Patrick
RSS feed for comments to this post