Your compressor has unloader valve issues? Since the unloader valve provides a critical function in the restarting of an air compressor you need to know more about your air compressor unloader valve or answers to any unloader valve issues your compressor has, and you’ve come to the right page to do just that.
Even if you are not sure what an unloader valve is you, like most of us with air compressors, have probably heard an unloader valve at work at one time or another.
Typical Unloader Valve Sound
Somewhere in the garage, the workshop or the plant, the “kathumping” of a reciprocating air compressor echoes throughout. Suddenly the thumping stops, and sometimes there is an audible… “pssssssschhhht”, the sound of air escaping, but just for a second or so.That is the unloader valve at work!
What the Unloader Valve Does
On a reciprocating compressor, compressed air can be trapped over the piston when the compressor reaches the cut out pressure and stops. If that air cannot escape, significant additional load is created for the start up of the compressor motor when the pressure switch turns the compressor back on and that pressure build up may be enough to prevent the compressor from starting.
For example, the last time you did a sit-up (or if you are like me, can you remember when you last did a sit up?), you may have folded your arms over your chest, or if you are particularly masochistic even held a weight to your chest. What you were doing by crossing your arms or holding a weight is increasing the load against which your muscles have to work to accomplish the sit up.
In the same fashion, compressed air, captured in the cylinder over the piston after the compressor shuts off, would increase the load against which the electric motor would have to work to start. If the load on the motor increases too much the compressor motor may fail to start completely, or it may pull too many amps, working hard against that additional and unneeded load, and fry a fuse or pop a breaker.
When the compressor shuts off the typical unloader valve opens and unloads the air that may be trapped over piston to atmosphere, and that motor overload problem is resolved.
Unloader Valve Location
The unloader valve is typically plumbed somewhere in the line between the compressor and the compressor tank, and quite frequently on the side of the compressor pressure switch, or as an integral part of the internal mechanism of the switch.
When the air in the compressor air tank reaches the cut out set point, and the pressure switch shuts the power supply to the motor off, at the same time, the pressure switch toggles the unloader valve to open dumping any trapped air.
In the photo the unloader valve is attached to the side of the compressor pressure switch. Not all are similar to this unloader, however.
Unloader Valve Differences
Unloader valves do not always work, or look, the same way.
The unloader valve is often part of the pressure switch assembly. It will, in many cases, be opened by a toggle extending from the side of the pressure switch as the switch operates, or from an internal mechanism inside the switch that opens and closes the unloader valve that is built inside the pressure switch.
Larger, more industrial air compressors, may have an entirely different unloader valve setup, yet, if the compressor is reciprocating, and has pistons, you will find one or more unloaders somewhere in the plumbing on that compressor.
Very small, fractional HP air compressors, may have no visible unloader valve. Some of these have a small hole in the line from the pump to the tank that is often under the cover. That hole is bleeding air all of the time the compressor is running, and then, when the compressor stops, that hole, the unloader for that compressor, continues to bleed air until all the air over the piston is gone. Then the bleeding of air should stop as long as the tank check valve is working!
When Unloader Air Hissing Continues
When the unloader valve operates, the relatively small amount of air trapped over the piston is voided. Usually a second or two of air escaping is as long as is needed for all the air trapped over the piston to escape.
Sometimes though, the air evacuating from the unloader valve does not stop The unloader valve continues leaking air after the compressor is stopped.
If there was no way to prevent it, when the unloader valve opened, all of the air already compressed into the tank would escape out the unloader valve which is open to atmosphere all the time the compressor is off.
To prevent that from happening there is a tank check valve installed, often in the fitting where the line from the compressor pump head enters the compressor tank. That check valve (also known as a one-way valve) keeps the compressed air in the tank when the unloader valve opens.
If air is bleeding from the unloader valve continuously, it is a good bet that the tank check valve has either failed or has not seated properly. The compressed air in the tank is bleeding back up out of the tank to the unloader valve, and out to atmosphere.
When the tank check valve isn’t sealing tightly or at all, compressed air will continue to bleed out until the tank pressure reaches the cut in pressure setting on the pressure switch, and then the air compressor will start to pump up the pressure in the tank again.
This cycle will continue until the check valve has been repaired or replaced.
If you think you’ve still got a problem with your compressor unloader valve, why not scan the questions and answers below?
Pretty much every compressor unloader valve issue has been asked about and answered in the existing comments.
In the event that your problem is not noted there, ask your unloader valve question at the bottom of this page.
And, if you can offer advice on any of the questions, it sure would be appreciated. Just add a comment. Then we all benefit from your compressor experience.