On a reciprocating, rotary screw, and even fossil fuel air compressors, compressed air can be trapped over the piston when the compressor reaches the cutout pressure and stops.

If that air cannot escape, a 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.

This highlights the importance of an air compressor pressure switch unloader valve.

If you’re interested in the broader principles of how an air compressor works – not just the unloader valve, you should check out our guide explaining how each type of air compressor works.

Now on with how unloader valves work, break and how to adjust them!

Table of Contents

What is an Unloader Valve?

Gas air compressor unloader valves are one of the numerous small, common and essential parts of an air compressor and the whole pressure maintenance process. They are a critical function in ensuring that the air compressor is able to restart and operate sufficiently.

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 and the sound you would expect to hear!

Where is an Unloader Valve Located?

The unloader valve will most likely be located inside the pressure switch or attached to it on most air compressors. During the operation of the air compressor, the switch turns the compressor off which activates the unloader valve. The air compressor unloader check valve keeps the entire tank from draining and has a pipe running to the unloader valve. 

**insert photo of unloader valve attached to pressure switch**

Above is an unloader valve diagram where the valve is attached to the pressure switch. However, the unloader valve can also be located next to the compressor on larger-scale air compressors. Here, a pilot unloader valve is used to control the emerging air pressure into the unloader valve.

How Does an Unloader Valve Work?

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, a significant additional load is created for the start-up of the compressor motor when the pressure switch turns the compressor back on. That pressure build-up may be enough to prevent the compressor from starting.

Compressed air captured in the cylinder over the piston after the compressor shuts off, increases 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 the piston into the atmosphere, and that motor overload problem is resolved.

Unloader Valve Differences

An important thing to note is that unloader valves do not always look or work in the same way.

The unloader valve is very 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.

As previously mentioned, 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 air 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!

You need to ensure when selecting an unloader valve that you find the right one for your applications, this youtube video below can help point you in the right direction!

Types of Unloader Valves

Continuous Run Valve

Some compressors are not able to stop and start frequently enough to supply compressed air to high-volume demand plants. If the cycle time between high and low air pressure set points is too frequent, this increases compressor wear and can reduce the life expectancy of some of the compressor parts.

If your air compressor cycles on and off more than 15-20 times per hour, you have reached the frequency-benchmark for that type of compressor. It’s time to consider moving from a Stop & Start type of air compressor to a continuous run style.

Pilot Unloader Valve

Pilot unloader valves are necessary for the proper functioning of most of the continuous run types of compressors. When an air compressor runs continuously, the air is continuously compressed into the tank causing the pressure to rise exponentially. This pressure level could rise past the pressure-safety limit which could cause potentially catastrophic results. This is where the pilot unloader valve comes into play, to allow the release of air pressure from the compressor head when the pressure reaches a set point.

This component allows for the activation of engine control and reduction in oil consumption. It does so between the air compressor unloader check valve and pump valve by leveling the pressure. This leveling includes diverting and discharging air pressure when the air compressor tank top pressure setting is reached and when the maximum air pressure setting is reached within the air compressor respectively.

Installation and Adjustment of the Unloader Valve

Installation

A general guide to installing a pilot unloader valve onto your air compressor can be understood here:

  • Place the valve ball into the pilot valve hole in the air compressor body and hand tightens the pilot cap assembly into the pilot hole body. Tighten the differential locknut.
  • Turn unloader screw assembly into the pilot cap assembly suitably to set the cut-out pressure. Tighten the pressure screw lock nut.
  • Ensure you check the operation of the pilot valve and the resulting pressures, adjustments can be made if necessary.

It is important to understand that this is a general guide for installing a pilot unloader valve and that it will certainly vary depending on the manufacturer and type of valve you have. Refer to the instructions provided with your type of valve or contact the manufacturer.

Adjustment

The cut-in and cut-out pressure inside an air compressor can be controlled by making adjustments to the unloader valve. A few important steps to follow can be understood here:

  • Ensure that the air compressor is turned off.
  • Turn the differential screw clockwise to loosen up the differential locknut. When the pilot seat of the differential screw is in contact with the ball, secure the locknut.
  • Adjust the air compressor by setting the load pressure to a minimum and then turn it off.
  • Slowly screw the pressure screw back into its place until you feel the air coming out the top of the valve. This then sets the bottom pressure so you must secure the pressure screw locknut.
  • Investigate adjusting the compressor until you are able to take not of your minimum pressure setting. Turning the pressure screw clockwise will help increase pressure and turning the screw counterclockwise will help decrease the pressure.
  • Repeat the above steps for the maximum pressure value.
  • Ensure all locknuts are secured thereafter.

Again, it is important to understand that this is a rough guide to adjusting unloader valves and you should seek to follow instructions provided to you and/or contact the manufacturer.

Problems, Questions & Answers

Unloader Air Hissing Continuing

When the unloader valve operates, the relatively small amount of air trapped over the piston is voided. Usually, one or two seconds 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 from continuing 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 of the unloader valve which is open to the atmosphere the whole time the compressor is off.

To prevent that from happening a tank check valve is installed. Often in the fitting where the line from the compressor pump head enters the air compressor tank. This check valve (also known as a one-way valve) keeps the compressed air in the tank when the unloader valve opens up.

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 the 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.


The pressure switch has reached set pressure – unloader valve needs adjustment?

by Norman

I have built compressors for several years but I am having a problem with the electric motor restarting after the pressure switch has reached set pressure and shuts the motor off.

I have seen some compressors which have a pressure valve that keeps the intake valve open until the motor restarts, and some that allow the motor to run free until the pressure drops.

I would like to find a supplier for one of these valves. I would like your help in this matter, thanks.


Hello Norman…

Thanks for your question.

What I think you need is an unloader valve, which automatically discharges the air pressure over the piston after the compressor has reached cut-out pressure.

If you don’t unload that pressure over the cylinder, it makes it very hard for the electric motor to start. Since the electric motor already has an inrush requirement greater than many electrical supply circuits, without the ‘capacitor’ (I think that’s the name) to provide a boost when the motor starts, the circuit breaker or fuse would pop each time the motor went to start.

The unloader valve mechanism is commonly part of a compressor pressure switch assembly. You can get a combo pressure-switch / unloader valve from a compressor parts supplier.

Further, all electric motors have an inrush current when they start, and then once they are going, draw less amps. Some motors come with starters that are basically (if I understand my electrician friend correctly) that allow a quick boost of energy to the motor when it goes to start. This reduces the inrush current, and helps prevent blowing fuses or popping breakers when the motor starts.

By not “unloading” the piston type compressor, you are adding load to the start up current requirements, and that may be why you are overloading the motor on start up and blowing the fuse.

The compressors have an unloader valve to bleed off the compressed air over the piston when the compressor stops, so that there’s less load when it goes to start.

Your electric cutouts are shutting off the motor, but not unloading the compressed air over the piston. You should be using a pressure switch that’s built for compressors, and when you do, it comes complete with the unloader valve.

Or, you can use the cutout to operate a small 2/2 valve that will perform the same function, but putting that into the circuit will probably cost more in materials and frustration than buying the pressure switch and plumbing the compressor that way.

Adjusting the unloader valve on a recip comp. 75hp

by Robert DeLaPlaza
(Orlando Florida USA)

I have a two stage compressor with an unloader valve, after several years of work the unloader valve got full of gummy residue, I think from the little amount of oil leaking from the compressor in to the tank.

One day I noticed that the compressor was running for too long to reach the stop pressure.

I checked the compressor while it was running, and it seemed like it was running at idle.

Then noticed a small leak at the unloader valve, and when I put my finger over the unloader valve, just were it was leaking, the compressor sound changed, and started pumping really well again and reached the stop pressure in less than 3 minutes.

Took apart the unloader valve and clean it really well with solvent, then I put it back together, after checking that all the o-rings were sealing.

After installing the unloader valve all hell broke loose… it is totally out of adjustment, it has 3 points were it can be adjusted.

1.- A knob on top, a real adjustment intended by the manufacturer. What it adjusts?

The valve has 3 sections

2.- So the top and middle sections can be adjusted and set with a locknut.
to adjust what? and how to set it ?

3.- The middle and bottom sections, can also be adjusted and set with a locknut.
to adjust what? and how to set it ?

when I change the adjustment on the bottom parts it chatters. Fiddling with it I can make it to unload the compressor but then it doesn’t pump until I move the adjustment of the valve again… I feel really lost with this.

HOW TO ADJUST A UNLOADER VALVE 3 Sections and top knob?


Hi Robert:

Gosh, this is impossible to do without having the switch here and playing with it on the compressor.

You don’t indicate the brand of compressor, so I can’t even refer you to the factory, if you could find it.

Go to your local “big box store” that sells compressors. They probably won’t have a compressor repair depot, but what they will have is a local company that they use for warranty repair of the compressors they sell. Find out who that company is.

Take your pressure switch to them and either ask them to show you how to set it, or given that you’ve taken it completely apart, it might be a better idea to ask them for a replacement for it.

Do you remember the pressure settings of the compressor before you started having problems? You want to set the OEM replacement switch to those same levels when you reinstall it.