Adjusting the compressor unloader valve

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 in 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 which allows 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.

Here’s more information on unloader valves. This page starts with a question about unloader valves, then continues as a complete explanation of what they are and what unloader valves do.

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 Rick
(So.San Francisco, CA)

How do you adjust the unloader valve on an old 75HP IR ES-1 compressor? I cannot seem to make it work.

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Hi Rick;

Short answer…why would you adjust it?

The unloader valve opens when the pressure reaches cut-out, and stays open until the pressure in the tank reaches the cut-in pressure level.

The pressure switch trips in response to the low pressure, turns on the electric motor, and at the same time, closes the unloader valve.

There is no adjustment needed.

If the unloader valve isn’t working, replace it. If it is part of the pressure switch, you may have to replace the pressure switch to fix the unloader valve problem.

Unloader valve leak issue.

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?

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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.