Has Unloader Valve Issues


Your compressor has unloader valve issues? The unloader valve provides a critical function in the restarting of an air compressor. Need to know more about your air compressor unloader valve? You've come to the right page.

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. This page provides information on what unloader valves are all about, information you want to know in the event yours starts acting wonkily and causing you compressor problems.

Somewhere in the garage, the workshop or the plant, the "kathumping" of a reciprocating air compressor echoes throughout. Suddenly the thumping stops, and sometime's 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 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, additional load is created for the start up of the compressor motor when the pressure switch turns the compressor back on.

For example, the last time you did a sit-up (or if you are like me, can youj 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. If the load on the motor increases too much the compressor motor may fail to start, or it may pull too many amps and fry a fuse or pop a breaker.

When the compressor shuts off the typical unloader valve opens and unloads the trapped air 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.

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.


Unloader Valve

In the photo this unloader valve is attached to the side of the pressure switch.

Unloader Valve Differences


Unloader valves do not always work 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 pressure switch as the switch operates, or from an internal mechanism that opens and closes the unloader valve.

Different manufacturers have different methods of tripping their unloader valves. Many of the smaller, franctional HP air compressors, have no actual unloader valve, but a small hole in the line from the pump to the tank. That hole is bleeding air all of the time the compressor is running, and then, when the compressor stops, that hole (the unloader) continues to bleed air until all the air over the piston is gone. Then the bleeding of air should stop.

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 of air escaping is as long a time needed for all the trapped air to escape.

Sometimes though, the air evacuating from the unloader valve does not stop coming out. The unloader valve continues leaking air after the compressor is stopped.

As noted, the unloader valve is typically installed in an air line running between the compressor pump head and the compressor tank.

If there was no way to stop it, when the unloader valve opened, all of the air already compressed into the tank would escape out the unloader valve as it 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 o the unloader valve, and out to atmosphere.

That 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.

There is more information about the unloader valve as part of the compressor pressure switch here. Please have a look, and then, if you still have a question about the unloader valve part of of the pressure switch, or just about compressor unloader valves, you can use the form below to ask it.

This is also a forum page providing information and a place to post a questions when your compressor has unloader valve issues.

If you think you've got a problem with your compressor unloader valve, why not scan the questions and answers at the bottom of this page. In the event that your problem is not noted there, do please add it using the form just below.

And, if you can offer advice on any of the questions, it sure would be appreciated. Then we all benefit from your compressor experience.

Thanks,

Bill

Problems with your compressor unloader valve?

Please if you have unloader valve problems, and the answer isn't already noted below, post the details.

Compressor has unloader valve problems?

Questions and answers are posted below.

Where can I buy an unloader valve? 
Bill where can I buy a unloader valve. Everything was working fine ,I had a couple air leaks including the odd tee that goes in the tank . so I was tightening …

Unloader Valve hissing upon operation 
Hi Bill, I have a Champion vr3-8 vertical compressor. 3hp 80 gallon. Anyway as soon as the compressor gets around 50psi it starts hissing out of …

Air coming out of unloader check valve hose 
Air coming out of unloader check valve hose when running. Just changed the oil with the proper oil recommended for the compressor . I started …

Compressor slow to almost a stop. 
Bill, I bought a horizontal compressor at auction and found it to have a rusted tank. I have an upright tank and with a few modification made the switch. …

K5 1 unloader valve 
Very good flow sensitive unloader.. But only lasting 3 weeks at best. My machine is running 8/10 hours per day. Simple enough to change.. Simple …

Unloader valve 
Bill, My compressor has an unloading valve that ends up on a circular device on the opposite end of the crankshaft from the drive pulley. I have see them …

5Z185 Unloader valve plumbing 
Hi I'm replacing the pump head on my 2nd-hand Speedaire 60 gallon tank. It's a 5Z185, 3HP vertical tank. The unit had tipped over and bent the crankshaft …

square d switch depressing unloader valve 
Hello,i replaced the square d pressure switch on my old craftsman compressor. The old switch has a set screw where the unloader valve sits,the new one,however,does …

LGM 125 unloader valve on 10HP DeVilbiss compressor. 
I re-installed the original LGM125 unloader on my 10 HP Devilries compressor. It operated properly after the compressor limit switch turned the compressor …

unloader line blows off 
I inherited a Kobalt 60 gal compressor (no. 221565)from the previous owner of my house and it ran fine (once I cleared the mud wasp nests out of the motor)until …

Can I replace the copper unloader tube? 
Hi Bil: Can I replace the copper unloader tube with a flexible metal braided line? Here's the situation. I have two oilless compressors. …

New unloader valve does not have compression fitting 
I just replaced the following on my Campbell Hausfeld compressor: CV209301AJ: regulator assembly CV303100AJ: Pressure switch assembly I did not …

Unloader valve starts bleeding off air through valve when top pressure is reached 
I just installed a new pressure air switch and am still have trouble with the unloader valve. When the compressor reaches max. pressure the unloader valve …

Compressor cannot unload even after the blowdown point 
Compressor cannot unload even after the blowdown point. On getting to the set point of blowing down, the compressor will continue to build up pressure. …

loss of pressure when compressor stops 
when the compressor stops at pressure set point the end point uses loose pressure , the pressure is not maintained when the compressor stops

New check valve, new bleeder valve, still not working 
I built a silent air compressor using a refrigerator compressor mounted on a craftsman 3 gal tank that was complete with the pressure gauges, regulator …

2 way pressure relief valve does not need an unloader 
If a compressor switch says it has 2 way pressure relief valve, does it mean that it does not need an unloader. A compressor switch I was looking at …

2hp compressor tripping breaker on 2kva generator 
Hi Bill Compressor runs ok on mains power (240) and also fills from empty on genset. Cuts out OK and dumps at 120 psi However it won't cut back in at the …

Dresser series 400 unloader valve plumbing ??? 
I just bought a used dresser series 400 air compressor and Im not sure how to hook up the unloader valve plumbing... right now it has the two unloader …

Buffalo Pneumatic compressor unloader valve no longer works - replacement? 
My 5HP compressor marketed as a Buffalo Pneumatic and purchased about ten years ago from Home Depot has a defective unloader valve. I've tried rebuilding …

My unloader valve is too noisy .. 
Hi Bill I have a silent compressor with the unloader built into the pressure switch. Is there any way to reduce/muffle the noise when the unloader operates? …

unloader problems compressor will not cycle 
Compressor will not cycle - hums tries to turn over but cannot - blows a new 20 amp circuit breaker - I've had it to the compressor repair guys twice - …

When running air keeps pouring out of air compressor unloader valve ... 
I wanted to make a "silent air" compressor out of my HF oil compressor. I took off the original (new) motor and replaced it with a motor/compressor from …

Air compressor will not autofill correctly ... 
At the start of the day with pressure at 0 psi, the compressor will start up fine and fill the tank to 150 psi. As air is used and drawn down, the compressor …

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Compressor shuts off and valve leaks ... 
I dont see any check valve it has the same set up as the unit you used to show the release valve __________________ G'day Steve. If air is bleeding …

Unloading valve doesn't close after changing air compressor motor ... 
Hi Bill, I bought a used 2hp air compressor with 24 liter tank and replaced it's motor with an air conditioner 1.25hp compressor. Now I'm having …

Compressed air keeps running - unloader not working 
dear sir i had solenoid valve issue in my electric screw compressor 30 KW, the unloading operation is not proper. when i clean solenoid valve the …

Compressor trips breaker but not when empty ... 
Hi,my compressor tries to start but struggles for around 10 seconds then it blows the trip. If the tank is empty it will start with no problem and run …

Compressor unloader valve bleeds air all the time ... 
I have a Campbell Hausfeld 13 gallon air compressor, model #WL604004AJ. The unloader valve (part #CW00800AV) dumps air 90% of the time. I installed …

Compressor unloader valve failing... 
On start-up, the compressor will fill regularly, but when it needs to refill, it struggles and sounds like it is cutting out, until the internal breaker …

Compressor won't kick on when pressure drops. 
My compressor will only start if there is zero pressure in the tank. I thought the problem was too much pressure on the head & sought to solve it by replacing …

Compressor will only start with little or no pressure in tank ... 
Bill; The compressor is a Norther Industial Tool 2 HP 8 Gallon. The compressor will only start with no or very little pressure in the system and …

Unloader valve malfunctioning... 
where can I purchase the unloader valve you show in your example. I need that exact one and the so called dealer says they can not get it. ____________________ …

My compressor unloader valve problems... 
I have a 60 gal 5 hp single phase compressor (energair) i bought at an auction. It has a 1/2" copper line off the compressor head to a tee in the tank. …

blows main breaker in house 
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Do I need an unloader valve on my switch... 
i have a presser switch with out unloader valve do i realy need a unloader valve

Compressor unloader valve is leaking continuously ... 
Unloader Valve wood not stop trowing air out _______________________ Bill says... Joseph, I doubt if it's your unloader valve that is causing …

Air Compressor Tank Fills and Loses Pressure and won't run 
I have a campbell hausfled 20 gallon air compressor. When I start it, it runs great and fills up the tank with air. After using my air gun for a couple …

My air compressor locked up on me? 
it was working but when i pluged it back in it locked up and now all it does is hum can you please tell me whats wrong with it? thanks shane _______________________ …

wont build up pressure under pressure 
Goes up to 130 psi fine, when it is time to build pressure back up motor stalls then flips breaker on power supply. Tank has to be empty to build pressure. …

Why does my compessor start pumping slow then speeds up 
It just started doing this. Sounds like it pumps slow and then speeds back up to normal speed. It does not always do it but the last few times I have noticed …

Sears compressor unloader valve not working... 
Hi! I have a problem with a compressor pressure switch which has a leaking unloader valve. The valve lets out air immediately when the compressor is …

Compressor unloader tube leaking ... 
Hey Bill, I've an old compressor which has been kept going with a quick service- it's a Gianesi Edilio Zavaterello- I have no idea what it's background …

Air compressor not unloading... 
I have a Husky (Campbell Hausfeld) 80 gallon 2 stage compressor and it does not unload when it reaches capacity and when I drain air down it kicks reset …

Air is leaking at the pressure switch end of compressor tube ... 
DEVILBISS Upright 5.5 hp. Model No. FB5520VP When shutting off at cut-out pressure, or when manually turned off, air continues to escape at the right …

Compressor struggling to restart... 
Hi There, I have a cheap 45 litre tank compressor. When i use the compresser at the start of the day , it will not pump and buzzes for a few seconds …

What the compressor unloader valve does... 
I am having no luck figuring this out. Hope you can help. I bought a new v-twin compressor with head unloaders. I know what unloaders do but I am …

Is unloader tripping breaker... 
The Swan tech came out today, and lengthened the discharge line a fare amount by going around the whole compressor unit, giving ample time for the motor …

Adjusting the compressor unloader valve... 
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 …

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