Leaking unloader valve when compressor is running

by Johan
(Lund, Sweden)

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 started. Due to this leak the compressor never reaches its turn-off pressure. I have tried replacing the pressure switch but I get the same problem also with this component. I have checked the one-way valve, which looks fine. (As I have understood it this valve is not likely to be the problem anyway, since the unloader valve lets out air already while the compressor is running?).

My suspicion is that I have done something wrong when mounting the pressure switch (particularly when mounting the unloader tube). I don't see what this error might be though. The tube is just "inserted" into place - maybe I've missunderstood something there?

The switch looks like the one found here (four port version):
http://www.cacheng.com/openwin_215_2801.html

Would you have any suggestion for how to troubleshoot further?

Thanks!

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Bill says...

Johan, if you haven't had the chance to review the page on this site, here is the info on unloader valves.

That your unloader valve is leaking air when the compressor is running indicates to me, since as you point out they are a simple, spring operated check valve, that you have something holding the ball open, allowing the air to escape.

The unloader valve is often opened (frequently by a lever moved by the pressure switch) by the depressing of a pin actuator that pushes the ball off seat inside the valve and allows air to flow.

In your case it sounds like either the ball is lodged off-seat by debris or something that is preventing the ball to seat and shut off the outflow while the compressor is running, or the linkage with the pressure switch is somehow compromised, resulting in the pin being depressed when the compressor is running rather than when it's shut off.

Cheers,

Bill

How to reassemble pressure unloader switch?

by Bill K
(Cornwall PA)

Inherited a broken air compressor with a disassembled unloader switch and busted crankcase, but otherwise like new. JB Welded the case, it compresses to 140psi just fine, now need to figure out how to reinstall the little spring inside the unloader valve that connects the switch inside to the steel lever forced down by air tank piston and large preset spring.

Any help appreciated .

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Bill says...

Hi Bill. The spring's purpose is to keep the check ball on the valve seat when the compressor is running. When the cut out pressure level is reached, the pressure switch and lever trips, and the ball is pushed off seat so the compressor can unload.

A photo of parts might have helped. I hope this general operating info helps.

Cheers,

Bill

Sears air compressor

by Tony Stutts

The compressor will start and build pressure up to the limit. After it cuts off I try to restart it and it runs slow briefly then trips the breaker. The capacitor has been checked and is good. What could the reason be for it not to start after it has pressure built in storage tank?

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Bill says...

Tony, it sounds as though the compressor is not unloading when it reaches cut out pressure.

Try this: pull the plug and open the drain valve to completely drain the tank. Now plug the unit back in. Did it start and run OK?

By dumping the air you are unloading the compressor (see Sitemap - Unloader Valve for details) and allowing an easier start.

Let us know if this works, OK?

Bill

Air compressor will not build pressure

My 22 gallon campbell hausfeld air compressor will not build pressure beyond 20 pounds. When the pump is running there is a small valve on the side of the check valve that air comes out of at high pressure (this is what I think the problem is). The air intake has steady and strong suction. Is it the check valve that needs replaced??

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Bill says...

Howdy. Off hand I'd say that your unloader valve is stuck open. It should be closed when the compressor is running, open when the compressor is stopped.

The unloader valve is often attached to the side of the pressure switch. At the risk of sounding dumb, (real easy for me :-), is it your pressure switch that you have a small valve on the side of, since I don't know of any on the side of the check valve?

Cheers,

Bill

Seems like unloader not working

by jimmy
(pensacola fl)

Have a 2 stage 30 cfm compressor with 10th HP sf 220 ,kicks 50 amp breaker ,@125 psi worked fine for aprox 100 hrs. At @ 75 psi or under no load motor runs good

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Bills says...

Sounds like a good guess, Jimmy.

Drain the tank down to zero, close the drain valve, and see if the compressor starts. Then, and if it does, when it gets to cut out pressure, listen for the unload.

If you don't hear it unload, then yes, your unloader valve is probably the issue.

Cheers,

Bill

My air compressor is leaking from the unloader valve while the motor is pumping but dont leak when it isnt running

by david
(west liberty ky.)

My air compressor is leaking from the unloader valve while the motor is pumping but don't leak when it isn't running

i have checked the check valve i have put a new jug and piston and all new seals and i put a new valve plate assy. on it and that the only thing that wrong it now please help thanks

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Bill says...

Hello David. Compressor problems really suck, huh?

When your compressor reaches cut in pressure, the pressure switch trips. In so doing, it starts the motor and closes, often by releasing a pin that allows the check ball inside the unloader valve to seat and seal off the air flow, the unloader valve.

When the compressor tank reaches cut out pressure, the pressure switch trips again, this opens the unloader valve to dump air over the piston, and shuts off the motor.

The only thing that is keeping your air in the compressor tank at this time is the check valve. If it leaks, air flows out the unloader valve, which stays open until your compressor starts again.

Since your unloader valve is leaking while the motor is running, some (most??) of the air that you are compressing is escaping out the unloader valve.

When your compressor stops, the unloader valve doesn't leak air because your check valve is fine, and even though your unloader valve is still leaking, there is nothing to escape as the check valve prevents that.

I think you either have some debris preventing the check ball in the unloader valve to seat properly, the unloader valve has completely failed, or maybe the pressure switch is not allowing the unloader valve to close, meaning that the pressure switch may be the issue. Watch the lever on the pressure switch to see if it moves when the compressor cuts in and out. If it does, then it's most likely a dirty or failed (the spring sometimes breaks) unloader valve.

Cheers,

Bill

Ship air compressor - bleeding air from unloader

on start up my air compressor keep bleeding air from the unloader a longer time than usual what make this happen

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Bill says...

Go to the Valves page, and click on the link to the Check Valves page.

I think this might help.

Cheers,

Bill

Head pressure release won't stop

by Chuck Kizer
(Loretto, TN USA)

When the compressor kicks off and the head pressure releases the air just keeps coming out. I have replaced the whole side cover assembly and it still does that. What is wrong with it???

Mfr. Model #3Z180

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Bill says...

Hi Chuck. I don't think that it's your unloader valve that is the problem, rather it's just venting air from the tank to atmosphere when the compressor is off.

The air is bleeding out of the unloader valve continuously because it's likely that your tank check valve isn't working properly.

More info on leaking unloader valves and on compressor check valves here.

Cheers,

Bill

This is the small valve that air comes out of when running

by Scott
(Pittsburgh)

I couldn't figure out where to put photos on my add on so I started this one. My compressor will not build past 20 pounds of pressure. When the pump is running air comes out of the small needle valve on the left.. I believe it is attached to the check valve. It screws in opposite the plastic line running to the front of the compressor. This is a campbell hausfeld 22 gallon oiless compressor. There are no other leaks present.

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Bill says...

Hey Scott, thanks for figuring out how to add a photo. The system I use won't allow you to add a photo after the thread has begun.

It is quit difficult to see in your photo what's what. I interpret the the red hose going into the top of the check valve is the line from the compressor head, yes?

Looking at the right of the check valve, I see a fitting, but I can't tell if that's got a line on it or not. From your comment, I surmise it does. Does this line go over to the pressure switch?

I am guessing, and it's just a guess as I really can't see in the photo, that the leak is from a PRV that's installed into the check valve housing. If you haven't, read the PRV page to understand what they are all about.

If it is the PRV, then it has a problem, as it's purpose is to blow open and vent over pressure in the system to prevent a catastrophic failure in the event the pressure switch fails.

I hope this helps.

Bill

Air compressor starts fine and fills to 120PSI, but won’t restart.

by Greg
(Saint Peters, MO, USA)

Air compressor starts fine and fills to 120PSI, but won’t restart. The motor seems to struggle to restart and then trips a 20amp breaker. I replaced the check valve, but still having the same problem. 5 hp 22 gallon upright compresser 919.165220

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Bill says...

It's either your unloader valve failing to dump air over the piston when the compressor stops, or the pressure switch, I believe.

I'm betting that your unloader valve isn't dumping air when the compressor reaches cut out pressure. If you aren't sure what that does, check the unloader valve page found as a link from the site map or the valves page.

Also, make sure you understand how the pressure switch works and interfaces with the unloader valve. The pressure switch might still be your problem.

Cheers Greg.

Bill