The air compressor unloader valve

The air compressor unloader valve provides a critical function in preparing your air compressor to restart.

A failed air compressor unloader valve is one of the most common reasons why your air compressor cannot restart after you have used air from the compressor tank. When the the air compressor unloader valve is working correctly, the pressure in the tank has dropped to below the normal cut in pressure level of the pressure switch, the pressure switch trips to allow power to flow through it, and the motor starts. If the unloader valve has failed when the motor tries to start quite often the motor will lug (struggle) to start and typically a breaker will trip or a fuse will blow.

What the air compressor unloader valve does

Compressor unloader valves are common pieces of equipment on many types of air compressors including reciprocating, rotary screw and even gasoline or other fossil fueled air compressors.

This Fix My Compressor website is focused on the do it yourself and small workshop air compressor and most of these are reciprocating type air compressors. A reciprocating air compressor will have at least one cylinder, and often may have two or sometimes even three cylinders. Reciprocating air compressors always have an unloader valve.

In the photo below the unloader valve connections are the brass fittings on the left sides of these Lefoo brand pressure switches. The pressure switch on the right incorporates an ON/OFF lever on the right side as well.

Lefoo air compressor pressure switches

When the reciprocating air compressor reaches the cut out pressure setting the power supply to the compressor motor stops, and as a result, the compressor pump stops. The compressor pump stops, regardless of where the piston in the cylinder is located, and that often means that there is compressed air trapped over the piston when the pump stops on cut out.

On other pages on this site we talk about how marginal a 120 volt air compressor motor really is, and the steps the motor manufacturers must take to ensure that, even with all compressor components working at their best, even being able to start.

If air is trapped over the piston on the air compressor, that adds load to the start circuit. Your 120 volt power supply hasn’t got enough oomph to start the motor without help from a start capacitor anyway, and that additional load on the compressor motor is typically sufficient to prevent the air compressor from starting.

The air compressor unloader valve vents trapped air

What the unloader valve does is that is responds to the compressor pressure switch turning the power off to the motor and allows any air that may be trapped over the piston, or pistons on a multi-cylinder air compressor, to vent to atmosphere, removing any load that the trapped air may have created over the cylinder piston.

How the unloader valve works

The pressure switch may have an external unloader valve like the Lefoo pressure switches above, or it might have an internal unloader valve the the Condor type switch shown below, which incorporates an ON/OFF switch on top though not all of them do.

Condor air compressor pressure switch

What you cannot see is the air fitting (could be quick connect or another style) underneath the Condor into which the air line from the compressor pump is installed.

Regardless of how the air line from the pump head is connected to the unloader valve in your pressure switch ~ an internal, underneath connection like the Condor or an external connection like the Lefoo (there are more brands, I’ve just used these two popular ones to illustrate the concept)  ~ the unloader valves do the same thing.

When the air pressure in the tank reaches the cut out pressure of that pressure switch, the switch trips to off, cutting power to the compressor motor.

Unloader valve opens

The process of turning the power off to the compressor motor is an electro-mechanical one. That means that electricity is driving it, but the end result is a mechanical movement. Something moves inside the pressure switch housing.

What is moving, typically, are a set of points which open and close depending on the pressure in the tank.

The movement of those points is connected to an arm or finger inside or outside of the pressure switch, and that arm or finger pushes or pulls on a small valve pin, which operates the air compressor unloader valve.

When the pin on the unloader valve is depressed, the air path through it is open to atmosphere, and any air trapped over the piston vents out. This happens when the compressor stops or cuts out.

Unloader valve closes

When the pressure in the compressor tank drops to the cut in pressure setting on your pressure switch, the pressure switch will trip again, and this time the arm or finger will typically release the pin on the unloader valve allowing it to shut, and prevent the air that is being compressed from escaping to atmosphere.

There are two unloader valves shown in the photo below. On the left is the Condor version. This unloader would typically be inside the pressure switch with just the brass insert fitting that is on the bottom of this unloader valve showing. The internal arm or finger to operate this unloader is not shown, but the pin which is depressed and released by that internal arm or finger is shown on top.

Condor-Lefoo unloader valves

The image on the right just above is the external unloader valve. You can just see the small pin protruding from the bottom of the unloader valve, and that little grey blob just below the pin is the arm or finger extending out from the pressure switch and is what depresses or releases the pin as the pressure switch trips on and off.

What about the air in the compressor tank?

I expect you have a question at this point? When the unloader valve is opened to atmosphere, why doesn’t all the air in the compressor tank escape too? For the answer to that, you will have to see the tank check valve page.

Your comments or questions are welcome using the form below.

 

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Comments

  1. rffs1ham says:

    I have replaced my pressure switch with a Pressuremate 034 -0197RP. When the pressure reaches the 130 psi lever the motor is shut off and air starts coming from the pressure switch. When the pressure leaks down to 90 psi the motor is switched on and the air leak stops. This is the second switch I have installed. What’s happening?

    • We can feel your frustration from here! What we think you’ve got is a bum tank check valve. Reason is, when the pressure switch trips off, stopping power to the compressor motor, the unloader valve is tripped open at the same time, to allow any air over the piston to escape. If you have a leaky or failed tank check valve, along with the air that comes off the piston head, all the air from the tank will leak out too, at least until the pressure drops to cut in, and the compressor starts. When the compressor starts, the unloader valve is tripped off, closing it. That’s what we think anyway, based on your comment.

      • rffs1ham says:

        Is the unloader valve in the pressure switch?

        • It depends on the make and model of the air compressor. If your air compressor has an external pressure switch, then typically the unloader valve with either be found attached to the side of the pressure switch. If the unloader valve on your air compressor model is integral with the switch, the unloader will be inside the switch cover, and the unloader line typically connects to the bottom of the pressure switch via a quick-connect type of fitting.

          Some small air compressors have a tube-like pressure switch inserted into the compressor tank. That type of switch does not have an unloader valve associated with it.

          • rffs1ham says:

            Is there a check valve in the 1/2 in. line that feeds the tank? I have an external pressure switch that had the unloader outside of the pressure switch. i’m confused with the what the tank check valve is. Is it’s function to keep air from the 1/2 in. feed line to the tank and flowing through the compressor and through the 1/4 in. line that goes to the unloader valve?

          • Please review the page Compressor Tank Check Valve linked from the right navigation bar which explains what it is and where it is on a typical air compressor.

  2. Danny V. says:

    Hello, i have a sears 31 gal air compressor that starts great, but after it cuts out it is under strain to restart and trips the circuit breaker. It does not hiss when it cuts outs. Any ideas on why this happens? When motor is cool and tank is empty it starts right away. Could this be a unloader or switch problem? Thanks.

  3. Hello. I have an air compressor where the relief valve starts popping open and closed when the compressor shuts off. Would anyone have an idea as to what I can do to repair that? The tag on the valve says, ” A popping relief valve is an indication of excessive interstage pressure.” I’m not sure what that means,or how to fix it. Thanks in advance for your help with this matter.
    Kenny

    • willyr says:

      A bit hard to be specific as we have no idea what make or model of air compressor yours is. If the PRV you refer to is in the line between two cylinders, then you quite likely have a problem with the intake valve on the second cylinder.

  4. aloha, , i have a Campbell hausfled 5 hp, 30 gal, (vt629901aj) compressor it has started to develop this straining sound when trying to build up pressure above 80 psi. i have cleaned and checked the valve in the tank, it builds pressure up to 80 psi at normal speed ,then the motor seems to slow to about half speed then it strains further and will stop. at this point i unplug it, , do i need a pressure switch or the unloader vavle, ?any help would be greatly appreciated, thank you john

    • And Aloha back to you. While there could be many things that may be the cause of the motor straining as back pressure from the tank grows, one of the most common is a failing run / start capacitor. Please see the pages on this site about what they are and how to check them. After doing so, if they are not the cause, please add a comment and we’ll on to the next logical source of the problem.

  5. I don’t really how to describe it. The compressor act like it’s in a bind. It will run pretty good then slow down as if in a bind, The compressor is full oil. The brand is a Kobalt from Lowes. thanks in advance

    • Does it run up to cut out pressure and then stop, or is it “binding” while the compressor is pumping up to cut out? Are you using an extension cord? (If so, don’t unless it’s a short, heavy duty one.) The “binding” may be a power supply issue, but it could also relate to back pressure buildup overloading the motor.

  6. I’ve taken my 3 gal compressor and removed the compressor from the tank. I them put an old refrigerator compressor on in its place, but now I’ve got air leaking from the unloader. It won’t allow the air into the cylinder to compress. Is it because of the low air flow? a bad check valve or the unloader?

    • I would think that if you have air leaking from the unloader valve that it is the tank check valve that is the problem. When the air compressor is off and the unloader valve is open then it is the tank check valve is what keeps the compressed air in the tank.

  7. Hi,
    I have a 2cylindr oil free Gentilin air compressor C330/20 which is malfunctioning due to air leaking from unloader valve. This unloader valve fixed with pressure switch.

    During pumping air leaking and compressor running continually and trying to fill up the tanks until the cut out point (10bar) has reached. But it won’t reached at cut out point because of air leaking, so it continually pumping and as the result air supplying tube to unloader valve ruptured. Because of continuous pumping air lines and piston heads are getting hot enough to burn if touched.

    Any help will be appreciated.

    Imran

    • Imran, though I am not familiar with the compressor, I did find the catalog and had a look, as best I could, of the setup. Based on what you are saying, it seems clear that the unloader valve in the pressure switch is stuck open, and, the result is exactly the symptoms you describe. If this were my air compressor, I would be replacing the pressure switch to resolve the problem. The easiest solution is to purchase the same switch from the distributor of the Gentilin compressor, though I suspect, as many compressor manufacturers do, they buy a standard switch from a switch manufacturer, in which case, you may be able to find a similar pressure switch with an integral unloader from any compressor or fluid power shop.

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