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.
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.
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.
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 about unloader valves at this point? That is, 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 though we can say that some air compressors unload all the time.
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