Air compressor check valves are one-way or a non-passing valves. In other words, whatever is in the line that the check valve is installed in, can only flow through the check valve in one direction. 

The air compressor check valve can be a source of problems on an air compressor. If you wish to know more about the different types, how they work, where they’re located or get help troubleshooting your air compressor check valve, this page will help.

If you’re having issues with your check valve, please head over to our troubleshooting page.

Table of Contents

What is a Check Valve?

An air pressure check valve can be found on almost any and every air compressor out there as they’re a very important part in the successful operation of air compressors. They’re used on many types of reciprocating piston compressors and rotary screw compressors. Some compressors even contain multiple check valves.

They have one sole function within the air compressor system and that is to let air flow from one side to the other whilst blocking air in the opposite direction. This allows for compressed air to pass through the check valve and have no way of going back again. They’re therefore, very crucial parts of the compressed air system.

Types of Check Valve

It’s very important to consider both in-tank and in-line check valves as they’re quite different for a number of reasons other than just their names.

Throughout the article, I will describe both further and separate any differences but here is a brief introduction.

In-Tank Check Valves

In-tank check valves have the very explicit purpose of preventing back flow through the compressor pump and are located directly screwed into the tank – essentially sealing the compressed air within the tank (which would otherwise leak out the unloader valve – more to follow on this).

In tank check valves tend to come with 1 female thread end, and 1 male thread end like the example below:

In-Line Check Valves

In-line check valves, yeah you guessed it, can be found “in-line” on the air pipe/hose. An in-line check valve doesn’t specifically serve the purpose of sealing the tank volume like a tank valve does but they generically prevent back flow in the same manner as one.

They tend to be both male or both female threaded ends so that they’re able to be connect in the middle of piping/hose runs like the example below:

How does the Check Valve Work?

How does the Tank Check Valve Work?

As the compressor is running, as air is being compressed, it flows from the pump head down into the tank through the tank check valve. Since the check valve is a one way valve, once compressed air gets into the compressor tank, in theory, the only way it should get back out, is via the discharge coupler into which you connect your air hose.

Upstream from the air compressor check valve, and typically in the same fitting, is an air line that runs over to the pressure switch and to the unloader valve.

When the compressor stops after reaching cut out pressure, as the pressure switch trips to cut power to the compressor motor, it commonly also toggles the unloader valve to open.

Unloader valve on the side of a pressure switch
One type of unloader valve on the side of a pressure switch

A brief summary of the unloader valve function is that when the compressor shuts off, air that may be trapped over the piston in the compressor head, vents to atmosphere through the now open unloader valve.

It works hand in hand with the compressor tank check valve.

In order for the unloader valve to work, there has to been an open line between it and the compressor head. That line is commonly teed off the tank check valve fitting.

Tank Check Valve Keeps Air In The Compressor Tank

When the unloader valve opens the compressor pump to atmosphere, if there were no compressor tank check valve, as soon as the compressor stopped and the unloader valve opened, not only would the air bleed off from over the compressor piston, all of the air in the compressor tank would bleed out the unloader valve to atmosphere too.

If the tank check valve is malfunctioning, it’s common to observe air flowing back out of the tank up through the compressor pump and out of the unloader valve.

The air compressor tank check valve’s purpose is to keep the compressed air in the tank so it’s there to be used by downstream air-tools.

Check Valve Allows Air Into The Air Tank

When the compressors components are operating normally, and when the pressure switch senses the tank pressure is low enough (the cut in pressure), the pressure switch relay closes, and turns on the compressor motor.

At the same time, the compressor pressure switch (usually it is a toggle on the pressure switch, but not always) closes the unloader valve.

Now the air compressor is running, air is being pumped into the air tank.

None of it can flow down the line and out the unloader valve.

The tank check valve is open as the compressed air generated by the air compressor pump is above the cracking pressure of the tank check valve – air flows only into the tank.

When the air pressure in the tank again reaches the cut out pressure setting on the pressure switch, the pressure switch opens again, the motor is shut off, the unloader valve is opened, and the tank check valve closes keeping the air in the tank – the cycle then repeats.

How does an In-Line Check Valve Work?

During the operation of an air compressor, air flows out from the tank into the compressed air line and, if installed, through an in-line check valve. Since the check valve is a one way valve, once compressed air gets into the compressor line, in theory, the only way it should get back out, is via the air-driven application at the end of your air line.

In-line check valves can sometimes have a 3rd port on them used for inspection, or if the application demands it for discharing air from the line.

As with tank check valves, in-line check valves have a specified cracking pressure which is the minimum upstream pressure required to open the check valve – allowing air to flow through it. Practically, this means that air won’t flow through the check valve until the cracking pressure has been reached between the compressed air source and the check valve.

To find out more about check valve sizes and how check valves are sized, check out my handy guide.

Why is the Check Valve Needed?

Check valves allow the compressed air to flow through them in one direction only. This is one of the many reasons why check valves are used on air compressors because they can keep certain areas of the compressor system pressurised or not.

Depending on the type of compressor, the size of it and its manufacturer, a single compressor can have anything up to 5 check valves on it. These check valves can also prevent the compressed air that has left the compressor through the piping to the storage tank back into the compressor. 

If you’re trying to troubleshoot your air compressor tank check valve, you should take a look at my guide to fixing, cleaning, testing and troubleshooting air tank check valves.

Where is the Check Valve Located?

Where is the Tank Check Valve Located?

While not true for every air compressor make and model, the normal location for the tank check valve is as part of the fitting where the pump head line enters the tank.

The photo of the 40400 check valve shows the various ports. The port on the top right of the photo is where this tank check valve threads into the tank.

Central Pneumatic 40400 check valve assembly
Central Pneumatic 40400 check valve assembly
Photo: Amazon.com

The port bottom right, with the ferrule and nut, is typically where the line from the pump head is connected. The small port, pointing right up, usually the line from the tank check valve that runs over to the unloader valve on the compressor.

It can be a difficult part of the air compressor to locate, if you’re having difficulty please visit our help page.

Where is the Line Check Valve Located?

An in-line check valve is typically located somewhere along the compressor piping or hose after the air has been compressed and released from the air receiver tank and before it reaches your required application.

You should find this a lot easier to locate than a tank check valve.

Check Valve location on Reciprocating Piston Compressors

On reciprocating piston compressors check valves can be found in numerous locations. There tends to be 2 individual or sets of check valves that are used inside the piston head to control the airflow to and from the compressor cylinder. The air is drawn in through the check valve at the inlet when the piston moves in a downwards motion inside its cylinder. When the piston moves back upwards (reciprocates) the air is compressed and then flows out of the cylinder through the second check valve at the outlet. 

Is there any repercussions of not having a check valve at the inlet? Oh yes, air will flow back out the air compressor inlet from following the path of least resistance. The check valves are crucial in ensuring that once the air has been drawn into the cylinder and compressed, it cannot go back out the inlet, only out the outlet. 

That’s not all the check valves on a reciprocating piston compressor however! There’s also another on located between the compressor and the compressed air tank or otherwise referred to as the receiver. Again the check valves, work in a similar fashion where they make sure that the air being pumped into the compressed air tank cannot flow back out the compressor when it shuts down. 

When the compressor shuts down, a blow-down valve comes into action to reduce the pressure inside the discharge pipe to 0 bar. This reduction helps the compressor restart its whole air compressing process. The blow-down valve is usually located on and as part of the pressure-switch. The check valves here ensure that only the small discharge pipe is blown down and not the whole compressor air tank. 

Check Valve location on Rotary Screw Compressors

On rotary screw compressors the check valve can be again found in numerous locations depending on the make and model of the compressor. 

Screw compressors have check valves integrated at their inlet valve for when the compressors operation comes to a stop. The compressed air that still remains within the system when it stops will try to find a way out, the inlet check valve ensures that this air (or even oil) cannot flow back out through the inlet. 

Another check valve on the rotary screw compressor is installed just after the compressor component, and as previously explained, this check valve ensures that the compressed air and possible oil mixture does not flow back into the compressor section of the unit. 

The final check valve can be found at the outlet of the air compressor unit. Any compressed air produced by the system is not allowed to flow back into the compressor, it should only go in the direction of the storage tank or receiver. 

FAQs (Frequently asked questions):

Does an air compressor need a check valve?

Yes, compressed air check valves are vital to the operation of air compressors. They stop the air from leaking from the system and moving in the wrong direction, as air should only travel from the pump to the air tank, and not able to escape from the air tank through the compressor pump or unloader valves. Check valves are crucial in allowing your air compressor to build pressure properly.

Where is the check valve located on an air compressor?

While not true for every air compressor make and model, the check valve can be located as part of the pump head line fitting that enters the tank. On a reciprocating air compressor for example, you could find sets of check valves inside the piston head to control the airflow to and from the compressor cylinder.

In contrast, a rotary screw air compressor will check valves integrated at their inlet valve for when the compressors operation comes to a stop.

What does an air check valve do?

They have one sole function of allowing air to flow from one side to the other whilst blocking air moving in the opposite direction within the air compressor system. This allows for compressed air to pass through the check valve and have no way of going back to the unloader valve and possible into atmosphere.

What is a check valve on an air compressor?

When the unloader valve opens the compressor pump to atmosphere, if there were no compressor tank check valve, as soon as the compressor stopped and the unloader valve opened, not only would the air bleed off from over the compressor piston, all of the air in the compressor tank would bleed out the unloader valve to atmosphere too.

So the check valve essentially stops the compressors air from bleeding off.

How does a pressure check valve work?

Since they only allow flow in one direction, a check valve relies on a pressure differential to work. They require a higher pressure on the input side of the valve than on the output side so that the valve can open. When the pressure is higher on the outlet side (or the input side pressure is not high enough), the valve will close. Ensuring the compressed higher-pressure air cannot flow backwards.


Additional information pages:

How to buy a replacement compressor tank check valve