Can I Plug The Line To The Pressure Switch

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This question popped up on a forum recently; ” can I plug the line to the pressure switch “?

The short answer is no, you cannot. Well, actually,  you can if you want, but then your air compressor will not work properly.

You see, there is absolutely nothing installed on a smaller lower cost air compressor unless it’s absolutely necessary. Otherwise, the air line from the tank check valve over to the pressure switch would not be there at all. Plugging that line would, in effect, make it disappear. Not good.

And why would you want to plug the line over to the pressure switch anyway? The questioner said that they had broken the line, a new one was $70, and they did not want to spend that kind of money on a replacement.

Here’s why you should not plug that line, followed by a lower cost solution.

What Line?

What line are we talking about? This first image shows the line in question. It’s connected on this compressor to the base of the pressure switch. It is the copper tube marked with a red triangle.

Air line to pressure switch -

This next image shows a similar line and its source. This line, identified by the red triangle, is aluminum.  It could also be copper as is shown in the first image.

The line plumbs from the fitting where the air line from the compressor pump enters the tank. The larger aluminum line coming in from the left in the second image is the feed from the compressor pump. The fitting into which both are plumbed – the brass “thingy” bottom center –  also usually contains a one-way or a check valve. The air line we are talking about runs from that fitting over to the pressure switch and is indicated by the red triangle.

unloader line from tank check valve -

What does the line do?

Because the line is connected to the fitting shown, air from the pump can flow both into the tank through that fitting, and out from the fitting along the smaller air line to the pressure switch.

A pressure switch, to which the line is connected, typically contains an unloader valve either as part of the switch, or mounted on its side. The line from the tank check valve plumbs over to the pressure switch, and it connects to the unloader valve.

When air is being pumped by the compressor pump, it flows down that big line to the fitting, and also flows over to the pressure switch via that smaller line.

Since, typically, while the compressor is running the unloader valve is closed, then the air travelling over to the switch is blocked by the unloader valve and cannot escape. The compressed air, then, flows only into the tank, increasing the pressure in the compressor tank, until that pressure level reaches the cut out pressure setting of the pressure switch. At this point the compressor stops.

The Compressor Unloader Valve Opens

Typically, when the compressor reaches cut out pressure and stops, the unloader valve then opens. Why?

If there is air trapped over the piston in the compressor pump, when the compressor tries to start next time the pressure switch sends power to the pump, that trapped air adds significantly increased load on the piston, and often the compressor motor cannot start against that added load.

The compressor typically goes off on thermal overload, or trips the breaker / blows the fuse.


If you were to plug the line over to the unloader valve, the next time your compressor tried to start at cut in, quite likely it would not. The trapped air over the piston would prevent the pump from cycling properly.


If the cost of an OEM line from the check valve to the pressure switch is prohibitive, don’t replace it with OEM equipment.

What you can do is visit your local compressed air fittings supplier (Google industrial air cylinders – these folks have fittings) and acquire the fittings necessary to replace the hard metal tube with a polyurethane tube.

Instant fittings, to which the polyurethane “quick connects” make the process fairly simple.

You will still need to spend $20-$30, but that’s better than $60-$70 for an OEM part.

The only thing you need to be aware of is that, if you are using your compressor for long periods of time (as in you use it to make money), then replace the part with the OEM metal tube. Polyurethane may not stand up to the high heat that is typically generated on a compressor when it is being run for extended periods of time.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)

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Craig Williams
Craig Williams
February 24, 2021 2:51 pm

Hi, I recently bought a 3 phase compressor that apparently was working. I tried to connect it up today and found some issues. The 2 pole switch was being used purely as a trigger for a wall mounted contactor stop start switch. This contactor was not supplied to me. So I invested in a new 3 pole switch that can transfer power to motor directly.. i then realised that this compressor did not have that thin pipe connected to the older switch at all. There is also no sign of any pipe fitting anywhere on the compressor. Nothing. I amm therefore confused as to how this compressor was able to cycle? It’s a 12hp motor. Triple head pump. Each head has an exit leading to what looks like a relief valve. 1 of the heads also has an exit going directly into the main tank. Any idea why the previous owner was able to get it to cycle without the air pipe going to the switch?

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Craig Williams
February 24, 2021 3:29 pm

No real idea. Haven’t seen the setup. However, it sounds as though the unloading is accomplished by the relief valve venting, but can’t be sure.

Anyone else?

December 20, 2019 6:04 pm

Hi. I’m in Australia and having trouble locating a new pressure switch. It’s a very old McMillan MC12 compressor and the switch was leaking profusely. (Diaphragm not sealing fully)

The pressure switch is a Furnass and does not have an unloader valve, that is in a separate line from the exchange pipe between the heads, mounted at the tank on a separate line.

I don’t seem to be able to find the same replacement pressure switch. Can I just put a blocking plug in the unloader valve on one that has it, given the unloader valve on the tank is still working? Great website, fantastic info. Thanks. cheers

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Shorty
December 20, 2019 8:57 pm

Thanks for our kind words.

If there is no unloader line, and air does not back-flow from the tank when the compressor is off, then why bother plugging the unloader outlet on the new switch? The unloader will only unload air if their is a flow back from the tank. Please clarify if I’m misunderstanding the issue.

Reply to  Fix My Compressor Moderator
December 20, 2019 11:44 pm

Thanks for the quick reply. That’s what I’m wondering – can I just use a pressure switch and ignore the unloader? I think there is an unloader attached to the tank where the pipe between the twin pistons has a line down to carry the air to the tank?? Looks like it is some kind of relief valve. Had it for years but should have looked more carefully while it was running. Something on the compressor gives a spurt of air at the end of a cycle, presumably to remove the pressure from the pistons and I’m assuming is in the valve where the inlet hose meets the tank. Definitely now was for air to be released through the pressure switch. Any ideas? Thanks and have a great Christmas break?? Cheers PP

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Shorty
December 21, 2019 8:54 am

If there is a functioning unloader, and it currently is not attached to the pressure switch or integral to it, then I can see no reason why you cannot use any pressure switch, with or without an unloader valve, as long as the pressure range and voltage suits.

Reply to  Fix My Compressor Moderator
December 22, 2019 10:08 am

Thanks! It’s taken me a while to read through some of your pages here and get a better understanding of how it all works. Really great site. I’ll track down a pressure switch and get it sorted. Once again, thanks for all your help.

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Shorty
December 22, 2019 6:12 pm

Most welcome. Cheers.

Dwayne Webster
Dwayne Webster
December 3, 2019 3:57 am

Thanks I’ve been looking for this answer for the last 6 hours on how to hook up a unloader switch ..the air compressor I inherited didn’t have an unloader valve on its power cut off switch it just had a check valve … I appreciate your time and trouble answering my question

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Dwayne Webster
December 3, 2019 10:10 am

Thanks for your kind comment. A portion has been added to the home page on this site!

Jason Hawks
Jason Hawks
October 19, 2019 7:03 pm

When an air compressor does not have a “Unloader Valve” a simple water pressure switch will be suffice. Roughly $10.

You’re welcome.

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Jason Hawks
October 19, 2019 7:13 pm

I’m not quite sure of the science behind your comment, but you are certainly welcome to your opinion. If a “simple water switch” is used, if there is no unloader, and the compressor setup requires one, then I’m afraid it won’t work. If you care to add more details about your posting, we’d share them.