Check Valves and Pressure switches



Only my second post and I am still in trouble! I have built a compressor using a fridge motor but it refuses to behave so I have done something wrong. The air goes from the fridge motor to a check valve which is fitted to the pressure tank. The check valve is also connected to the unloader valve on the pressure switch. When I turn the thing on air goes into the tank but also blows through to the unloader valve so releasing air constantly! It seems that the unloader valve is permanently open whether the power is on or off and I have tested this by blowing through the tube. I spoke to the man who sold me the pressure switch and he said it was the check valve. Changed the check valve and same problem occurs. Can anyone help please - I have googled through the night with no luck - simply cannot find any detailed explanation of how the unloader valve actually works. I can try to send some photo's if that would help. Many thanks.
Here's one i made buddy
To view this content we will need your consent to set third party cookies.
For more detailed information, see our cookies page.
A photo would help.
My set up is motor through non return valve, into tank then another port is used for the release( over pressure valve) and auto shut off.
I had the same problem when i built a silent compressor. If the unloader valve is soft start the compressor might not be producing enough pressure to close. Hold your finger over the unloader output hole until the tank reaches about 20psi. The valve should close when you remove your finger if that's the problem.
Hi there. Im building my own silent comp. Same problems. So I've blocked off that little hose. But I have different problem aswell. My fridge compressor is not reaching lower pressure limit on switch. It only can pump up to 75psi. It seems that all pressure switches have lower limit of 80psi. Did anyone of you came across that kind of a problem?
That is odd, fridge compressor can generate pressures of over 500psi. Are you sure you are not losing air somewhere or that the intake lines are not pinched (on the fridge compressor you have usually two intake port and one output port) if the intake ports are pinched the compressor cant get air to push it back out. The other aspect is air displacement, fridge motor usually displace less air than the noisy motors, and so take more time to pressurise the same volume of air and if the air is being used at the same time, it might not be able to keep up, that is why you see more than one motor on big silent compressors.

An other reason might be that the compressor you got was shot (too worn) and so is losing pressure internally.

P.S. As a safety note due to the high pressures that can be generated by a fridge compressor, it is important to have a safety valve in your setup in case the pressure switch fails to stop on reaching the max pressure desired. It will prevent a catastrophic failure of your system and save you from possible arm.
Hi there! I had a leak. Do pressure is sorted :). Also I have a safety valve fitted, so all is good and working nice. Thanks for good advice! Now plan is to connect an additional air tank to increase the capacity. Tank came off of a lorry and it has its own safety valve.
Another possible culprit is the valve stem inside the unloader valve. If it's the wrong length for your pressure switch model, it'll result in the same "always open" problem.

I had this problem with mine. The pressure switch I bought came with an unloader valve with the wrong type of hose connector, so I bought another unloader valve with the right connector type to replace it. Although the unloader valve I bought was made by the same company and rated for the same model of switch I'd bought, the valve stem was too long, so it was always depressed even when the pressure switch plate was relaxed.

I swapped the stem with the one from the valve that came with the switch (it's just a little brass dowel pin that drops right out of the valve when it isn't installed in the switch), and now it works fine.