Compressor with head unloaders. Do I use a check valve with unloader port or a switch with a unloader on it?

An unloader valve when talking compressors, is used with engine driven compressors when it would be impractical to stop the engine when maximum pressure is reached and restarted when it drops significantly and so the system is designed to keep running and when the maximum working pressure is reached the unloader valve sends compressed air to the special compressor cylinder head where the piston of a small pneumatic ram lifts the compressor intake valve from it's seat preventing it pumping until the pressure falls.

This system is also used with electrically powered units when for reasons of light weight and/or economy the electric motor is under-powered. The unloader valve keeps providing air to the compressor head until the pressure falls which is inevitable as there is invariably leakage...

Adequately powered electrically powered compressors do not need the unloader valve system and instead use a pressure switch that if correctly fitted does not leak or at least should not leak. If your unit leaks listening through a short length of hose will help locate the area of the leak and a little soap solution will narrow the search even further (disconnected from the mains power).

Check the pressure switch. The unloader valve is actually in the pressure switch where the small air line from the tank check valve attaches to it. When the pressure switch turns the motor off it then lets the head pressure bleed off so the motor can restart the next time pressure drops in the tank. If it's not working and bleeding off the head pressure the cold start valve (the one with the pin in it) also won't open so the motor can restart and warn up properly.

you must have wired the new pressure switch incorrectly .when the pressure switch opens it will release the head pressure so your compressor must be running with the pressure switch contacts open should be 4 connections 2 marked motor which is from your compressor and 2 marked line which is your power coming in

The unloader port should only unload the compressor head, so there is a valve leaking in the system, probably where the fitting that the unloader valve tees into and that also takes the compressor head output to the receiver. With the receiver at pressure, if you remove the unloader line there should not be any air coming out of it.

When a compressor reaches set pressure, there should be a release of air from discharge hose thru the unloader. This release of air unloads the compressor head pressure. The unloader is usually located within the pressure switch. Easy to check. Follow discharge hose from compressor head to tank. There should be a brass/steel checkvalve at the tank. The checkvalve allows air to enter tank, however not allow air to be released back into discharge hose. If this valve is leaking, air will be released from the unloader constantly. Follow 1/4" tubing from checkvalve to unloader and listen for leaks. Repair as needed. If air is not leaking from checkvalve then check unloader mechanism with-in pressure switch. Parts are available at www.toolpartsdirect.com Good luck

This is usually a problem with the check valve. The unloader valve on your pressure switch is designed to bleed the pressure off the head of the compressor. With a leaking check valve the pressure isn't relieved from the head and the pump can't turn causing the motor to overload and blow the breakers.

To troubleshoot your problem try the following:

Trace the 1/4" line that connects to the unloader back to the tank. You'll find a fitting there. Remove the lines and then the fitting. Sometimes cleaning that valve will alleviate the problem. If not you need to replace the check valve.

Hope that helps.
-under_psi

Piston air compressors have to unload to permit a restart, the two mot common types are an unloader valve or free air unloaders.

with an unloader valve this system uses a check valve mounted in the discharge line as it enters the tank, when the pump stops this check closes and does not permit the tank pressure to return to the pump. a unloader valve also opens to allow air pressure in the discharge line to vent, this valve is either on the pressure switch or on some models is on the pump, and is a centrifugal type.

Free air unloaders or head unloaders are a set of fingers that hold open the intake valves on the cylinders to permit the piston not to pump air against a head load, and once the pump is running the fingers retract to allow the pump to function as normal.

It sounds like the unloader valve it not working correctly. The unloader valve bleeds off the pressure between the pump head and the tank check valve. Without it every time the compressor starts the piston would be working against the max tank pressure. This would cause what your describing. However when the unloader valve is not working the pump will shut off with in a couple seconds. If your pump is working longer then that I would guess that the unloader valve may be bleeding off some but not all the pressure.

On most smaller craftsman compressors the unloader valve is comprised of a piece of tubing running from the cylinder head or the check valve to the switch. At the switch you will see a plastic tip with a pin running to a small trip lever. Make sure that it's moving freely and moving through it's full range of motion. It's impossible to describe how this looks and feels to someone who has never messed with one of these.
After you have checked it and made sure it's working, if you still has trouble after this please let me know.

That little copper line is an unloading line, to bleed compressed air out of the compressor head so that the motor can more easily start the compressor. Pressor switch should be threadded into top of air tank, and the unloading valve is built into the side of the switch. When the tank reaches the high pressure setting the switch disconnects power to the motor and opens the unloading valve at the same time. The unloading valve should be connected to the compressor discharge line through a check valve with an unloading valve port upstream of the check valve. These valves are usually threaded into the top of the air tank as well. Sounds to me like your valve was leaking and someone bypassed it.

Yes, your compressor will work just fine with a Load Genie continuous run unloader from www.grainger.com. The part number is 4tk14 with a cost of about 45.00. This unloader has a built in check valve so you can remove the present in-tank checkvalve and save it for later use. All that you need to do is install the unloader at the tank and compressed air in to top of valve. (you may have to buy extra fittings to make it work with your tubing size) Adjust valve to open just before the pressure switch cut-off setting. The unloader will bypass the compressed air and will have less stress on the motor. You will not need to use the present unloader at the pressure switch. The only caution is to manually unload the Load Genie upon initial start if there is pressure in the line. Good Luck with your project.