Your reciprocating piston compressor will have a smooth start up, thanks to the unloader valve.

The air compressor unloader is valve is a small, but important, part of the reciprocating piston compressor.

It blows off the pressure inside the discharge pipe when the compressor stops.(There is another type of unloader valve, which can be found at the inlet of rotary screw compressors, also called inlet valve or loading valve).

How does it work and why is it there?

Air compressor unloader valve.

The output of the piston compressor is connected through a pipe to the air receiver. When the compressor stops, the pressure in this pipe is blown off to 0 bar (atmosphere).

This is the ssssssshhh sound you will here when the compressor stops. At the same time, a check-valve makes sure that not the whole air receiver (tank) flows empty, but only this small pipe.

Why? When the compressor starts up again, and this pipe would still be under pressure, the electric motor will not be strong enough to turn the compressor.

Where to find the unloader valve

On smaller units, this valve is often mounted on, or inside, the pressure switch. When the pressure switch switches off the compressor, the unloader valve is also actuated.

Small modern-type unloader valve,
using a solenoid valve. Photo: Atlas Copco

The check valve (which prevents the whole tank from draining) is often located right at the point where the compressor discharge pipe is connected to the storage tank.

From this check valve, a small tube or pipe runs to the unloader valve on the pressure switch.

On newer compressors, a simple solenoid valve is sometimes used to blow down the pressure.

On bigger compressors, there is often a big unloading valve, which is controlled by air pressure from a small pilot valve. In this case, the unloader sits somewhere besides the compressor.

Big unloader valve (left) which is controlled by a small pilot unloading valve (right, below the pressure switch). On the left side of the big valve we see the compressor output (top) and the connection to the air tank (bottom)

Unloading valve problems

A lot of people come to this page because they are having problems with their unloading valve. A common fault is that the unloading valve stays always open, or the opposite: the unloading valve does not open anymore.

If the unloader doesn't open anymore, you will get startup problems. Plus, you will not hear the typical blow-down of compressed air when the compressor stops.

If the unloader stays open, you will hear a continues leak of compressed. I usually sounds like the pressure switch is leaking, because the unloader is often located on/near/inside the pressure switch.

Buy an unloader valve

Unloader/blow-down valves are pretty universal. Which means that you usually can easily buy a replacement valve from a different brand.

There are some things to watch out for: it must have the same physical connection for both the compressed air input, and for the connection to the pressure switch.