Hi,

Thanks for your question. You are right, the orrifice in the filter housing is the vent of the blow-off valve.

When the compressor goes into 'unload mode', the internal pressure is blown off. This is done by the blow-off valve.

Sometimes the blow off valve is integrated in the unload valve (most older Atlas Copco compressors for example), sometimes it is a separate valve.

In your case, it is a separate solenoid valve. The Ingersoll Rand UP6 has a solenoid valve for load/unload and a solenoid valve for blow-down (but they are electrically actuated at the same time).

After blow-down (and with running, unloaded compressor), there is a tiny bit of air drawn in by the compressor element. The same ammoount of air is blown-off through the blow-off valve. So a balance existst inside the compressor.

Now, to ensure that only the compressor is blown off, and not your complete compressed air system, there is a check-valve installed near the outlet of your compressor.

This check valves makes sure that compressed air from your system can not flow back into the compressor ( and blow -off).

Now, there is one more thing to know: your compressor always needs a little internal pressure. To ensure there is always a little internal pressure, the check valve I talked about before has an extra spring, which ensures a minumum pressure inside the compressor. Therefore this valve is called the minimum pressure valve. (

And there is your problem: your minimum pressure valves is leaking. At unloading (blow-off) air is leaking back from your compressed air system into the compressor, and is then blown-off.

The blow-off valve can be found on the side manifold block where the oil filter and oil separator are installed if I am not mistaken.

Please see attached screenshots of the manual/partlist.

Good luck and let me know how it works out.