Is the Schrader valve on my expansion vessel faulty?

Mar 30, 2012
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Something weird is happening. I place my pressure instrument on the Schrader valve of the expansion vessel to measure the pressure inside the vessel. Reading: Nil. I repeat the exercise with another pressure instrument. Reading: Again Nil.

How can this be? because I am certain that there is at least 4 bar in the vessel.

Are my pressure instruments defective? Certainly not, both instruments show the exact pressure in the tires of my car.

I have a small black and decker air compressor. You type in the desired pressure you want to inflate to and then hit start. The air compressor starts up and when it reaches the desired pressure it automatically shuts off.

I set the air compressor to 3 bar, attach it to Schrader valve of the expansion vessel and then press the start button on the compressor. The compressor temporarily starts up and then immediately shuts off displaying that the pressure in the vessel is already at 4 bar.

So there is 4 bar pressure in the vessel. Why do my pressure instruments read Nil?

By the way, when I depress the needle in the middle of valve, it goes in just fine but no air comes out. Consequently I cannot reduce the pressure in the expansion vessel even if I wanted to.

Has anyone encountered this type of problem? Is this typical behaviour of a valve?
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Feb 4, 2011
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Assuming the vessel is fitted to a C/H boiler inside the pressure vessel is a diaphragm with water at one side and air at the other side.
If you loose the air the diaphragm moves across leaving very little space for air. This usually causes water to come out of the PRV and through the wall when the system is hot, and for the water pressure to keep falling when the system cools.
To set this up you need to drain some water out of the system and reduce the system pressure to zero then pump up the schrader valve to around .8 of a bar then re pressurise the system to around 1 Bar.
If the diaphragm is leaking your problem will return but if it is leaking usually water comes out of the schrader valve when pressed in.


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