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The diaphragm can fail, hence filling with water and no longer doing it's job.
@user69121 The expansion vessel has a Schrader valve that I pressed and air escaped and not water. The valve is at the bottom.
@Bobbo Yeah this is at least 10 years old so was thinking replace the vessel and the over flow valve, for peace of mind and hassle free hopefully.
@Mr_Sworld It does have a valve, all I have here is a track pump that is now broken. So new pump.
lynx-
The expansion vessel has a Schrader valve that I pressed and air escaped and not water. The valve is at the bottom.
Sounds like the diaphragm is OK then, or you would have got water out.
How much air pressure is the important part.
With water pressure on, but heater switched off, run a hot tap for a few seconds to bleed off any over pressure.
Check the air pressure on the schrader valve. This will be the same as the water pressure/prv pressure on the other side of the diaphragm.
Switch off the water and run a hot tap. Repressurise the air to the same as the water pressure noted previously.Turn on the water and heating again.
Any expansion above incoming pressure has maximum volume available before relieving.
user69121
I have heard that the expansion vessels can fill up with water. Not much help I know but it's pretty much the sum total of knowledge in that area!