Wosgarnon?

[ChrisR]

2 year old WB combi. Pressure gauge sits at about 2.7 bar, always has been and doesn't move much at all. No leak from the prv. Air from the expansion vessel when you poke the valve.

(Yes I know the answer!)

I was only there because of a blocked trap, see other post, not relevant.

 

[chrishutt]

Overpressurised expansion vessel. If precharge pressure was 2.7 Bar no expansion would be absorbed by the vessel until the system water had reached that pressure, which it would do quite quickly on heating due to lack of expansion space below the precharge pressure.

 

[ChrisR]

Yes It was, first time I'd seen one that high from new. But now you've opened a can of worms...

no expansion would be absorbed by the vessel until the system water had reached that pressure

All the system water would be at that pressure, even when cold.

lack of expansion space below the precharge pressure.

There isn't any such space.

Point is that the expansion vessel is FULL of air, so there's plenty of space for the water to expand into, so the pressure remains much the same.

It does beg the question of how it came to be the way it was. If the installer filled to 1 bar, then what happened?

 

[chrishutt]

All the system water would be at that pressure (2.7 Bar), even when cold.

Would it? When filling system from empty, the pressure gauge rises to say 1.0 Bar. This is sufficient pressure to displace air in the system and operate any water pressure switch.

So we start with the cold pressure being say 1.0 Bar. and no air in the system except that in the expansion vessel which is on the other side of the diaphragm and at 2.7 Bar. The diaphragm would be pushed against the wall of the expansion vessel by this precharge pressure, so negligible system water (or air) would enter until system pressure rose to the same level.

On heating up of the system water (assuming no air remained in the system) expansion could not be accommodated so pressure would rise rapidly until the 2.7 Bar pressure was reached, at which point the air within the expansion vessel would start to compress and absorb any further expansion, so system pressure would then stabilise.

On cooling, the system pressure would surely return to the 1.0 Bar level. This does not appear to be the case in your example so I assume that the system had also been overpressurised, or......

Of course another explanation for the whole phenomena might be that there is a mains leakage into the system with mains water pressure itself at 2.7 Bar hence no increase to trigger the PRV.

 

[ChrisR]

As I said, the EV WAS overpressurised and the system had only ever shown "about 2.7" bar.