Adding a second Expansion Vessel?

[Duker996]

I was at a job looking at an unrelated issue when the lady mentioned her blow off keeps running outside. Apparently the last plumber replaced expansion vessel(and checked the precharge was 3bar) on the cold feed into the cylinder, 3bar pressure reducing valve and 6bar blow off in the hope to cure it.

The cylinder temp/pressure relief valve, htg relief valve and hw relief valve are all connected into one 22mm pipe. The cylinder valve has a tundish which isn't showing any signs of water, the htg pressure stays well below 3 bar and when disconnected shows no sign of letting by.

So it's all pointing to the kit on the mains pipe feeding the cylinder. I didn't take note but I want to say the vessel is around 24l and is positioned about 4 metres from the cylinder. Is it a case that an extra vessel may be needed as it's over pressurising?

 

[Harvest Fields]

If it is unvented. You need to be G3 registered.

 

[Sharp Point]

You’re obviously not trained up on unvented, so don’t touch it and get someone to check it over who’s G3 qualified.

 

[Best]

The vessel supplied with unvented cylinder (or identical replacement) will be correct size for the unit and nothing else needs added.
Something else is faulty and it could also be elsewhere in the cold and hot water system in the house.
As said, don’t touch anything unless you have the unvented qualification or you could get the blame

 

[chris watkins]

Why not go do the unvented course? it is only one day off work (with some home study before hand) then you would fully under stand how all the parts go together & why.

 

[Duker996]

At which point did I say I don't have the G3 qual? I do have the qualification and understand exactly how all the parts work. I have just never encountered this problem before so asked for advice, which I'm never too proud to do. Don't be so quick to judge chaps!

I think you may have nailed it Best, turns out the issues started after they had a new bathroom fitted so my guess now is no check valves on the feed to the new shower.

 

[Best]

At which point did I say I don't have the G3 qual? I do have the qualification and understand exactly how all the parts work. I have just never encountered this problem before so asked for advice, which I'm never too proud to do. Don't be so quick to judge chaps!

I think you may have nailed it Best, turns out the issues started after they had a new bathroom fitted so my guess now is no check valves on the feed to the new shower.

Maybe no balanced cold then?

 

[Duker996]

It definitely wasn't taken off the same reducing valve that goes to the cylinder, so unless they installed a separate one (which I doubt) my guess is the mains cold water is pressurising the hot.

Never encountered it before.

 

[SJB060685]

Like Best and you say Duker I’d be suspicious of something downstream back pressurising. When I used to do site work the guys used to put one PRV under sink on incoming and then obviously the one in cylinder cupboard with balanced draw off. Some people even install pressure gauges so you can see exactly what’s going on

 

[chris watkins]

A vessel installed 4M away from the cylinder is very unlikely to be the right side of a check valve if it is on the cold feed.
Could it be there for another reason other than expansion ?
I diagram would be useful as I am struggling to understand what is going on from your description Duker996.