Adding a second Expansion Vessel?

Duker996 · Mar 6, 2019

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 · Mar 6, 2019

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

Sharp Point · Mar 6, 2019

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 · Mar 6, 2019

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 · Mar 7, 2019

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 · Mar 7, 2019

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 · Mar 7, 2019

Duker996 said:
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 · Mar 7, 2019

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 · Mar 7, 2019

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 · Mar 7, 2019

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.

SJB060685 · Mar 7, 2019

chris watkins said:
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.

Good point Chris, I overlooked that

Harvest Fields · Mar 7, 2019

Duker996 said:
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.

You didn’t say that you was G3 registered either!

Best · Mar 8, 2019

Photos would also be good.

JasetheMace · Mar 8, 2019

chris watkins said:
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.

Maybe it's a megaflo?

SJB060685 · Mar 8, 2019

It’s a Megaflow, don’t they have an internal air gap?

JasetheMace · Mar 8, 2019

SJB060685 said:
It’s a Megaflow, don’t they have an internal air gap?

That would be my guess! If expansion vessel is 4m away, more likely to be heating expansion vessel rather then hot water.

chris watkins · Mar 9, 2019

JasetheMace said:
That would be my guess! If expansion vessel is 4m away, more likely to be heating expansion vessel rather then hot water.

The OP has their G3 so I would expect them to be capable of knowing the difference between heating & hot water vessels.

Even if it is a Megaflo with an internal air bubble for expansion addition vessels can & are often added, for instance when the baffles has failed or there is a secondary return installed which increases the amount of water being heated.
The worry here (as far as I can tell from the description) is that it has been installed on the cold feed 4 Metres away from the cylinder so it is very likely to be on the wrong side of the check valve which is built-in just before the pressure relief safety valve. If it is then this will isolate the vessel from the source of expansion so it can't work & the pressure will continue to cause the safety valves to open!!

Duker996 · Mar 10, 2019

All sorted now chaps.

As Gary said it was due to not having balanced supplies at the new shower and them clearly not fitting check valves. There was no way of me getting to any of the shower pipes so a 3bar pressure reducing valve on the incoming main has cured it.

No idea why the vessel, relief valve, reducing valve etc was placed in a small roof space 4 metres away but it is all correct and how it should be, no check valve between the vessel and cylinder.

First time I've seen this issue in my 19 years plumbing, hence why I was a bit baffled by it!

Thanks all for the help

Adding a second Expansion Vessel?

Duker996

Harvest Fields

Sharp Point

Best

chris watkins

Duker996

Best

Duker996

SJB060685

Plumbers Arms member

chris watkins

SJB060685

Plumbers Arms member

Harvest Fields

Best

JasetheMace

SJB060685

Plumbers Arms member

JasetheMace

chris watkins

Duker996

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