Water meter and combi boiler back pressure

10 Aug 2016
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United Kingdom

I have a Worchester 440cdi combi boiler which regularly dumps water out due to over pressure.

- There is 3.3 bar pressure at the property.
- There was no pressure reducing valve fitted to the inlet of the boiler.
- There was no external expansion vessel on the inlet of the boiler.

I fitted a pressure reducing valve to the inlet of the boiler. I closed the mains water, released all the pressure by running a tap. I then closed the the inlet and opened the mains, and set the valve to 1 bar.

As soon as i open the inlet to the boiler the pressure on the reducing valve starts to increase until it reaches 6 bar.

I assumed this is down to back pressure so I added a 8 litre expansion vessel pre-pressurised to 1.6 bar. Here is what i have now:

Mains in (water is metered) ----> pressure reducing valve (set to 1 bar) ----- [Expansion vessel] ------ Boiler inlet.

The same thing still happens though. As soon as i open the inlet to the boiler the pressure gauge on the reducing valve increases to 6 bar. If i run a tap it gradually reduces back down to around 1.7 bar, but when i close the tap up it goes again.

Any ideas what could be going on here?
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I forgot to mention with the boiler off and the system cold the exact same thing happens. I can't understand how there could be a pressure build up of 6 bar when nothing in the system is hot or pressurised to that level. Any ideas?
I think your confusing your mains pressure and the boiler system pressure, your boiler should have either a built in filling loop and gauge or external filling loop and gauge, you open the filling loop till the gauge reads 1 bar and then close it, hardly surprising the gauge is going up and down if you have left the filling loop open.

No, i'm not talking about the boiler system pressure. This is all about inlet pressure.

There is 3.3bar pressure from the water mains. There is a water meter which means there is a not return valve in that meter at the mains.

There will be back pressure from the boiler which is supposed to be absorbed by the main, but due to the Non return valve this can't happen. Advise is to fit a expansion vessel on the inlet to absorb that pressure which is what I have done. But why is there 6bar+ back pressure still? It's possible the expansion vessel isn't large enough but on a cold system that's not even heated yet, this doesn't make sense.
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Its physically impossible to have 3.3 bar incoming pressure and 6+bar without heating the water, where and what type of gauge are you using ?
Exactly :/

Ok so i have:

Mains (3.3 bar with water meter) ----> A ---> Boiler inlet

A is a honeywell pressure reducing valve set to 1 bar:


1) turn off mains
2) run tap to empty the boiler and inlet pipe - the pressure at A drops to 0
3) close boiler inlet.
4) Turn on mains and pressure at A is steady at 1 bar (which is what it's set at).
5) open boiler inlet, pressure at A increases till it reaches 6 bar.

All water is cold and boiler is OFF.

What's going on :/
Oh and I closed the CH flow and return and repeated the above to isolate that as the source of the pressure (unlikely as it is), but the same happens. With the boiler off where on earth can that pressure be coming from?
Yeah definitely the right way round. The flow indicator is left to right (left being the water mian in and right being the pipe to the boiler).
You have adjusted the prv the wrong way, its now on maximum, turn the adjuster fully in the opposite direction and follow the instructions that came with the valve, you must have at least 6 bar coming into the property, now wait for the "dynamic flow " posters that will be along shortly.:)
Nope it's 100% the right way round. Double triple checked. But I think i may have found the issue.

Mains -> pressure reducing valve ---S----> boiler inlet.

S is a sink that sits between the mains and the boiler (we're talking 1 meter from mains to S and 2 meters from S to boiler).

Hot water is not working in the rest of the house since this problem. But at S and only at S i get steaming hot water coming out of the cold tap.

It looks like the heat exchanger may have a leak in it and is mixing the heating water with the mains water?
If that were the case the boiler gauge would show 6bar, I did not say the prv was the wrong way round I said you have adjusted it the wrong way and its now on max, have you replumbed your kitchen or moved any pipes since putting in the prv ?
No other pipework has changed.

I adjusted the PRV to max (pressure) which is minimum (turn) on it to try to take the PRV out of the equation as it was working before i added the PRV.

I removed it totally and everything works as normal again :/

I have no idea what is going on here. Any ideas? I guess the PRV was acting like a non return valve due to the pressure on the boiler side - is it possible the expansion vessel was not large enough to handle the back pressure?

I have about 1000m of underfloor heating being run off the boiler, and a 8L expansion vessel.

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