Vaillant EcoTEK 831 plus - Overpressure

[Ren_egg]

Hi All, I recently had an issue where the boiler turned off due to low pressure in the CH circuit. I have since found that the pressure vessel diaphragm had failed so I put in a new one and pressurised the air side to 1 bar. This has not fixed the issue. When the boiler is heating it will go up to 2.8 bar and occasionally 3 bar and discharge out of the PRV - that explains the low pressure problems that follow.

I've set it to a low starting pressure and it's getting me by, but I'm quite confused as to why the pressure is rising so much. Although the pressure vessel may be undersized for the system this problem didn't occur for the last two years. It will also happen with only 2 rads operating. I don't think it's the filler loop valves leaking past as the pressure never rises when it's cold. Is there a way that the mains pressure is leaking in only when the boiler is heating?

 

[CBW]

Stop messing around with your boiler, it can be dangerous. Did you replace the membrane and put air in or replace the vessel and put air in?

 

[Ren_egg]

A complete vessel replacement. I'm not messing around with gas or electricity, I can handle replacing a system component on the water side.

 

[Ren_egg]

I'm thinking of closing the stop cock and opening a basin tap to depressureise the mains coming in then and running the boiler (at a normal starting pressure). That would rule out water coming in through the mains side.