Strange boiler pressure loss scenario

[emjayjay]

Hi there. We have a Vaillant system boiler which has recently had a new expansion vessel and PRV. However, we still have some kind of pressure issue. The pressure remains pretty constant with daily checks and similar usage patterns (typically 1.4 or 1.5 bar) for about 4 weeks, but then suddenly drops right off and errors. This has happened for 2 months in a row - constant pressure for 4 weeks and then suddenly pressure loss. We are also monitoring the PRV exit pipe and there is no water loss. We have no visible water leaks and even if we did would expect gradual pressure loss over a period of time. Any ideas? Thanks!

 

[jurassic sparkk]

Isolate both flow and return from the boiler and inspect the pressure for the next few days.

If it drops pressure it's the boiler that would need looking at.
If it remains the same then you probably have a small weep on the pipework.

 

[CBW]

As above, could be leaking down the main heat exchanger.

 

[emjayjay]

Thanks all. The perplexing part is that the pressure is fine for about 30 days (checking daily) and then collapses on day 31!

 

[Johntheo5]

Thanks all. The perplexing part is that the pressure is fine for about 30 days (checking daily) and then collapses on day 31!

If the precharge pressure and filling pressures are the same then there can be a very rapid fall in pressure but I wouldn't expect it to collapse from say 1.0 bar to 0 bar, if it only takes seconds (say 10) to top it up then suggest getting the precharge and filling pressures checked, the filling pressure should ideally be 0.5bar higher than the precharge pressure, a 10L EV will then have a 2L water reserve and if the pressure falls from 1.5bar (cold) to 1.0bar and if you lose this 2L in 30 days then you certainly have a problem.