Glow Worm 30SI boiler flashes 3 bar when hot

[captain_pugwash]

Hi
I have a 30SI boiler that has started to exhibit a problem that it goes from 1 bar cold to flashing a warning of 3 bar when hot. I isolated the boiler then drained it down and connected my electronic pressure gauge/pump to the expansion vessel valve at the back of the boiler. It read almost zero so I set the pressure to 0.5 per the manual and it quickly fell back to almost nothing so I think it has perished (boiler is about 14 years old). Oddly no water comes out of the valve so not sure how it could fail and yet not pass water through?

The 30SI is a system boiler and sits next to my Ariston unvented water tank which has a Varem 18L white expansion vessel which is on the pipework right next to the boiler. The Glow Worm boiler manual does state that because the built in vessel requires complete boiler removal it can be easier to add an external one. I already have this but I notice on the Varem that the pressure is 3.5 bar so this would be no good to absorb the expansion from the boiler as it opens the PRV at 3 bar and of course then the boiler will leak and probably cut out when cold if I don't keep topping it up.

My question is can I reduce the pressure in the Varem unit to the 0.5 of the 30SI setting? Or is the 3.5bar required for the Ariston tank and if lowered may cause its PRV to blow?

Thanks

 

[Elkato531]

The white expansion vessel is attached to the potable water feeding the cylinder. Do not interfere with it as it is an important safety feature of the unvented system. The boiler expansion vessel may well be ok. It needs to be pressurised to approx 1 bar with the drain point on the boiler open.

 

[captain_pugwash]

thanks for the reply - yes I now see having taken a closer look at the pipework that the Varem EV is sitting on the cold water circuit to the unvented tank so nothing to do with the expansion of the hot water in the boiler circuit. I did have the drain valve open on the boiler when I tried to pressurise the boiler EV and it just wouldn't hold the 0.5 bar that the manual recommends - but was hopeful it was okay when no actual water leaked out when I pressed in the valve needle. I will see if it will hold 1 bar as you suggest.

I'm reasonable at plumbing and I have some space to put in a t section on the boiler return pipe just under the boiler so if the boiler EV is perished I wonder if anyone has experience of doing this - I guess I just match the capacity to the current one and ensure it operates at something under 1 bar?