AlphaCB28-Overheat/Pressure Drop

I have what seems to be (judging by lots of posts on here) a fairly common problem with my Alpha CB28 combi boiler. Within minutes of the CH starting, the needle on the pressure dial goes up to in excess of 2.5/3 bar before the relief valve operates and sends excess water out of the pipe outside. This means that when the boiler is cold again there is not enough pressure to operate. Have re-pressurised etc but realise there is a problem which needs sorting. From what i have read the expansion vessel needs attention, and maybe also the PRV. Is this something that I may be able to do myself, and if so how ? If not, how much should I expect to pay for a pro to sort it out ? Any info would be gratefully received. Cheers.

Doesn't sound like a PRV fault, although if the pressure were to get so high that it operated then you may well end up replacing it.

Certainly does sound like EV fault. If within the boiler housing then you might be wise to let a heating engineer do the work, although it's not illegal for you to tackle it if you're not letting the house.

Or, a handy alternative is to ignore the faulty EV and fit a new one where it's easy for you to get at and to maintain in the future.

If outside the boiler casing then it's more straightforward - just replace it.

I have the suspicion that you have not repressurised it properly in accordance with the instructions in the FAQ section of this site!

Did you pressurise to 0.9 Bar with the relief valve or a bleed screw open as you did it???????????????????

If so did you then repressurise the system to 1.5 Bar AND see if the gauge on the foot pump went up to 1.5 Bar. ( Thats not mentioned in the FAQ but checks that the feed tube to the EV is not blocked. )

Tony Glazier