Boiler de-pressurising, no leaks visible

Mr.Generous

once the expansion vessel loses pressure it is unable to regulate the boiler pressure so it goes up until the PRV (pressure release valve) lets it out. If this is the problem you will see boiler pressure go from about 1 bar or less to over 3 when you switch on heating.

It will need a new expansion vessel (although you could try re-inflating) and a new PRV and probably a few washers. Its usually easier to take the boiler off the wall to do a vessel change, and 2 men usually needed. Cost me £170 to get this done on a Worcester Bosch Greeenstar recently.

Edvin_Paus

That's what it cost me roughly too for a combi boiler.