I have a 5 year old Halstead Ace High boiler recently when the central heating is running the pressure has been rising from 1 bar to 3 bar within the first hour of the central heating coming on. The pressure is releasing through the prv like it should do, i have no visible leaks and have checked the expansion vessell which is charged to 1 bar. it seems that the boiler is only happy running from a static 0.25 bar which isn't right, a mate of mine seems to think i need a new expansion vessell but when the pressure is released from the expansion vessell through the charging valve there is no evidence of water contamination.has anybody got any ideas, thanks

dave, February 2009