My boiler needs re-pressurising daily. I have checked the expansion tank @ 1 bar. I cannot find any leaks. I have installed liquid leak sealers into the system and I still have to fill the system every day.
if its that old the diaphragm in the expansion vessel is probably ruptured, any water coming out of the schrader ( air ) valve.? if so it's knacked.( answer above is also correct)
richard ( heating eng'n'r ), January 2009
Unless you depressurised the radiators/boiler the expansion vessel pressure will be the same as the system pressure. The expansion vessel should be measured with zero system pressure reading on the boiler front panel. The water must be going somewhere. Either you have a leak or it is going out past the system overpressure valve. Does the system pressure go up to 3 bar when the central heating is hot? (this will cause loss of water through safety valve). If so recharge expansion vessel.
