The Expansion Vessel may have lost it's air charge, this should be checked using a tyre gauge on the vessels Schraeder valve, you will need to release the water pressure to zero first, by operating the boiler safety valve, the air pressure should be approx 1-00 Bar +/- 0.2bar, this will need pumping up with compressed air if low, close of safety valve and recharge with water to approx 1-00 Bar on system pressure gauge, this will most likley rectify the fault
www.elanar.co.uk, April 2009
