Sounds like you need to get the expansion vessel recharged with a foot pump, the boiler pressure needs to be at zero before you do this though. Not a job for a novice unless you know where to locate the vessel and how to drain the water off first. Depending on which boiler you have, the vessel is usually located at the top and down the back of the boiler and has a valve the same as a car tyre, drain the boiler down to zero pressure, attach a foot pump with a long hose and pump to between .5 and 1 bar, remove pump. repressurise boiler and this should cure problem. If water comes out of the vessel when attaching the footpump to the valve then the vessel has burst and needs to be replaced. Most of the time they only need recharging though. Hope this helps.
Taliskerman
March 2011
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