Exspantion Vessel pressure may be deflated causing the pressure to rise in system to over 3 bar then releasing hot central heating water out of the relief pipe(drain off) to outside.first thing to do would be to drain the heating down until the gauge on front of boiler is 0.get hold of a pressure gauge then behind the boiler at the top is a red vessel,and there is a valve(like a tire valve).make sure the gauge says its 0.8 bar,if not pump it up (with a bicycle pump)until you get that.im pretty sure its 0.8 but check your manual just to be 100%.once done fill back up then test.if thats all ok,check the relief valve if its leaking all the time,if so.......replace that.good luck
heatfactor
November 2007