I have an overflow pipe dripping at the back of my house, on investigation, there is hot water coming into the expansion tank in the loft from the down pipe that feeds the boiler. I have blanked off the main water supply to the expansion tank and yet the water level in the expansion tank still rises. I've notice that if I turned off the water supply to hot water cylinder the water level on the expansion tank remained stable.
Any idea on how to fix this problem? The make of my boiler is Myson Marathon 1000B.

Steve Tam, January 2008

Thought the problem was solved! Same thing happened tonight anybody got any other ideas???

M. McKnight, February 2008

I am having the same problem and am on here trying to verify a solution I have just got of a friend. Apparently it is a problem with air in the system which causes the heating system to push water into the expansion tank. Symptons are banging/rattling pipes/radiators, which I have noticed. Solution I have been given is to shut down all the radiator starting from the furthest from the bolier with the exception of the 1 closet to the boiler and to bleed the radiators working away from the boiler. I am going to give this ago tomorrow will post the results. Hope this helps!

M. McKnight, February 2008