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
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