overflow pipe?

If the overflow is from the small central heating tank in your loft then the problem may well be caused by a split in the heating coil of the Hot Water Storage Tank in the airing cupboard.

Normally isolated from one another a hole in the heating coil connects the main storage loft tank to the small header tank. The water now conncted via the split tries to find its own level - the big tank tries to fill the smaller one - water level in the large tank drops but its ball valve lets more in - if you turn off the flow into the big tank and the two tnks eventually settle at the same level - this will be your problem.

Peccavi
February 2010