Do you have separate tanks--one for the domestic hot water and the other a smaller header tank in the loft?
If the header tank is back filling and overflowing the chances are that you have an indirect cylinder Big one in airing cupboard--this has the heating coil inside as part of the central heating system it transferes heat from the central heating curcuit to the domestic hot water and is kept topped up by the header tank . Its likely that the coil is leaking if the water level in the domestic hot water tank is above the central heating tank it will back fill via the coil and try to even out the water level and overflow the header tank in the process. Fix is a new cylinder.
tom, December 2009
