the big tank is your cold water store either just for the hot water or hot and cold (not kitchen sink cold though and possibly not for bath/basin either). have you had any taps changed for mixers lately? only reason i ask is that if the mixer mixes in the body of the tap rather than at the point of exit AND you have cold mains and gravity hot at that tap then there is a chance the cold mains is forcing its way back up the hot pipe into the hot cylinder which in turn forces the hot water back into the cold store tank in the loft - would explain why the tank is filling up and why its warm...
its just a thought - without seeing it its difficult to say for sure..
random man, December 2007
