I have a cold water tank in my attic that keeps filling up and over flowing, with warm water from my HW storage tank which is just below it. I have had a new boilerstat fitted to my combi boiler, a new storage cylinder, new ball cock. also my central heating overflows into the expansion tank please any ideas? The only thing I have had changed befor it all strated to go wrong was an extension with three small radiators and a shower fed by the storage tank all at ground level the storage tank is up on first floor. 
