My 1980's house will be empty for six weeks this winter and, due to its age and the fact that I've had to adjust and/or replace toilet cystern valves to stop overflows over the past few years, I'm considering turning off the water at the mains but leaving the heating on low. I have a 2-year-old "British Gas 330" condensing boiler supplying radiotors and hot water, which is fed by a header tank in the loft. My domestic water supply is also via another tank in the loft -- not direct from the mains. My question really is, how much water does the central heating system use, assuming no leaks? If I turn off the water, the heating system will have it's header tank full and so should last for six weeks, shouldn't it? If it does empty the tank for some reason I'm confident that the boiler's control system will switch it off. What are your views/advice, please?