I have what I suppose is a standard hot water supply system. That is, there is an electrically-heated copper hot tank gravity-fed from a cold tank, and there is a stop-c*ck in that supply. The hot feed to the taps exits from the top of the hot tank, and the feed is branched off to an expansion pipe which exits into the cold tank. Presently, if I want to do something simple to the hot supply (e.g.change a washer) I have to close the cold feed c*ck, and drain down the hot tank, which is a pain.
My question is this: can I put a stopcock into the expansion pipe to prevent air ingressing when doing this trick, thus allowing only a small amount of outflow until pressure is equalised? There is an obvious danger that the c*ck could be left closed, leading to an explosive pressure build-up, but are there regulations to prevent this being done? (I'm in the UK). What is the alternative, apart from living with the inconvenience? The expansion pipe is not easy to get at, at its outlet over the cold tank, if sticking a bung in is one answer.