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-cock 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 cock, 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 cock 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.