Hot supply question.

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.
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If possible I would go with the bung as if you forget it is there it will just blow out of the pipe instead of blowing up your house. If you forget to open the valve and the thermostat sticks a very bad thing can happen. I have seen what a 50 gal. tank will do when it pops and it ain't pretty. Better safe than sorry.
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Dale
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I assume the cold feed from the header tank (i.e. NOT the mains water feed) feeds into the bottom of the hot water tank ?
Why not put a stop cock in that pipe?, then you shouldn't have to drain down anything as there will be no pressure in the system to "push" the hot water out of the top of the hot water tank.
This is how my system is set up and it works fine.
Regards,
Paul. Leicester. UK.
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Xmas,
I've just re-read you OP and I think I may have misunderstood where your stop-cock is !
I've done a quick picture you can find at http://www.esna.org.uk/temp/water.bmp which explains what I have, this seems to work for me. If I do any work on the hot taps I just shut the stop-tap shown on the picture and it immeadiately stops any flow out of the taps (apart from the water in the pipes).
Regards,
Paul.
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