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When needing to plug a tank outlet, usually find it is hexagonal or internally ribbed so rubber bung is ineffective. What's needed is a mushroom shaped stopper where stem would locate in hole and dome could be pushed against outlet with length of plastic pipe braced off opposite wall of tank.
I won't trust a gate valve, half the time they don't shut fully anyway
Reply to the thread, titled "turing of gravity hot water" which is posted in Bathroom Advice on Plumbers Forums.
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