On Wed, 7 Oct 2020 16:50:48 +0100, John Rumm wrote:

Or you can sometimes put a rubber bung in the bottom of the cold
tank,
in the pipe that feeds the hot cylinder.

+1

+1

and another bung in the vent pipe will save needing to drain the content
of the feed pipe though the cylinder as well.

Or all of the pipe work above the tap in question. If air can get in
water can get out. Worst case is that it takes a while for air to
glug into the pipe work leading you into a false sense of securtity.
Then at some point the level drops below that required to glug and
the flow rapidly increases as the pipes empty...

--
Cheers
Dave.