Tony Williams wrote:

BTW: My old plumbing book has a gruesome tale about
a plumber who blocked a vent pipe with a cork, to try
and reduce the amount of water lost during a drain
down. The resultant vaccuum caused parts of the
plumbing to collapse inwards. Blocking a vent pipe
may not be good news.

Good point sir! Probably the copper hot water storage cylinder, which
will be crushed by atmospheric pressure if you were to drain it without
relieving the vacuum (hose out bathroom window, no hot taps open).

On unvented hot water storage cylinders the mandatory T&P relief valve
also acts as a vacuum breaker valve (I think, goes off to get book to
check).