On 17/08/2018 07:50, The Natural Philosopher wrote:

Once the cold feed to the cylinder is shut off, you will still get
several litres of hot water out. However let it run and that will
reduce to nothing as the pressure vessel de-pressurises.

Until someone flushes a loo...

then the loo will allow air into the cold side of the systemÂ* and the
hot water will all come out.

Not in my experience, but in theory the head of water within the pipes
could push a small amount of hot water out of the cylinder. There's
nothing wrong with TNP's theory, so I guess it's stymied by anti-syphon
valves in the toilet cistern.

Just to be on the safe side and to avoid an argument with TNP, leave the
hot tap open and flush all the toilets in the house before changing the
washer.

My 2p-worth: turn the boiler off today and change the washer tomorrow!