On 17/08/2018 11:17, NY wrote:
"The Natural Philosopher" wrote in message
news
On 16/08/18 22:21, Andy Burns wrote:
John Angus wrote:

Surely there is a valve on the output of the pressurised cylinder
that stop hot water coming out of that that can be used to change
a hot water tap washer ?

If you turn off the mains pressure cold going into the cylinder,
there will be no hot water coming out of the cylinder ...

That depends on where you turn it off.

Critically, on whether there is a possibility of unsealing it
downstream of where you have switched it off.

No. Once you have turned off the cold water supply somewhere, the only
water that will come out is in the expansion vessel and in the pipes
above. Once the pipes/expansion vessel are empty, there is no way that
any more water can come out of the cylinder because the outlet is at the
highest point.

Hmm

There are 3 forms of unvented system where both the primary and
secondary systems don't have a header tank.

1) Combi boiler.

2) Unvented/pressurised cylinder.

3) Thermal store.

With all 3, simply turning off the cold water feed stops any flow
through the hot water tap. [1]

There may be an expansion vessel to stop back return onto the cold
supply, that will then need a pressure relief valve, which of course is
mandatory for the Unvented/pressurised cylinder.

If the OP has another system then all bets are off! Unvented can mean
other types of system too.

[1] I have seen a standard tank collapse where the cold feed and
expansion were frozen, causing a partial vacuum in the tank to collapse
it. For the more informed, is there any mitigation of this for 2 and 3?
Are tanks so designed to not 'implode'?