Pete wrote:
I've just turned off the hot water at the stopcock in the airing
cupboard (one of those red-wheel jobs.

gate valve

Something has broken, and it won't turn back on again.

ought to open and close it once a month to avoid such issues.

I've removed the wheel, and the centre stem is just turning endlessly
without pulling the shutter (don't know the right terms!) back and
openening the pipe.

No hot water tonight is going to be popular. Is there a way forward
without draining the whole system down?

not really, new gate valve is needed. couple of hours at most, including
nipping down to a shed to get the gate valve.



RT