"Holly" wrote in message
...

IMM wrote in message
"Holly" wrote in message

re valve:
One can be fitted on the draw-off pipe.

That might actually be quite a good idea.

If the cylinder boils the expansion will go backwards

expansion go backwards??!! but I know what you are getting at....

and the anti-vacuum valve will open to prevent implosion.

so why wouldn't it either not expand further in the first place when
it reached that state, or simply suck the air/water/steam back in
again??

When it boils it has to go somewhere, so it goes back up the cold
feed. With no water replacing the water moving back up the cold feed

When the pressure ceases to be a problem the water why wouldn't the
water just stop moving back up the cold feed?

That is where is expands to, back up the cold feed. UK cylidners can't
expand up there, and have either: aa air pocket that requires periodic
re-instating or an external expansion vessel.

the cylinder will implode.

I'm not saying that these cylinders can't or don't implode, just can't
see why they would in this situation.

If water boils and it expands up the cold feed, which is at the bottom, the
water requires replacing from the top, otherwise implosion. What prevents
this is an anti-vacuum valve. The passage the water expands out of in an
overboil situation ideally should not be the same passage water enters the
cylinder.

If the cylinder is indirectly heated via a boiler there are two levels of
protection on the boiler side: the cylinder thermostat and the boiler stat.
If a cylinder stat set to 55c fails the cylinder will heat to the boiler
stat of 82C. Only if both fail will it overboil. An immersion has only one
safety level, the immersion stat. After that the cylinder own controls have
to provide safety.

In your situation, at the very least I would install an anti-vacuum valve on
the draw-off directly above the cylinder.