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?

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.

Holly