In an earlier contribution to this discussion,
John wrote:

I am by no means an expert on these things. But, AIUI, an unvented
hot cylinder has to have some trapped air above the water, to allow
for expansion when the water is heated - otherwise it would burst.

Have you blocked up your safety pressure relief valve then?


I assume that's not a serious question! I was giving a simplified
explanation of how these things work to someone who had never come across
one. I suppose I could have said "otherwise the pressure would build up to a
dangerous level". Obviously, the pressure release valve would open before
the cylinder burst in the case of loss of air bubble - but that only happens
in exceptional circumstances, and I was describing the *normal* operation.
--
Cheers,
Set Square
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