"Set Square" wrote in message
...
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.

It was "serious" insofar as the pressure would not rise until the cylinder
burst because the (3 bar) valve would protect it. You and I know it would,
but the uninitiated might have considered your response as a true statement
of fact and run screaming down the road whenever they saw an unvented
cylinderg

Unvented cylinders are multilevel protected - Back up limit thermostats,
Over pressure safety pressure relief valves, Over temperature relief valves
etc. The biggest problem is keeping them running not stopping them