"Christian McArdle" wrote in message
...
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?


Note that only some unvented cylinders require this. I believe Heatrae
Sadia
Megaflo is one. Other designs use conventional pressure vessels.

Christian.