French Pressurised Hot Water Cylinders - fitting in the UK

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.

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.

Third and last try at this question. I now understand how UK cylinders work. I now see the problems and possible solutions to the situation we have here in our house. I'm just questioning your theory here. In the case of the cold feed being open, when the pressure ceased to be a problem why wouldn't the water just stop moving back up the cold feed?

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I still think this is nonsense, "it's the law of physics Jim" but I do accept that I could be wrong. Go on, convince me, when the pressure ceased to be a problem why wouldn't the water just stop moving back up the cold feed?

Holly