Boiling immersion.

Is the heater at the top or bottom of the cylinder?

It's a conventional top entry one which goes most of the way down the cylinder. The only slightly unusual thing is the cylinder is tall and slim to fit the available space - although still a stock size.

My only guess is that once the boiling occurred, *all* the water in the cylinder gets forced into the header tank. But it can't have happened that quickly as it didn't overflow.

I see TLC sell thermostats with an extra cutout which are the norm these days - can they be retro-fitted to an older immersion?

I suppose that could be possible. 3kW wouldn't reheat the cylinder

Not sure on that one. Would be convenient if it does :-)

Once the vent is spurting water it circulates between the header tank and cyclinder. I wouldn't expect an overflow at all, unless the the thermal expansion of the entire volume of water is enough to cause it.

As to only tepid water an hour later, are you sure that immersion extends way down the tank? A tall thin tank and the immersion on the dome at about

Either that or there was still some circulation after you switched of the immersion and you noticed the boiling fairly quickly, so the header tank was still full of cold water. Check that there is a decent gap between the end of expansion pipe and the water in the tank to prevent gravity circulation.

AFAIK, yes :-)

The one I fitted "fitted":-) If you get my drift.

Have you tried this again under control to see if you did have a full cylinder of boiling water?

Adam

As I said earlier I had a decent shower out of it - then left it on, which is when it boiled. So it is heating a decent amount of water.