Boiling immersion.

[Dave Plowman (News)]

I've been doing some CH mods so have it drained down. And decided to use the backup immersion for the first time in years. It's in a header tank fed open vented storage system. Switched it on about an hour before I wanted my shower which was fine and at normal sort of temperature - but it's a temperature controlled one anyway. Then sort of forgot and left it on. A couple of hours later there was a strange noise - soon realised the storage cylinder was boiling. And pumping water and steam into the header tank. Switched off the immersion. Carefully ran off some boiling water, Should have made tea, I suppose. ;-) About an hour later went to do the washing up. Only tepid water. Where had the tank of near boiling water gone?

[Andy Hall]

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

[Dave Plowman (News)]

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?

[Andy Hall]

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

*that* quickly.

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

[Dave Liquorice]

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

45 degress angle would perclude having a really (30 or 36") long one...

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.

[Andy Wade]

AFAIK, yes :-)

[ARWadworth]

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

[Dave Plowman (News)]

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.