My mother's central heating had to be switched off for repair this weekend, so she switched on the immersion heater for hot water.
After a time (not sure how long) the water in the cylinder started to boil. It was clearly audible. The water out of the tap was scalding hot.
The cyclinder is foam covered and has a white cylinder thermostat strapped onto the front of the cylinder.
I apologise for the lack of technical information, but I know little about heating systems.
I am concerned because I would expect the water to stop heating at a specified temperature. If we had not heard the cylinder boiling it would have carried on heating. I am also concerned that even with the boiler now switiched on again that this could still happen.
ok, don't want to fear monger here but there was a recent news story about a similar thing but this cylinder/immersion heater combo did end up really hurting someone.
Immersion heaters usually have a temperature sensor in them and cut out when too hot. Sometimes they fail, sometimes they don't have them. Best thing to do is either get a heating engineer out to check it electrically or simply have him replace it.
Replace it yourself if you feel competent but please remember to turn off the electricity when you are messing around with the supply and to drain down the cylinder or you'll have a hole in the ceiling before you know it!
If you need help on how to do the job in detail just ask.
The thermostat you see on the side of the cylinder controls the water temp linked to the central heating boiler and has no connection to the immersion heater. Turn the element off and the water should return to normal.
The immersion stat is either adjusted incorrectly or faulty and needs to be attended to as it is very dangerous because if left to boil, the header tank may collapse as it did to a family last year, and scald people in rooms below. In fact, if I remember correctly, it killed a child in the bath.
The boiler and heating system will probably be ok, the fault is probably with the immersion heater - this should NOT be used, as the tank feeding the cylinder will probably be plastic - these are meant for cold water storage, and as the child was killed a couple of years ago when the platic tank overheated and split extreme care must be taken.