Scalding/dangerous hot water from immersion heater

24 Oct 2010
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United Kingdom
We moved into our new home yesterday (1999 build) and found there was no hot water. In the end we contacted the previous owners who informed us the heating is ran by the boiler (working fine) and the hot water is ran by the immersion heater which they left swotched constantly on.

We turned on the immersion heater (top part, as there is a top switch and bottom switch). A short time later we had hot water - yey!

However, at 2am, there was loud bubbling noises coming from the tank and the loft. I immediately turned off the immersion heater and found:

1. Pipe coming from top of immersion too hot to touch.

2. In loft the tank was bubbling like a boiling kettle and steam was coming from it.

3. When i turned on the hot water, it was scalding hot - no kidding, i couldn't touch it (you could have made a cup of tea with the water!)

I drained off some of the water through the taps and fortunatley we are all still alive this morning. I cannot find a thermostat anywhere, so can anyone advise what has happened? Do we need to get an engineer out?

(sorry for questions, but previous owner has not left any instruction manuals for system!)

Here are some pics of the system:
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TURN OFF THE IMMERSION HEATER NOW !! dont use it until the thermostat has been changed, the water in the cylinder is boiling which is filling the tank in the loft with boiling water, run the hot and cold taps until the water runs cold and dont use it again until its fixed
Switch off the top immersion heater, and look for a control for the lower one. It may only be a fuse or MCB in an off-peak CU / fusebox, and turn it off / remove the fuse. That should remove the immediate danger.
Edit - just noticed the switch apparently for the lower immersion in your photos - turn that off now.

It looks like you have two immersion heaters fitted in the cylinder. Normally, the lower one is run on cheap rate (Economy 7?) electricity ro heat the cylinder overnight, and the upper one is just used as a boost, switched on for an hour or so if you run out of hot water during the day, not left on constantly.

From your description, one of the heater thermostats has failed, so one heater (or maybe even both?) are running constantly.

Further investigation depends on your DIY skills and kit. Do you have a multimeter?
Do you feel confident to use it?

If not, an electrician may be able to help you out.

All replacement thermostats now include a cutout to help stop this situation arising.
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OK, at 2am, i switched off the top switch due to the problem. This morning we had no hot water. I then stitched on the bottom switch and a few hours later we have correct temperature hot water.

Should i leave the bottom one switched on and just use the top for quick bursts? Or do you think there is still a problem with the temperature control?
get both thermostats in the immersions replaced with new ones so that they will incororate an overheat stat. They're often easily replaced. It could save a life, no joke.
Secondly, who in gods name decided to put gas central heating in and a direct cylinder?? Why not save up to get your cylinder replaced with an indirect and piped to work from the boiler. It will save you money long term in electric.
....who in gods name decided to put gas central heating in and a direct cylinder??....
There appear to be two unused cylinder tappings just about where you would expect the primaries to be. If those tappings look like open holes in the cylinder the OP may have an indirect cylinder, just one not connected to the heating circuit. if they are plugged, the chances are it's a direct cylinder.

The costs of electricity versus gas make further investigation well worth while.

And, in the mean time, DO change those thermostats!
They are under the blue covers on the immersions. Turn off the power to the immersions at the switches and preferrably at the fuse board / CU as well before opening them.
Secondly, who in gods name decided to put gas central heating in and a direct cylinder?? Why not save up to get your cylinder replaced with an indirect and piped to work from the boiler. It will save you money long term in electric.

You're not the first to recommend this - ok, so looking at the set up i have, what would this cost and who would do it?!
ok, i've took the thermostats out and now have 2 x BT7 thermostats. On the net i have found these:

Are these OK to use? Or can anyone recommend a better one?

Also, when i took them out, there were 2 wires going into the thermostats, 1 white, 1 grey. When replacing the thermostats does it matter what goes where? ie is it like a 3 pin plug, live, neutral etc?)[/url]
can't fault backer stats or immersions.

one white one grey :confused:
live (brown) from supply cable will go in oneside of stat then a fixed wire
(sometimes brown or white) go's into otherside of stat, thats soldered to one term of the element.
neutral (blue) to other term of element.
earth (green/yellow) to earth term.
OK, 3 cables come from the flex - blue and earth are connected to element and brown was connected to thermostat. Coming out from other side of thermostat was the white cable which again connects to the element.

When i removed the thermostat i undid the white and brown cable and the thermostat slid out.

So, when i put the new one in, does it matter which way around the brown and white re-connect to the thermostat?

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