Overheating Immersion Heater

[Moonraker]

I mentioned this problem in passing in another thread. Two years ago I fitted a new element to my immersion heater and all was well until three months ago when the water became dangerously hot. I  lowered the thermostat setting but was still getting a couple of sinkfuls of scalding water before it quickly ran cold. The cut-out button didn't pop up.

Three months ago British Gas fitted a new thermostat but the problem continued. Then I discovered that the vent had become blocked and that the cylinder had distorted and sprung a seam - presumably because of the excessive heat.

Last Monday British Gas fitted a new cylinder, thermostat and connecting pipework but the water still ran far too hot, even when I lowered the thermostat to 48 degrees.  I checked inside the wall switch and there were a couple of loose screws which I tightened. Then the thermostat failed to work at all! The plumber came this evening and said he would fit a new one tomorrow.

In the other thread Kerching commented "I would be v surprised if a faulty switch made it overheat...more chance of a knackered or incorrectly wired stat!".

I've read of bad batches of thermostats, but what are the chances of a two-year-old one and two brand new ones failing. And even I can wire a thermostat with reasonable confidence and the last two were installed by different British Gas fitters. I'm very doubtful that a fourth thermostat in three months will be satisfactory.

And I'm not sure how hot the water has to be before the cut-out pin pops up, but it never has, even though the water was scalding.

Looking forward to your comments!

Moonraker

 

[ProDave]

Several issues.

Presumably this is a normal copper hot water cylinder fed from a header tank? If so it's nigh on impossible to get pressurised and burst. The vent pipe is normally 22mm, and it would take a LOT for the vent pipe to fur up to the point of being blocked. I suspect it's been plumbed seriously wrongly without a vent pipe?

So first confirm it is a vented tank fed from a header tank?

If instead it's a pressurised unvented tank, then they rely on an over pressure and over temperature relief valve to prevent them over pressurising in the event of a fault. these are supposed to be serviced regularly to verify they operate correctly. If it is unvented, confirm it's had it's annual service.

Is it heated by JUST ONE immersion heater? if there are two (economy 7 over night, and daytime boost) perhaps you have been diagnosing a fault in the wrong one?

It's pretty elementary for an electrician to confirm the thermostat is switching on and off. One would have hoped the BG man did that.

And as you have already been told, loose wires in a switch will result in arcing at the terminals and probably the switch getting hot, but if anything will result in LESS heat going into the tank, so that's not the cause of your problem.

Perhaps a photo of the tank and the immersion heater will resolve any doubt about what you have?

EDIT:

A bit more thinking. 

You refer to a relatively small amount of scalding hot water before the water runs cold.  This suggests to me an immersion heater hear the top of the tank.  If it were a heater at the bottom of the tank, you would have nearly a whole tank of near boiling water.

This confirms my suspicion that you have a tank with 2 heaters. The bottom one for overnight heating at the cheap rate, and the upper one for a daytime boost.

I strongly suspect you have been checking / replacing the bottom overnight heater, without realising it's the upper boost heater that has the problems.

All just speculation until we get more details from you.

 

[Moonraker]

Thanks for your speedy response. I've lived in this house from 1976 (and watched it being built) and haven't had this problem until now. To add to my woes, at the end of last year I had leaking pipes under the kitchen floor and the leak-detection guy went up into the loft and reported that the vent into the header tank was furred up and that he had cleared it. It was he who noted that the cylinder had become distorted at its base and that a side seam was weeping,.

You'll have gathered that I'm very naive in these matters, but I'm not aware of any " over pressure and over temperature relief valve" and I've always assumed that the cylinder is vented. My brand-new cylinder is labelled as "vented".  It is supplied from the header tank.

There's only one immersion heater.

With all my previous plaintive attempts to adjust the thermostat I've always heard the click, on and off. But after today's foray into the switch there is only silence. The British Gas guy did twirl a screwdriver backwards and forwards to adjust the thermostat but didn't say if he actually heard a click. I've just turned the immersion switch on again, but there's no response (by which I mean the sizzling of water heating).

All the thermostats were supplied by British Gas and to my amateurish eye look the same make.

Moonraker

 

[Apache]

I suspect M107 could have sorted this too!

I would never use a company such as British Gas for this sort of work. They are usually very dear for average service.

 

[ProDave]

You have confirmed you have a vented tank from a header tank.

The plumber that cleared the furred up vent pipe? how did he discover that?  If there is any doubt, the whole vent pipe needs replacing, not usually a big job as it's usually a short run.  The vent pipe should terminate ABOVE the water level in the header tank. I wonder if yours dips down into the water?  Either way I suggest getting the suspect vent pipe replaced by a plumber.

Is your one immersion heater in the top or the side?

Top mounted ones seem to give much more problem, though usually the secondary thermostat nuisance tripping. Tanks with side mounted thermostats tend to be more reliable. They can sometimes be harder to find in the smaller sizes but I bought and fitted one not long ago.

I would replace the complete immersion heater and thermostat. If the BG man did not use any for of electrical test instrument to verify the operation of the thermostat, then as suggested above, he didn't do the job properly. In fact because he didn't fix the problem, I would be demanding a refund of what you paid to BG.

 

[M107]

Moonraker does the switch for the immersion have a neon light if so does it illuminate when switched on? if it doesnt (or if you havent got a neon on the switch),  check that the fuse wire has not blown in the blue fuse, its the one 3rd in from the left on the consumer unit (fuse box).

 

[Moonraker]

Reference Pro Dave's bit more thinking, added after my reply, post 3:

there is definitely only one immersion heater and that's fitted into the top of the cylinder. There is quite definitely no immersion heater fitted anywhere else. All this work is being done free under a British Gas Homecare agreement, though I read in today's Daily Mail  of a customer who had three call outs in a year (as I've done now) and his weekl;y charge went up by £7 a week!

Moonraker

 

[Moonraker]

I would replace the complete immersion heater and thermostat. 

That's exactly what British Gas did seven days ago when fitting the new cylinder!

Moonraker

 

[SPECIAL LOCATION]

Decent thermometer and test meter to verify exactly what is switched on...

and exactly if or when the thermostat actually switches off...

and at what temperature is whats needed in order to establish next course of action I think?

Guinness

 

[Moonraker]

Moonraker does the switch for the immersion have a neon light if so does it illuminate when switched on? 

Yes, there is a neon light and it comes on when I switch on the heater now even though there's no warming of the water.

This is the first time that the thermostat has died on me, all the others went on working, though the cut-out button never has popped up. That's a bit worrying in itself, as the water was scalding.

Moonraker

 

[ProDave]

How much do you pay for this BG homecare agreement? is does not seem like you are getting value for money.

I agree a new immersion heater and new thermostat should not behave like this.  Are you SURE when they changed the tank they really fitted a NEW heater and thermostat, not merely swapped over the old heater and stat from the old tank?

That's one of the issues of a scheme like that, I'll bet each visit is by a different person, so you have no continuity, and none has any knowledge of what happened before.

It's a shame you didn't ask here first, I would very very strongly advise anyone fitting a new tank should avoid one with the immersion heater entering the top of the tank.

 

[mikep]

I must admit that they do tend to play up, I had a problem with a screwfix one changed the first one after 24hours due to overheat stat tripped, fitted another went open circuit after 2 days got me money back and purchased one from Toolstation then worked fine.

I have taken some cylinders out when fitting Combi boilers and central heating systems to houses, had their original Immersion heaters & stats in still probably over 20 years or more old, still working, seems that things are not made no where near as well as they used too.

 

[sellers]

That's exactly what British Gas did seven days ago when fitting the new cylinder!

Moonraker

So they say.

If this is heated via the element only then you have a problem with the stat within the element or the wiring withing the element.

If it is heated by both the boiler and the element is a back-up.  You may have a problem with the stat associated with the boiler control wiring, or the boiler wiring is incorrect.  There isn't much else it could be?  Both stats should switch the associated heating off, simples.

 

[Moonraker]

I agree a new immersion heater and new thermostat should not behave like this.  Are you SURE when they changed the tank they really fitted a NEW heater and thermostat, not merely swapped over the old heater and stat from the old tank?

Yes, I'm sure it's a new heater and thermostat, as I saw the old heater -  which was only two years old -  and I retained the stat - which was only two months old - for myself as a spare, though perhaps I should junk it..

I'll be logging off in a tick, so thanks for all your comments so far. After the latest thermostat (perhaps from the same batch as the last two) has been fitted tomorrow, I shall be keeping a very careful eye on it.

Moonraker

 

[Moonraker]

If it is heated by both the boiler and the element is a back-up.  You may have a problem with the stat associated with the boiler control wiring, or the boiler wiring is incorrect.  There isn't much else it could be?  Both stats should switch the associated heating off, simples.

I seldom use the boiler to heat the domestic supply - yes, I know it's cheaper if I did.

Moonraker

 

[ProDave]

I seldom use the boiler to heat the domestic supply - yes, I know it's cheaper if I did.

Moonraker

Ah, now we know there IS a boiler as well as the immersion heater.

Surely you are using the boiler at the moment to heat the house.

Could it be the controls have gone wrong, and even though you THINK the boiler is not heating the tank, it is?

 

[Doc Hudson]

I seldom use the boiler to heat the domestic supply - yes, I know it's cheaper if I did.

Moonraker

If you can afford to just use an electric immersion heater for heating your water when you could be using a gas boiler then money clearly isn't an issue.  Have you proved that the boiler isn't also heating up the water, even though you think it is not being used?

Doc H.

 

[Moonraker]

Ah, now we know there IS a boiler as well as the immersion heater.

Surely you are using the boiler at the moment to heat the house.

Could it be the controls have gone wrong, and even though you THINK the boiler is not heating the tank, it is?

I seldom use the boiler to heat the house because I spend most of my time huddled over a 40-year-old electric bar fire in one room and when I do use the boiler it's nearly always only in the evening, whereas I use the immersion first thing in the morning when the boiler is quite definitely off!

 

[Doc Hudson]

I seldom use the boiler to heat the house because I spend most of my time huddled over a 40-year-old electric bar fire in one room and when I do use the boiler it's nearly always only in the evening, whereas I use the immersion first thing in the morning when the boiler is quite definitely off!

Seldom use? so when actually is the boiler on, is there a timer switch, wasn't it originally programmed to come on at certain times to heat the water? are you sure it is not coming on at times that you have forgotten about?

Doc H.

 

[sellers]

Now we know you have a boiler it opens up a can of worms.  Try 2 days without your boiler.  Switch off the boiler at its supply and run solely on the element, see if you have the same problem.