Immersion heater boiling

[grayham]

Hi all, wonder if anyone can help?

A few weeks ago I managed to leave our immersion heater one all night (thought I switched it off, but assume I must have switched it just as the timer was switching somehow... anyway...). Woke at 2am to hear it boiling away. As bad luck would have it, the overflow pipe in the loft from the cold water tank fell off, and so we had a bit of leakage into the spare room. So two failsafe mechanisms (three if you count stupid me not switching it off) failed. The overflow pipe is now more securely attached, but obviously I want to make the immersion work properly too.

Obvious solution - replace the immersion thermostat. No problem, bought one, installed it and connected it exactly as the old one. However... it hasn't fixed it! The element is powered regardless of the thermostat setting and water temperature. If I fiddle when the water is pretty warm, then I can hear the thermostat click when I rotate the temperature setting, and if I disconnect the stat from everything, and stick a multimeter on the terminals then the contact between the two is definitely being made / broken along with the clicking. So the new stat appears to be working. So the only conclusion I can draw is that the circuit connecting the mains supply, element and stat must be wrong in some way (either connected wrong or failed somehow). The heater isn't wired up simply with the mains supply going to the element, with the stat interrupting the live feed. Instead the stat is connected to a couple of other terminals. I'm no electrician, but my guess is that this is some kind of relay type thing, so that the full current doesn't go through the stat, but the stat's open/closed status is translated to chunkier wiring inside?

Any ideas as to what could be wrong? In case what I've described above makes no sense, I've put a couple of pictures up. One is a dodgy camera phone photo, but because that's not very clear I've also drawn a diagram describing it.

Photo:

http://picasaweb.google.com/graham.searle/House/photo#5236886539137448978

Diagram:

http://picasaweb.google.com/graham.searle/House/photo#5236891538981706258

Thanks very much for any help! I realise, of course, that just replacing the whole immersion heater unit might be the sensible solution, but it's hassle I could do without if it can be solved by simply switching a few wires around!

 

[theoutlaw]

Hi Graham, my first post on here, so hello all.
The live wire from you main supply needs to go to the right hand terminal of the thermostat and the white wire coming out of it goes to the binding post marked + on the element.
Basiclly the thermostsat breaks the cicuit when the temp range selected has been reached.
Make sure you isolate the elec before doing it though.

Fingers crossed it should work ok.

Tony

 

[theoutlaw]

I will nip up and take a pick of mine before you start, so you can see the comparison. Dont move.


Tony

 

[powell30]

First things first your lucky you didn't have a serious accident. I assume the feed tank was plastic.......2 old people died in a council house a few years back........water boiling in the cylinder vented into a plastic tank and it eventually split and ...well, scalded the couple to death......

Check the electrics from isolation switch to immersion. I would guess it could be the immersion itself thats faulty.......they corrode over time and short. If so, then you must replace

 

[grayham]

Hi, thanks for the replies. Sorry for my ignorance, but if the element itself had corroded and shorted, wouldn't that have led to (a) poor (if any) water heating performance, and (b) likely higher current that would trip the circuit?

Also, Tony, I understand that a simpler wiring arrangement as you suggest should work in the case where you basically have access to two terminals directly connected to the element, the thermostat, and nothing else, but what is the round grey bit on mine to which the stat is currently connected? Presumably it's there for a reason?

Thanks again!

 

[theoutlaw]

Your diagram is the best way to do this because my phone hasnt got a flash.

In your pic there are two white wires, unscrew the one that goes into the right of the stat, put the live/brown wire from the mains in here.
on the other side of the stat the white wire come out to a two poled component on the immersion. From this a wire goes to the live on the immersion, where the live/brown was originally.
Your Heater has been wired up wrong there is not a fault with it.
Any probs pm me and I will ring you.

Tony

 

[theoutlaw]

Hi, thanks for the replies. Sorry for my ignorance, but if the element itself had corroded and shorted, wouldn't that have led to (a) poor (if any) water heating performance, and (b) likely higher current that would trip the circuit?

Also, Tony, I understand that a simpler wiring arrangement as you suggest should work in the case where you basically have access to two terminals directly connected to the element, the thermostat, and nothing else, but what is the round grey bit on mine to which the stat is currently connected? Presumably it's there for a reason?

Thanks again!

This is a component the live goes through, do not bypass it, its there for a reason.

PM sent

Tony

 

[theoutlaw]

After our chat on the phone, how did it go Graham?

Tony

 

[grayham]

Hi Tony,

Got home pretty late last night, and quickly rewired the immersion as you suggested. Got up this morning and found that it hadn't worked at all - no power had got through to the heater at all. The stat is new and was definitely working last time I waved a multimeter at it, so the obvious conclusion is that the round grey component (whatever it is!) is breaking the circuit. Had to run off to work, so didn't have time to get the multimeter and test whether the two terminals on the grey thing are connected, but my guess is that they aren't. Of course, I could just bypass it, and I assume all would then work fine since the circuit would be simple, and only involve components that I understand and know to work. I'm not keen to simply bypass it without knowing what it is though! My best guess is that it might be an additional thermal cutout, which might have failed in a "safe mode". If it is then it's probably fine to bypass it, since the new stat is a modern one with an independant stat and safety cutout. Ho hum. Will probably end up being easier just to replace the whole heater, but I'm loathe to do that because it may be hassle getting the old one out.

Cheers,
Graham

 

[theoutlaw]

Hi Graham, I believe it could be a self resetting thermal cut-out, as long as the thermostat is working ok on your cylinder, to by pass it wouldnt be an issue, I have tried it on mine just now, and there seems to be no difference apart from the extra over heat protection it gives.
You could try it when you know you will be around for and hour or two, just incase the thermostat isnt working.

Tony

 

[grayham]

Yeah, I think I'll try it this evening. I can easily check whether the thermostat is working properly (should be - it's new, and according to the manufacturer's spec it has a secondary safety cutout which should be plenty safe) by simply running the heater until the water's over 40C, then turning the thermostat temperature control and checking that the element switches on and off.

Cheers for all your help Tony - will let you know if it works!

 

[grayham]

Hi again - just to follow up - all working fine now. Weirdest thing, got home on Friday evening, got the multimeter out and started fiddling around. The round grey component (presumably a thermal cutout device, which I assumed had failed) has connection between its terminals. Also checked the resistance of the heater element (18ohms - about right for a 3kW heater). Wired it up the way you'd suggested again (which I thought I'd done on Thursday night - maybe I'm going mad, or maybe I'd forgotten to switch the timer on, but it hadn't ot warm then for one reason or another!).

Anyway, it's all working fine now - have checked that the heater switches off when I turn the stat down (can easily hear when it's on, provided the water's already pretty warm). So I'm a happy bunny - no need to mess around taking out the heater (and buggering the cylinder knowing my luck!).

Thanks very much again for your help Tony. Much appreciated.

 

[theoutlaw]

Cool, glad you got it sorted in the end Graham.

Tony

 

[ChrisR]

I believe it could be a self resetting thermal cut-out

No it isn't, it's a NON self resetting termal cutout. You have to press the button in the middle, which is presumably what you've done without realising it.
All new thermostats have them built in, with a very small button, so the original is redundant.
It would have been worth looking at the FAQs.
The original should have been wired like this:

 

[Agile]

If you look back throught this posting its a sorry case of the blind leading the blind!

The non resetting stat is there for a very important safety function and the OP was told to ignore it !

Its very lucky that he has probably now got it connected properly but a photo would be reassuring!

It would be boring for me to explain all the safety issues but gas and electricity do need to be treated carefully to ensure that dangerous mistakes do not occur. The best way to avoid problems is to engage someone who is competent.

Tony