Immersion heater puzzle

[Penners]

Our immersion heater doesn't seem to be working properly.

I have taken off the cap and checked its components as follows:-

* There is current into the thermostat

* When it's cool enough to demand heat, there is current out of the thermostat.

* With the thermostat removed from the circuit, my ohmeter shows a circuit between the terminals of the heater element itself. So I take it to be intact.

* However, when current flows out of the thermostat, before it supplies the heater terminal it flows (via the white cables in the picture) through a gizmo (indicated by the yellow arrow). I can find 240V on the "in" terminal of the gizmo but zero V on the "out" terminal.

So my questions are as follows:-

1. What is the gizmo?

2. Is it something that would allow current to flow under some circumstances but not others? Or does the fact that it is not passing current now indicate that it's faulty?

3. If the gizmo is faulty, can it be replaced or do I have to replace the whole Immersion heater? I can see what appears to be a hexagonal nut underneath it - does this mean it's demountable without allowing water out of the tank?

All advice very much appreciated.

 

[Flyfisher]

When you say 'current' do you actually mean 'voltage'?

I don't know for sure but I'm wondering if the gizmo is some sort of thermal fuse that has blown and therefore interrupting the circuit and preventing the heater from working. I can't think of anything else that would be required for such a simple circuit.

Though I'm not really sure why it would be necessary given the usual thermostat. Belt and braces because of new regulations perhaps? Although I've just checked our immersion heater, installed about 5 years ago, and it has nothing like that.

Hmm. Not very helpful really. Perhaps our stripey friend will know?

 

[MdB]

How are you measuring currents? and what readings are you getting?

1) The gizmo is a thermal fuse (http://www.flickr.com/photos/cocojen/6328510945/). It is a short in normal operation and will be open circuit on over temperature. It is there for when the thermostat fails and stops the tank from boiling.

2) Unless your tank is very hot - it should have zero volts across it. It may also be a once only operation device and once tripped will always be an open circuit and never reset. I don't know. Either way, if it has failed, it is likely that the thermostat has failed also.

3) You could potentially replace the thermal fuse but removal / remounting doesn't appear to be easy. Remember though that the thermostat may have failed also. I wouldn't bother wasting my time thinking about it, I would just drain the tank and then replace the whole unit (approx £20 - £30). To undo, you are likely to need a correct ring spanner. http://www.ebay.co.uk/itm/IMMERSION-HEATER-CRANKED-SPANNER-/200795729905?pt=LH_DefaultDomain_3&hash=item2ec05baff1
Remember to drain tank first...

 

[steveb]

google shows that a T24 LC is a thermostat - comes in types of "auto reset" and "manual reset"

http://www.lightcountry.com/download/thermostat/T24.pdf

Not sure which one you have but my guess is that :

a) If it's a manual reset then you need to reset it somehow (push it?)

b) If it's auto-reset then it could be knackered

it may have tripped for a good reason so usual warnings about not messing with stuff you don't understand apply but it appears you are well past that point anyhow.

-Steve

 

[Penners]

Many thanks for those views, everyone. Here are my answers to a few questions:

Yes, when I say "current" I'm being inaccurate. What I mean is that I have set my multimeter to its 250V scale and read 240V across the terminals.

I don't think the tank is over-hot, so I doubt that the thermal fuse (thanks for that identification) is likely to have blown or jumped out (assuming it's re-settable) due to its designed function. But it may of course be faulty. However, the logical way to proceed now, I imagine, is to see whether I can press the little button in the middle and get it to reset.

If it continues to give trouble, then I will replace the immersion heater, as MdB suggests. I was just trying to avoid this if I could - I always seem to have trouble unscrewing immersion heaters, despite having the correct ring spanner. Last time I tried (different house, different cylinder), it was so tight that I ended up distorting the copper around the boss in the cylinder!

Lunchtime now, but I'll try resetting the fuse afterwards, and report back.

Many thanks again for the advice.

 

[Flyfisher]

Well done on the google searches chaps. I must be losing my touch because I also tried that but only came up with a T24LC TVS Diode Array device, which has no real application for immersion heaters!

 

[FamilyWiggs]

Well done on the google searches chaps. I must be losing my touch because I also tried that but only came up with a T24LC TVS Diode Array device, which has no real application for immersion heaters!

I just came up with loads of articles on liquid chromatography.

It's a wonder how Penners ever completes any jobs... first there is the extended KitKat consideraton phase, followed by photoraphing exhibits and uploading to PPUK. After which it is lunchtime.... :lol:

 

[Flyfisher]

I've just remembered that I have a DHW cylinder on my scrap metal pile, complete with immersion heater & thermostat because I don't have one of those big hex spanners to remove it. I've just taken the wiring cover off and it doesn't have one of those thermo/fuse thingies. The tank is, I think, about 15 years old so perhaps there are new regulations nowadays.

 

[Penners]

Click!

I pressed the little tit in the middle and the cutout reset itself! Whoopee!

I suppose I mustn't cheer too prematurely. I had drained quite a lot of the hot water out of the cylinder, to ensure that the thermostat would be demanding heat, so I need to wait until it's fully up to heat, to make certain that the cutout doesn't operate again. It may be faulty.

But so far, so good. The kettle is singing nicely (I'm ignoring the fact that this may suggest a calcium build-up!).

Thanks very much for all your advice, chaps. If you don't hear any more on this from me, it's good news!

 

[Moneypit]

I saw this post and thought straight away "I know the answer to this one for once" but soon noticed that it had been answered already. I think I can clear up the reason why some have one and others don't.
We had our cylinder replaced last year and had a new immersion heater while we were at it. Our plumber told me that all new heaters now have to have a cutout fitted - a result of that tragic incident a few years ago when a young toddler died from scalding injuries following an overheating/expansion/overflowing event.

 

[jocelyn plummer]

Click! I pressed the little tit in the middle and the cutout reset itself! Whoopee!

OOO be careful Penners, must set a good example to the younger generation :roll:

 

[Penners]

Sorry, Joss. It's the result of excessive association with rough builder types, on this forum.

 

[Penners]

I just came up with loads of articles on liquid chromatography.[/quote]
Damn! If I'd had that stuff earlier it would be have been of immense help in pressing the t... button.

It's a wonder how Penners ever completes any jobs... first there is the extended KitKat consideraton phase, followed by photoraphing exhibits and uploading to PPUK. After which it is lunchtime....

You can't rush genius, FW. It takes its own time. Particularly when it has to be imparted to a gamma semi-moron by the more capable and skilled contributors to this forum.

But you've reminded me - it's time for a Kit-Kat.

 

[88v8]

I had two new stats tripping out, no Reset button, they were OK initially but tripped out when I had drawn a bath-full and they were reheating. Rather annoying when one is in the bath and expecting to top-up.
I ended up putting an old stat in there, and when I say 'old' I mean 35 yo, works OK now. However, I know it won't last for ever, as I have a stainless (= cheap) cylinder, and this needs some special type of stat :?

Ivor

 

[Penners]

Ivor - I can only think that your two thermostats were different from mine, because mine doesn't have a thermal cutout. This item is fitted to the immersion heater itself, not the stat.

 

[Lime]

A common cause for the over-temp stat to work is air trapped in the cylinder around the immersion heater head.
Another is a loose electrical connection.
And yet another is if the heating element is furred up.

 

[Penners]

Thanks, Lime. I found all the electrical connections to be sound, but the system does have a tendency towards general bubbling and gurgling - indicating trapped air. There is an automatic air vent, but I've never been sure how effective those things are.

It is also possible that there may be some limescale build-up. I shall just have to monitor the system (which is now running on immersion heater only) and see whether the cutout trips again.

 

[Lime]

The vent for the escape of air on the cylinder should be (and normally is) at the top of the cylinder.
(The hot water feed is commonly tee'd off the vent pipe too.)
It is possible that when the vent pipe was installed or altered it was inserted too far down into the tank in some way.
This is not normally possible but sometimes the wrong type of screwed connection is used that allows the vent pipe to pass right through into the water.
Similarly the threaded section may be too long and protrude into the water.
Both situations can cause an insulating bubble of air to collect around the top of the heating element within the tank resulting in overheating.

 

[Penners]

Yes, this is a conventionally designed system, with the vent pipe coming out of the top of the cylinder and curving over into the top of the header tank. I've seen all elements of it in the past, and I;m happy that there is no incorrect design in that way. I cannot, of course, speak for the entry of the pipe into the top of the cylinder.

Thinking about it, the intermittent gurgling tends to come from the secondary side - central heating - so I suppose this is not going to affect the over-heat cutout in the way that air in the primary would.

 

[Lime]

Thinking about it, the intermittent gurgling tends to come from the secondary side - central heating - so I suppose this is not going to affect the over-heat cutout in the way that air in the primary would.

Now that is something I'd stop.
It is an indicator of corrosion.
The gas is probably hydrogen generated by the corrosion of steel or iron in electrical contact with brass and copper immersed in an agressive conductive fluid that was once plain tap water.
It needs to be drained and a good inhibitor put in to the system to stop corrosion throughout the piped heating system.
Fernox inhibitor is the best IME and don't skimp on quantities.
http://www.fernox.com/products/water+treatment+chemicals/inhibitors
A few pounds spent today will save £1000s later.