Immersion Thermostat wire burns out

[brixtonia]

I have a 4kW immersion heater in a thermalstore - very recently replaced by a plumber as the previous one was leaking. Supplied with a cotherm thermostat and a second pocket for a safety cutout into which the safety cutout from the previous immersion was fitted and seems to be working fine.

I smelled burning and removed the cover to find that the insulation on the wire between the cutout and the stat was melting. The end closest to the stat was clearly more affected. The stat has deformed around the point the wire connects to it and the stat has ceased to work.

The wire between the safety and the stat is exactly the same as the wire from the spur to the immersion which has been in place since I can remember. The connection seemed sound but hard to tell now.

One thing I had noticed after it was installed is that it was cycling on and off quite rapidly.

Just a faulty stat?

 

[marconi]

Usually, the electric immersion heater is rated at 3kW. You mention the IH is now rated at 4kW after it was replaced by the plumber. This would be something to clarify first.

A 4kW IH draws more electric current - about 16Amps - whereas a 3kW one draws about 12Amp. It is important that the cabling, connections, thermostats , switches and supply from the consumer unit are suitably rated for the power of the IH.

If you send me details of the thermal store I will look up the power of the IH which should be fitted.

 

[brixtonia]

Thank you. It has replaced a 4kw immersion and the stat is / was 20A rated. I've replaced the stat and it does not cycle in the same way. I think it must have been a fault with the stat.

 

[ColinACJL]

I have a 4kW immersion heater in a thermalstore - very recently replaced by a plumber as the previous one was leaking. Supplied with a cotherm thermostat and a second pocket for a safety cutout into which the safety cutout from the previous immersion was fitted and seems to be working fine.

I smelled burning and removed the cover to find that the insulation on the wire between the cutout and the stat was melting. The end closest to the stat was clearly more affected. The stat has deformed around the point the wire connects to it and the stat has ceased to work.

The wire between the safety and the stat is exactly the same as the wire from the spur to the immersion which has been in place since I can remember. The connection seemed sound but hard to tell now.

One thing I had noticed after it was installed is that it was cycling on and off quite rapidly.

Just a faulty stat?

You can get clamp meter for a about £25 online.

 

[brixtonia]

So the new stat has also burned out. The live connects first to the safety cutout, then into the 20A temp control stat, then to the element (using 2.5mm wire). It's always the wire between the two stats which seems to overheat. The wire insulation melts and the terminal melts at the point at which it connects to the second stat (20A temp control).

The 4kw immersion element seems fine with resistance testing at about 14-15ohms. The immersion replaces another 4kw immersion.

Any suggestions?

 

[johntheo]

Have you checked the voltage, also suggest (when repaired) check voltage drop between two above mentioned terminals.

 

[brixtonia]

Have you checked the voltage, also suggest (when repaired) check voltage drop between two above mentioned terminals.

Thanks. I will check when I have a new stat. Just noticed that the safety cutout (in line before the variable temp stat) is 16A rated. It operates fine and has been in there for years, but could its rating possibly be causing a problem for the stat?

 

[johntheo]

I don't think so as its not doing any switching normally, your measured 14 ohm resistance means a current flow of 16.43A at a nominal 230V (if that's the UK voltage), normally, heat damage with fairly high loads is caused by lose connections but I think that is very unlikely, now at any rate, I would measure the volt drop (immersion on) between the furthest away terminals first, if no volt drop then problem elsewhere, maybe the control stat is rubbish and causing very high resistances while switching (unlikely) but if you get a volt drop of say 5V then this will result in ~ 80W heat being generated somewhere between the two stats, you can then narrow it down by probing separately across each set of stat terminals, you say it's the wire between the two stats that's overheating which might point to a control stat problem but see what readings you come up with.

 

[brixtonia]

I don't think so as its not doing any switching normally, your measured 14 ohm resistance means a current flow of 16.43A at a nominal 230V (if that's the UK voltage), normally, heat damage with fairly high loads is caused by lose connections but I think that is very unlikely, now at any rate, I would measure the volt drop (immersion on) between the furthest away terminals first, if no volt drop then problem elsewhere, maybe the control stat is rubbish and causing very high resistances while switching (unlikely) but if you get a volt drop of say 5V then this will result in ~ 80W heat being generated somewhere between the two stats, you can then narrow it down by probing separately across each set of stat terminals, you say it's the wire between the two stats that's overheating which might point to a control stat problem but see what readings you come up with.

Thanks. I will try all of this once I get a new stat. Only doubt I have about it being a faulty stat is that this has been a consistent problem with various stats. Even on the previous immersion. But then what else could it be? I don't suppose there is any mileage in changing the order of the cutout and the temp stat? Or even putting the cutout after the element?

 

[johntheo]

Is the old stat(s) actually burned out or can you hear it switching in/out if you turn the dial?.

 

[marconi]

Post some pictures please which includes details of the burned out link wire and its terminals.

Did you make up the link wire? If you did what did you use?

Also send a picture or details of the IH element, control stat and safety stat.

The cotherm stats I have looked up are rated at 16A which is suitable for a 3kW IH element but unsuitable for a 4kW element. Are you sure the two stats are rated at 20A?

 

[brixtonia]

Is the old stat(s) actually burned out or can you hear it switching in/out if you turn the dial?.

No switching sound but when measured with a multi meter at ambient temperature it shows no circuit up to about 60C then about 3 Ohms resistance.

 

[brixtonia]

Post some pictures please which includes details of the burned out link wire and its terminals.

Did you make up the link wire? If you did what did you use?

Also send a picture or details of the IH element, control stat and safety stat.

The cotherm stats I have looked up are rated at 16A which is suitable for a 3kW IH element but unsuitable for a 4kW element. Are you sure the two stats are rated at 20A?

The link wire is the positive wire taken from a new section of 3 core 2.5mm2.

The stats I have used are Cotherm TSD 00706 and TSD01106 - both rated 20A (and printed on the side) and, importantly, variable to 80C (as this is a Thermalstore).

The safety is a Sunvic VKL 2352 with manual reset (16A). It's been in place for a while but seems to work fine.

Can't get a photo just now and it's dismantled but as previously described mains live runs into Sunvic, then into Cotherm, then to immersion terminal. Neutral fixes to the opposite terminal. Contacts are good.

 

[johntheo]

No switching sound but when measured with a multi meter at ambient temperature it shows no circuit up to about 60C then about 3 Ohms resistance.

Don't think that means alot because if the closed contacts of the thermostat had a resistance then they would be generating ~ 550watts in the stat itself when wired in series with the 14 ohm element.

 

[marconi]

Is there a reason why the tank is fitted with a 4kW element rather than the typical 3kW element? Could you use a 3kW element instead or are you in a hurry to heat up the tank?

20A rated safety and control stats such as those Cotherm types are on their limits to switch and pass 20A for long periods of time and will inevitably deteriorate through concentrated electrical heating of their contacts and connections. The physics of electrical heating is that it is proportional to the current squared not simply the value of the current. This means Ohmic heating as it is sometimes called rises rapidly with each increment of current eg (read as I/Isquared) 1/1, 2/4, 3/9, 4/16...16/256....20/400.

Those 4 kW elements require much more robust stats rated at say 30A - 40A to give reliable service. For 4kW and higher elements , ordinary stats like the cotherm ones you have are used to switch an electromagnetic switch called a contactor on and off. The stats only then have to handle the very small current required to power the contactor's electromagnetic operating coil; the contactor has substantial switch contacts and surfaces to safely and reliably close and open the immersion heater circuit through which flows 4000/230 = 17 Amps. In the early stages of heating cold water the current is likely higher than 17A because the element's resistance is lower than at warmer water temperatures.

One also needs to employ the correct power cable to the element which has heat resistant insulation suited to higher operating temperatures. It is sold as 'immersion heater cable'. eg:

Doncaster Cables Immersion Heater Cable 3183TQ 2.5mm2 x 1m Coil - https://www.travisperkins.co.uk/cable-lengths/doncaster-cables-immersion-heater-cable-3183tq-2-5mm2-x-1m-coil/p/759014

Finally the terminations of the flex must be made using ferrules or ring crimps to maximise conductivity of electricity.

I believe then the combination of 4kW IH element and those Cotherm stats is unsuitable. One could stick with the Cotherm stats if you swapped to a 3kW element. To use a 4kW element you require a 30-40A contactor.

I have not looked up your profile but if you are not an electrician then this is a job for one to complete for you. Hope this helps.

 

[johntheo]

Still and all, the measured element resistance is 14 ohms or 16.43A at 230V, so, is it really outrageous to use a 20A stat to switch this?. Daughter's 2.5kw immersion still has the same 15A skol (or shol?) stat switching it for the last 16 years, so 15A switching 10.87A.

 

[marconi]

Is the immersion heater element supplied via a residual current detector ?

 

[marconi]

With reference to the 20A control stat I discovered this while searching for its technical specification: ​

Cotherm TSD01106 Thermostat​ Cotherm TSD01106 20A Control Thermostat A Cotherm brand adjustable control thermostat with a range of approximately 5°C - 85°C. The TSD01106 is a rod thermostat with single pole break-on-rise action rated at 20A 240v. The stem (rod) is 265mm long and 7.5mm in diameter making suitable for use in all standard 280mm x 8mm (11" x 5/16") thermostat pockets. It is ideal for use in commercial products for direct control of single phase loads up to 3kW or to operate a contactor for higher loads or three phase applications. Note: This device does not include a safety cut-out and is only designed to control the water temperature. This thermostat is NOT SUITABLE for DOMESTIC without there bring an additional a safety cut-out as required by law. Condition: New and unused in original manufacturer's packaging.

Delivery​ Orders received before 2pm Monday to Friday will be despatched the same day. Orders received after 2pm Friday, Saturday or Sunday will be dispatched the following Monday. Orders received after 2pm will be treated as having arrived the following day. All goods are dispatched, professionally packed, on a signed for service. In most cases our carrier can leave the goods in a safe place, or with a neighbour for no extra charge. Should you require this service please detail it in the notes when placing your order together with a contact telephone number. If you do not receive the goods within three working days (Monday to Friday) of placing your order we would ask that you contact us. We can then investigate the problem and authorise a replacement where necessary. If you wish to purchase more than one listed item, we can usually ship these together for a reduced overall carriage charge. In this instance please contact us before paying so that we can advise you of the combined carriage charge. Hours​

 

[marconi]

And the 16A safety cut out is under-rated for a 4kW element.......

 

[marconi]

I have a 4kW immersion heater in a thermalstore - very recently replaced by a plumber as the previous one was leaking. Supplied with a cotherm thermostat and a second pocket for a safety cutout into which the safety cutout from the previous immersion was fitted and seems to be working fine.

I smelled burning and removed the cover to find that the insulation on the wire between the cutout and the stat was melting. The end closest to the stat was clearly more affected. The stat has deformed around the point the wire connects to it and the stat has ceased to work.

The wire between the safety and the stat is exactly the same as the wire from the spur to the immersion which has been in place since I can remember. The connection seemed sound but hard to tell now.

One thing I had noticed after it was installed is that it was cycling on and off quite rapidly.

Just a faulty stat?

One thing I had noticed after it was installed is that it was cycling on and off quite rapidly.

By the second it I reckon you mean the control stat. I think this control stat is acting akin to a thermal flasher device viz:

What is a Flasher Unit? | what is a Flasher Relay? - https://www.arc-components.com/blog/blinking-lights-what-is-a-flasher-unit.html

One might assume the control thermostat stat senses the temperature of only the water in the tank - but it does not. Alas the control stat will also be subjected to the Ohmic heating effect of all its electrical components through which the IH element current flows. This heating effect will increase if the IH element current is higher as is the case for a 4kW element vis a vis a 3kW one.

 

[johntheo]

With reference to the 20A control stat I discovered this while searching for its technical specification: ​

Cotherm TSD01106 Thermostat​ Cotherm TSD01106 20A Control Thermostat A Cotherm brand adjustable control thermostat with a range of approximately 5°C - 85°C. The TSD01106 is a rod thermostat with single pole break-on-rise action rated at 20A 240v. The stem (rod) is 265mm long and 7.5mm in diameter making suitable for use in all standard 280mm x 8mm (11" x 5/16") thermostat pockets. It is ideal for use in commercial products for direct control of single phase loads up to 3kW or to operate a contactor for higher loads or three phase applications. Note: This device does not include a safety cut-out and is only designed to control the water temperature. This thermostat is NOT SUITABLE for DOMESTIC without there bring an additional a safety cut-out as required by law. Condition: New and unused in original manufacturer's packaging.

Delivery​ Orders received before 2pm Monday to Friday will be despatched the same day. Orders received after 2pm Friday, Saturday or Sunday will be dispatched the following Monday. Orders received after 2pm will be treated as having arrived the following day. All goods are dispatched, professionally packed, on a signed for service. In most cases our carrier can leave the goods in a safe place, or with a neighbour for no extra charge. Should you require this service please detail it in the notes when placing your order together with a contact telephone number. If you do not receive the goods within three working days (Monday to Friday) of placing your order we would ask that you contact us. We can then investigate the problem and authorise a replacement where necessary. If you wish to purchase more than one listed item, we can usually ship these together for a reduced overall carriage charge. In this instance please contact us before paying so that we can advise you of the combined carriage charge. Hours​

A bit strange because they have a 4kw (at 230V) element here with a ST7 thermostat which doesn't look anything special, no mention of a contactor.
The OP has a 4kw element which he measured at 14 ohms which is more than likely 4kw @ 240V = 3.8kw @ 230V or 16.5A and is using a 20A stat!.

14” TITANIUM SINGLE IMMERSION ELEMENTS - 4KW - 1 3/4”BSP - 230V - SIDE ENTRY WITH STAT - TSDR7

IDL091009SC 091009SC Heating and Ventilation Ideal Elements Domestic & Industrial Elements Heating and Ventilation Water Heating Immersion 4KW Ideal 230V

www.eurosales.ie

 

[marconi]

Today I sent a question on current rating and immersion element power to the Backer Electric Company Limited. They confirmed as Johntheo mentioned above that their Backer stats rated 20A can indeed directly control a 4kW IH element. Whether other makes of stat can is another matter.

brixtonia: 1. The question about the rcd is to see whether the 4kW element albeit a new one is leaking current to earth - unlikely if new but maybe this is a cause of overcurrent.

2. I'd love to have some pictures of the installation to study. Are push on spade terminals used for any of the connections? Are they bare metal or shrouded types?

3. What is the supply voltage?

4. Does the installation have a large solar pv system feeding into the grid which might be elevating the local mains voltage? Are the large solar pv arrays nearby which might be raising the supply voltage?

5. Forgive me - is the wiring definitely 2.5mm2 csa?

6. The 16A safety stat is under-rated.

7. What temp do you have the control stat set at?

8. What is the length of the link between the control and safety stats?

Probably enough for now. But some images of the point of failure on the control stat external terminals would be useful to see.

 

[brixtonia]

Today I sent a question on current rating and immersion element power to the Backer Electric Company Limited. They confirmed as Johntheo mentioned above that their Backer stats rated 20A can indeed directly control a 4kW IH element. Whether other makes of stat can is another matter.

brixtonia: 1. The question about the rcd is to see whether the 4kW element albeit a new one is leaking current to earth - unlikely if new but maybe this is a cause of overcurrent.

2. I'd love to have some pictures of the installation to study. Are push on spade terminals used for any of the connections? Are they bare metal or shrouded types?

3. What is the supply voltage?

4. Does the installation have a large solar pv system feeding into the grid which might be elevating the local mains voltage? Are the large solar pv arrays nearby which might be raising the supply voltage?

5. Forgive me - is the wiring definitely 2.5mm2 csa?

6. The 16A safety stat is under-rated.

7. What temp do you have the control stat set at?

8. What is the length of the link between the control and safety stats?

Probably enough for now. But some images of the point of failure on the control stat external terminals would be useful to see.

Firstly to answer your questions:

1) Not on an RCD.
2)

3) 240
4) No solar PV.
5) That's what I specified when I bought it.
6) yes the 16A cut out stat is underrated - nevertheless it has been working for about 4yrs.
7) Temp control set at 75-80C as per manual supplied with UFH and Thermalstore.
8) 100mm approx.

 

[brixtonia]

This may be resolved although I am not sure why.

I purchased a Sunvic VK4351 20A rod thermostat which I mistakenly believed had a safety cutout but did not - it is just a straightforward 20A stat. I fitted this with the 20A cotherm TSD01106 stat (mains>sunvic>cotherm>immersion) which is the same as the one supplied with the immersion. (I appreciate that this is a little pointless but I wanted to see if having removed the 16A cutout things worked OK). The connection into the 20A Cotherm quickly began to overheat as before and melt the insulation. It also cycled perhaps 10s on 40s off as before.

So I removed the Cotherm and replaced it with the Sunvik VKL2352 cutout with manual reset (the 16A cutout which has been working fine for a couple of years). So (mains > new sunvic 20A stat > old sunvic 16A cutout > immersion). And it's been working fine for a week.

I was struggling to find an 11" (or 7") rod stat which was 20A rated and had a variable temp range up to 80C and an inbuilt safety.

It seems to be the Cotherms which have been causing the problems.

 

[brixtonia]

With reference to the 20A control stat I discovered this while searching for its technical specification: ​

Cotherm TSD01106 Thermostat​ Cotherm TSD01106 20A Control Thermostat A Cotherm brand adjustable control thermostat with a range of approximately 5°C - 85°C. The TSD01106 is a rod thermostat with single pole break-on-rise action rated at 20A 240v. The stem (rod) is 265mm long and 7.5mm in diameter making suitable for use in all standard 280mm x 8mm (11" x 5/16") thermostat pockets. It is ideal for use in commercial products for direct control of single phase loads up to 3kW or to operate a contactor for higher loads or three phase applications. Note: This device does not include a safety cut-out and is only designed to control the water temperature. This thermostat is NOT SUITABLE for DOMESTIC without there bring an additional a safety cut-out as required by law. Condition: New and unused in original manufacturer's packaging.

Delivery​ Orders received before 2pm Monday to Friday will be despatched the same day. Orders received after 2pm Friday, Saturday or Sunday will be dispatched the following Monday. Orders received after 2pm will be treated as having arrived the following day. All goods are dispatched, professionally packed, on a signed for service. In most cases our carrier can leave the goods in a safe place, or with a neighbour for no extra charge. Should you require this service please detail it in the notes when placing your order together with a contact telephone number. If you do not receive the goods within three working days (Monday to Friday) of placing your order we would ask that you contact us. We can then investigate the problem and authorise a replacement where necessary. If you wish to purchase more than one listed item, we can usually ship these together for a reduced overall carriage charge. In this instance please contact us before paying so that we can advise you of the combined carriage charge. Hours​

Interesting - especially as this was the stat supplied with this 4kW immersion.

 

[marconi]

Good evening from London. Thank you for the information. That element has 3 separate elements wired in parallel. Could you give me manufacturer and part number for it please so I can learn more about it?

 

[brixtonia]

Good evening from London. Thank you for the information. That element has 3 separate elements wired in parallel. Could you give me manufacturer and part number for it please so I can learn more about it?

Made in Italy I believe.

 

[brixtonia]

With reference to the 20A control stat I discovered this while searching for its technical specification: ​

Cotherm TSD01106 Thermostat​ Cotherm TSD01106 20A Control Thermostat A Cotherm brand adjustable control thermostat with a range of approximately 5°C - 85°C. The TSD01106 is a rod thermostat with single pole break-on-rise action rated at 20A 240v. The stem (rod) is 265mm long and 7.5mm in diameter making suitable for use in all standard 280mm x 8mm (11" x 5/16") thermostat pockets. It is ideal for use in commercial products for direct control of single phase loads up to 3kW or to operate a contactor for higher loads or three phase applications. Note: This device does not include a safety cut-out and is only designed to control the water temperature. This thermostat is NOT SUITABLE for DOMESTIC without there bring an additional a safety cut-out as required by law. Condition: New and unused in original manufacturer's packaging.

Delivery​ Orders received before 2pm Monday to Friday will be despatched the same day. Orders received after 2pm Friday, Saturday or Sunday will be dispatched the following Monday. Orders received after 2pm will be treated as having arrived the following day. All goods are dispatched, professionally packed, on a signed for service. In most cases our carrier can leave the goods in a safe place, or with a neighbour for no extra charge. Should you require this service please detail it in the notes when placing your order together with a contact telephone number. If you do not receive the goods within three working days (Monday to Friday) of placing your order we would ask that you contact us. We can then investigate the problem and authorise a replacement where necessary. If you wish to purchase more than one listed item, we can usually ship these together for a reduced overall carriage charge. In this instance please contact us before paying so that we can advise you of the combined carriage charge. Hours​

Can you provide a link to this info? I'd like to share it with the supplier.

 

[brixtonia]

Incidentally, my supply is single phase so I guess that must have been the problem. Why would single vs three phase make a difference to the amp rating of the stat?

 

[brianmoooore]

If this was fed by a three phase supply, you wouldn't be able to control it directly with a single 'stat, which possibly explains why there appears to be two 'stat pockets.
You could also fit two stats with your single phase supply, controlling two elements with one, and the third with the other. This would bring the 'stat current well within the limits of a 16A version.

 

[marconi]

Can you provide a link to this info? I'd like to share it with the supplier.

I will see if I still have it.

 

[marconi]

Cotherm TSD01106 Thermostat - 280mm Stem, 20AMP | eBay - https://www.ebay.co.uk/itm/183661179944

It was this company advertising on ebay to sell the control stat that included the advice I pointed out in red. Perhaps email or call them for further information. If you learn more from this company on why the advice was given would you mind sending another post so others can know too please?

Regards
Marconi

WH (Water Heating) Co Ltd
Trevor Guy
2 Horsecroft Place
Harlow
Essex
CM19 5BT

Phone: 01279641722
Email: [email protected]

 

[johntheo]

Total resistance was measured at 14 ohms so 3 elements in parallel = approx 42 ohms/element but total current through stats is still the same, "4kw" immersion is 3.779kw @ 230V = 16.43A. or 4.114kw @ 240V = 17.14A so new stat is just doing what it's supposed to do and not burning out?. (single phase supply).

 

[brixtonia]

Wrote to supplier who replied:

The thermostats are designed for single phase elements, or three phase elements wired in single phase configuration up to 4kw.
To use with elements wired in three phase configuration, the thermostat needs to switch a contactor.
So, the thermostat you received with your 4kw element is suitable for single phase use.
The 4kw element is rated at 17.5a and the thermostat is rated at 20a.
We have never had a thermostat burn out. Please check your wiring as the problem is not with the thermostat.

However, it seems to contradict the instruction leaflet (not received with the immersion - emailed after) which seems to say that higher loadings than 3kW need a contactor?

 

[Avo Mk8]

I'm suspicious of the binary pricing of the Cotherm thermostat in the marketplace - around £12 general outlets, around £30 from reputable suppliers, one of whom publishes this footnote with the details of the stat:

SANTON S8203 THERMOSTAT TSD01106 11" 5-80'C COTHERM
This part is compatible for use with:

• Santon DF range R25/R45/R70/R115

PLEASE BE AWARE: This a genuine Heatrae Sadia manufactured part, unlike many of our competitors.

The genuine part seems to have a 'properly approved' 20A resistive rating, so I would have thought capable of handling the 4kW load. Perhaps your leaflet covers the, dare I say, " Chinese Export" version