Immersion thermostat keeps tripping

[Little Clanger]

The thermostat in one of my immersion heaters keeps tripping. Immersion element, thermostat and cylinder are about 9 months old. Thermostat is set to 65°. This is the lower immersion heater in a dual heater Economy 7 cylinder.

 

I swapped the thermostat for the one from the upper immersion heater, but that tripped at some time during the night.

 

These are Tesla units. I've ordered a Backer replacement, but does the fact that two thermostats tripped indicate that the element is at fault?

 

We live in a hard water area, but the elements are incoloy-type, and nearly new.

 

Any suggestions?

 

Thanks 

[Nickfromwales]

8 minutes ago, Little Clanger said:

The thermostat in one of my immersion heaters keeps tripping. Immersion element, thermostat and cylinder are about 9 months old. Thermostat is set to 65°. This is the lower immersion heater in a dual heater Economy 7 cylinder.

 

I swapped the thermostat for the one from the upper immersion heater, but that tripped at some time during the night.

 

These are Tesla units. I've ordered a Backer replacement, but does the fact that two thermostats tripped indicate that the element is at fault?

 

We live in a hard water area, but the elements are incoloy-type, and nearly new.

 

Any suggestions?

 

Thanks 

Hi. They will be under the 1st year manufacturer warranty, so contact the cylinder supplier and ask for a free replacement. When it arrives, swap them over and job done. 
 

Monitor the performance of the replacement, and hopefully there will be no issues moving forward. 👍

[Little Clanger]

32 minutes ago, Nickfromwales said:

Hi. They will be under the 1st year manufacturer warranty, so contact the cylinder supplier and ask for a free replacement. When it arrives, swap them over and job done. 
 

Monitor the performance of the replacement, and hopefully there will be no issues moving forward. 👍

Thanks, Nick.

So you think the element has gone?

[John Carroll]

2 hours ago, Little Clanger said:

The thermostat in one of my immersion heaters keeps tripping. Immersion element, thermostat and cylinder are about 9 months old. Thermostat is set to 65°. This is the lower immersion heater in a dual heater Economy 7 cylinder.

 

I swapped the thermostat for the one from the upper immersion heater, but that tripped at some time during the night.

 

These are Tesla units. I've ordered a Backer replacement, but does the fact that two thermostats tripped indicate that the element is at fault?

 

We live in a hard water area, but the elements are incoloy-type, and nearly new.

 

Any suggestions?

 

Thanks 

You say the thermostat keeps tripping, do you mean the lower immersion is tripping the RCD (earth leakage trip) or that the hi limit stat is just switching out the immersion and requires resetting?.

[Little Clanger]

56 minutes ago, John Carroll said:

You say the thermostat keeps tripping, do you mean the lower immersion is tripping the RCD (earth leakage trip) or that the hi limit stat is just switching out the immersion and requires resetting?.

The latter, John.

The reset button on the immersion thermostat keeps popping out (but stays in for a while after being clicked back)

[SteamyTea]

Can you try the thermostat on the top element, see if the same thing happens.

[Nickfromwales]

3 hours ago, Little Clanger said:

Thanks, Nick.

So you think the element has gone?

Could just be a “Friday unit”. That simple.

 

Why lose any sleep? Just ask for a free replacement, fit, forget, go to pub. 👌. 

[John Carroll]

42 minutes ago, Little Clanger said:

The latter, John.

The reset button on the immersion thermostat keeps popping out (but stays in for a while after being clicked back)

Is the hi limit stat part of the control stat or a separate stand alone stat?, whether or which, ensure the (normal) control stat setting is not higher than, say, 70C, normally set to 60/65C, post a photo.

[Little Clanger]

1 hour ago, SteamyTea said:

Can you try the thermostat on the top element, see if the same thing happens.

Good idea, I'll try that. Ty

[Little Clanger]

38 minutes ago, Nickfromwales said:

Could just be a “Friday unit”. That simple.

 

Why lose any sleep? Just ask for a free replacement, fit, forget, go to pub. 👌. 

😉

[Little Clanger]

19 minutes ago, John Carroll said:

Is the hi limit stat part of the control stat or a separate stand alone stat?, whether or which, ensure the (normal) control stat setting is not higher than, say, 70C, normally set to 60/65C, post a photo.

The limit stat is part of the control stat. It's set to about 60/65 now.

Will send photo later.

Thanks 

[Nickfromwales]

These are all in one hi-limit plus regular stats combined. 
 

You just slide the whole thing out with some types, and leave the chassis in situ (dry replacement) or the lots got to be changed with cheaper units (wet replacement).

 

It’s just a duff unit most likely, but direct cylinders do have a tendency to go through lower immersions sooner as that’s the one doing the heavy lifting. Just not this much sooner   

[John Carroll]

Just wondering why a thermostat module that works in the upper immersion should trip due to apparent over temp  in the bottom even if the (bottom) heating element is faulty.

[Nickfromwales]

34 minutes ago, John Carroll said:

Just wondering why a thermostat module that works in the upper immersion should trip due to apparent over temp  in the bottom even if the (bottom) heating element is faulty.

Because…….its faulty. If neither were faulty you’d not be posting here   
 

Just sounds like a manufacturing defect, so the heats getting to where it shouldn’t be.

[Little Clanger]

4 hours ago, John Carroll said:

Is the hi limit stat part of the control stat or a separate stand alone stat?, whether or which, ensure the (normal) control stat setting is not higher than, say, 70C, normally set to 60/65C, post a photo.

Voila

[John Carroll]

9 minutes ago, Little Clanger said:

Voila

 

Voila!!, 

If not singing Voila, have you checked that the bottom stat works OK in the upper immersion?.

"I swapped the thermostat for the one from the upper immersion heater, but that tripped at some time during the night"

 

[Little Clanger]

 

2 hours ago, John Carroll said:

 

Voila!!, 

If not singing Voila, have you checked that the bottom stat works OK in the upper immersion?.

"I swapped the thermostat for the one from the upper immersion heater, but that tripped at some time during the night"

 

I've moved the thermostat from the bottom immersion into the upper immersion, and put it on 'Boost'.

It hasn't tripped yet, but the upper immersion doesn't stay on for as long as the lower one, of course, because it reaches set temperature more quickly.

[Nickfromwales]

3 hours ago, Little Clanger said:

 

I've moved the thermostat from the bottom immersion into the upper immersion, and put it on 'Boost'.

It hasn't tripped yet, but the upper immersion doesn't stay on for as long as the lower one, of course, because it reaches set temperature more quickly.

🙄🤐

[John Carroll]

On 05/03/2025 at 21:04, Little Clanger said:

 

I've moved the thermostat from the bottom immersion into the upper immersion, and put it on 'Boost'.

It hasn't tripped yet, but the upper immersion doesn't stay on for as long as the lower one, of course, because it reaches set temperature more quickly.

 

Please keep us informed of the "fix".

 

Of course the upper immersion doesn't stay on as long as the lower one as it will only be heating ~ 50% of the cylinder vol and maybe, even less, so only on for 1/2 the time or so.

What's puzzling, to me at any rate, is that the (lower) stat is presumably switching the power off via its Hi limit stat which is part of the combined stat, yet a known perfectly good combination stat, the upper one,  isnt switching off the power (in the lower immsersion)  via its  control stat, yet is switching the upper immersion off.

The only logic I can apply to this is that the lower immersion isn't wired (properly) through the stat and just depends on the time its programmed on, the stat will then open the hi limit stat but this then, under those conditions shouldn't prevent the bottom immersion switching in again later.

I presume you found out first  that its not in fact switching back in due to the hi limit stat operating, which brings me back to square one.

 

What are you renewing?, is it the lower heating element?.

Edited Friday at 08:29 by John Carroll

[Little Clanger]

12 hours ago, John Carroll said:

What are you renewing?, is it the lower heating element?.

Initially, I'm renewing the thermostat in the lower immersion heater, with a different make (a Backer to replace a Tesla).

 

A continuity test on the element gave me 21 ohms, which I understand is about right.

 

Confusingly, it was working fine last night - heated a whole 162 litre cylinder without tripping.

 

By the way, a note to NickfromWales - I haven't been able to follow your advice about returning the cylinder because I bought the cylinder without any immersion heaters fitted. I bought the immersion heaters from Screwfix, but didn't keep the receipt, so can't return them.

[John Carroll]

21 ohms @ 230V, UK standard voltage?  gives a power output of 2.519kW, it requires 9.42kWh to heat 162L from 10C to 60C so a max of 3 hrs 44min to achieve this, and just over 4 hours if stat set to 65C.

Have you tried the bottom stat in the upper immersion?, you can easily monitor that during the day by draining off some water to get it to cut in on boost.

Can you post a photo of the wiring at/around the stat.

Edited Friday at 21:44 by John Carroll

[SteamyTea]

Could it be the sleeve the stat goes in is bent and touching the element?

[John Carroll]

It could I suppose but again one might think that the control stat should switch out the power before the hi limit, I think the hi limit is set somewhere around 80/85C, one might then expect the cylinder to not heat to its SP temperature, there's only one way to check it out, remove the heating element.

[John Carroll]

9 hours ago, John Carroll said:

21 ohms @ 230V, UK standard voltage?  gives a power output of 2.519kW, it requires 9.42kWh to heat 162L from 10C to 60C so a max of 3 hrs 44min to achieve this, and just over 4 hours if stat set to 65C.

Have you tried the bottom stat in the upper immersion?, you can easily monitor that during the day by draining off some water to get it to cut in on boost.

Can you post a photo of the wiring at/around the stat.

I see you have tried the bottom stat in the top with no problems.

[Little Clanger]

12 hours ago, John Carroll said:

Can you post a photo of the wiring at/around the stat.

Voila encore