Immersion Heater Thermostat scorching

StephenB · Feb 4, 2016

When I removed the cover from the immersion heater in order to adjust the thermostat I found the thermostat had very deep scorch marks and I could see sparks when I switched the power back on. However, both the thermostat and immersion heater still seemed to be functioning properly. I swapped the thermostat with the one from the top (booster) immersion heater that had never been used. When I switched on again there was immediately a smell of melting plastic and brown scorch marks began to appear on the thermostat plastic cover. I swapped them back again because at least the original thermostat had melted all the plastic it was going to melt and didn't smell of scorched plastic any more. Both the immersion heaters are Cotherm model ELE-14-IN-800-FAB and the thermostats are model TSR00014. Has anybody seen this problem before? Is it normal? Any suggestions what to do about it? I don't want to leave these scorched thermostats in place even though they still seem to be working to control the water temperature. Any suggestions welcome.

Leesparkykent · Feb 8, 2016

StephenB said:
You're right, when I replaced the stat with the (never before used) one from the booster, that started scorching as well. This suggests that the problem is with the connection between the element and the stat, and that it's on the element side. I don't want to replace the element as that involves draining the water, so I need to be sure I've solved the connection problem on that side before fitting a new stat. Do you have any suggestions on the best way to clean and tighten the connectors on the element? Thanks, StephenB

Best to get a plumber in to change the immersion if you are not sure. As davesparks pointed out especially if its an unvented cylinder.

StephenB · Feb 10, 2016

Lucien Nunes said:
You could try pinching the element socket contact slots shut slightly with needle nose pliers, then slide the stat in and out a couple of times to burnish the surface of the points that actually make contact but I suspect it will continue to burn out. When the contacts overheat, the metal loses its spring temper and will never grip the blade on the stat properly again.

Thanks for this. I'm a bit scared of anything to do with plumbing but I decided to check out replacing the whole immersion heater, element and stat. It turns out I can get a complete 14" titanium unit from Screwfix for £28 whereas I've been quoted £61 just for the Cotherm stat from Newey & Eyre. It seems I'd better start trying to improve my plumbing skills.

StephenB · Feb 10, 2016

davesparks said:
Be careful when you are draining the tank if it is an invented one, it needs to be done correctly otherwise they can implode which is a little bit scary and a big bit dangerous.
I saw one implode when a plumber didn't do it right, I think the whole street heard the bang it made.

Thanks for this, and your previous comments. Not sure what you mean by an invented tank. Here's a photo of the tank. Is this what you mean? I assume I should open some of the hot taps after shutting off the cold supply and before opening the drain, so the tank can back-fill with air from the hot taps while it's draining. Is there something else I should know?

[ElectriciansForums.net] Immersion Heater Thermostat scorching

Not sure why the tank appears horizontal in this photo. It's obviously vertical from the position of the inlet/drain, the two immersion heater covers, and the text on the label. Thanks again, StephenB

ruston · Feb 10, 2016

Yes , need the air to be replaced as you draw water off or you will cause a vacuum if it is unvented , and may collapse the tank.
Incidentally. It does say on that label 'to use only manufacturers recommended parts'.
if this is a sealed system I would be very careful , as there are regulations as to who can install these.

Geordie Spark · Feb 10, 2016

Murdoch said:
and there was me thinking you are a 6 inch nail man!

He was - until he realised scrap 2.5 is cheaper meaning more money for Hobgoblin. :8:

davesparks · Feb 10, 2016

StephenB said:
Thanks for this, and your previous comments. Not sure what you mean by an invented tank. Here's a photo of the tank. Is this what you mean? I assume I should open some of the hot taps after shutting off the cold supply and before opening the drain, so the tank can back-fill with air from the hot taps while it's draining. Is there something else I should know? View attachment 31461 Not sure why the tank appears horizontal in this photo. It's obviously vertical from the position of the inlet/drain, the two immersion heater covers, and the text on the label. Thanks again, StephenB

It should have said unvented, not invented, which is what that tank appears to be.

You need to read the manufacturers instructions carefully to establish the correct procedure for draining it and refilling, otherwise you risk damaging the tank or upsetting the pressurised diaphragm inside (if present in that one)

As someone else mentioned you are also technically supposed to be qualified and registered to carry out any work on these.

StephenB · Feb 15, 2016

davesparks said:
It should have said unvented, not invented, which is what that tank appears to be.

You need to read the manufacturers instructions carefully to establish the correct procedure for draining it and refilling, otherwise you risk damaging the tank or upsetting the pressurised diaphragm inside (if present in that one)

As someone else mentioned you are also technically supposed to be qualified and registered to carry out any work on these.

Thanks Dave. Okay, I give up. I found the installation instructions on the Dimplex website and decided I need to get a Dimplex approved plumber in. Unfortunately the nearest one is in Swindon, 35 miles away. I'm waiting for them to call back to fix an appointment. In the mean time I'm relying on the booster element for hot water as I'm worried that the sparking on the bottom one will set fire to the insulation.