Immersion heater removal

11 Jan 2009
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United Kingdom
Hi all,
Our central heating broke down yesterday so while waiting for BG in next couple of days , I thought I'll use the immersion heater for hot water and that tripped the electrics instantly on connection. I had a look at the (top entry) immersion heater and notice a very tiny amount of water so looks like a replacement is required.

What's the best way of unscrewing the heater as it won't move using a box spanner
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Keep the cylinder full and try a sharp tap or two on the spanner to TIGHTEN the heater. This can break the scale that is resisting your attempts to loosen the heater.
I'll give the tightening approach a try on the box spanner, I can't see a 86mm slogging spanner.

Just incase .... would a (cheap) blow torch work if the above method fails me?
Last edited:
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Hurray tightening did it!

Before putting the new one in, Are there any check I can do to make sure its actually ok. This current one certainly never worked in 10yrs
Disconnect incoming live and neutral. With multimeter,test resistance across element terminals, should be 18-20Ω .
Have you removed the immersion heater from the cylinder ?
Did you do that before establishing what caused the electrics to trip ?
@terryplumb , While still in the cylinder yesterday, I disconnected it from its original fused socket and wired it to standard 13 amp plug and it tripped as well. This ruled out any existing wiring problems. It was when I inspected the terminals that I realised there was a very thin film of water at its base ..... so I think this was the cause of the trip. So looks like the solder joint was weak.

@polesapart, I am getting 20 ohms on the terminals so looks good at least.

However I screwed the new heater back in and there is a leak :( I did use some ptfe tape as well as the washer.

I am going to take it apart again now so hoping for more tips on making sure there are no leaks.
Clean the cylinders mating surface .
Make sure the new seal that comes with the element is still good. Thread sealing compound on the immersions threads , if you have any,or several wraps of PTFE ,tightly wound , if you don't.
1. Don't just check the resistance of the element, also check that the element is not shorting to the body of the heater.
2. With immersion totally disconnected from the mains:
2a. With multi-meter on continuity range, connect one meter lead to earth terminal on immersion.
2b. With other meter lead, check continuity between each element terminal and earth. There should NOT be continuity.
2c. Repeat test on the highest resistance range on the meter. Resistance should be many megohms, or infinite.
3. If either test fails, immersion heater must be replaced.
4. Re. leaks:
4a. Make sure there are no remnants of the old washer stuck to either the heater or the sealing face of the hole it screw into.
4b. Make sure there are no deep scratches to either face.
4c. If using the old immersion, use a new washer. E.g. Toolstation 73273
5. Wipe some silicone grease in the threads of the boss the heater screws into. Will make it easier next time!
ok, off to Screwfix to get a wire brush and fernox jointing compound. I have them somewhere in the shed but I can hardly get through its door let alone find it.
I wouldn't use a wire brush on the mating surface !

Ahh ok!, old tooth brush then.

@oldbuffer, I'll do the extra tests once I get the new Thermo back out.

Which of these 2 should I use with the PTFE/washer ?:-

Fernox Jointing compound
Fernox LS-X Leak Sealer
Fernox jointing compound.
Is there any trace of the old washer left on the cylinders mating surface?
Fernox jointing compound.
Is there any trace of the old washer left on the cylinders mating surface?

I already scraped it off.

Out of curiosity. is the tight seal meant to be on the thread or mating surface ?.
Also does the ptfe go on clockwise or anticlockwise ?

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