Immersion heater element sizing?

29 Jan 2008
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United Kingdom
Hi all

I have an immersion heater which has never worked, I’m guessing due to a broken element as there is power to the switch on the wall (led lights up) but the water never gets hot unless the boiler is on.

I’ve removed the cap but nowhere on the element does it say what size it is!

The cylinder has a label that says ‘Indirect Cylinder 36 x 18’, and the element goes into the top.

Sorry if this is a stupid question, but is a 27inch Heating element ok for this? Like this one:

Ideally I want to get all the bits before removing the old one so I can do the job in one go.

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It could be that the thermostat under the immersion heater cover has failed. If it's a modern one, the thermostat might have tripped and be resettable. Turn the power off, remove the cover and post a photo, before assuming it is the element which has failed.
Top entry immersions are 27" to allow them to heat the majority of the cylinder. Side entry, obviously the cylinder is only 18" diameter so an 11" is used.

Good call from Harry, check the stat is working and there's not s reset that popped out before condemning the element. If you do need to replace it, undo the existing with care, they can be an absolute pig to get out, and you can risk damaging the cylinder if it's really tight!
Hi @Harry Bloomfield and @Hugh Jaleak, thanks for your help.

Here’s a couple of pictures. I did see a couple of vids about the thermostat but there doesn’t appear to be a switch to push down.

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Immersion looks ancient, so wont have a cutout. It's these earlier types that are so dangerous, if the stat fails, then it keeps heating, to the point where it boils the water. This then causes the water in the cold storage cistern above to boil too eventually, which leads to the cistern collapsing and dumping it's contents on to whatever is below. Tragically in 2 separate cases this has been a Mother and a Baby.

The new replacement types should have a cutout, much safer in the event of a fault. Worth noting though, if you have a gas boiler, that remains the cheapest way of heating your hot water, immersion is better kept for a backup in the event of a boiler breakdown.
That is quite an old one, it has no trip button and marked in degrees F.

If you have a meter - Make sure the power is off and with meter set for continuity, there should be continuity across the two 'stat terminals.

Likewise see what the resistance is across the two immersion heater terminals. It should be around 20 to 25 Ohms.
Thanks for the feedback guys, I appreciate the guidance.

The new element from Screwfix has a Thermal cut out, so hopefully will be safer.

To be honest, I’m mid-build of a diy extension but I need to decommission the Gloworm boiler - which is just as old as the immersion - as the wall the boiler is currently on is getting knocked through.

My plan is to get the immersion working so that we can have hot water for the next few months once the gas man disconnects the boiler (will live without the heating). Then a few months down the line the immersion will be removed as we’ll get a combi boiler in the (to be built) kitchen.
How would I know if my immersion heater has a safety cutout? Installed in 2010 so I'm assuming it does. At what temperature does it kick in?
How would I know if my immersion heater has a safety cutout? Installed in 2010 so I'm assuming it does. At what temperature does it kick in?

Turn the power off, remove the metal cover and take a look at the thermostat. If it has the cutout, there will be some sort of button to reset it.

I will have to guess at the temperature it should trip, which is hotter than the hottest you can set the adjustable - it should trip at around 85C.

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