Wiring two immersion heaters

2 Sep 2003
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United Kingdom
I apologise in advance for posting the same message in two forums, I wasn't sure if this was more of an electrics based problem after I'd posted it in the plumbing forum

I have replaced my hot water direct cylinder (100 litre) with a larger capacity cylinder (125). The original had 1 immersion heater that was wired up to an on/off control switch.

After removing the old cylinder and connecting the new one to the water pipes I then connected each of the two immersion heaters to a junction box and then connected this (using the original immersion heater cable) to the control switch. Unfortunately the bottom immersion heater's cut off keeps tripping so I only heat the top half of the cylinder.

Does my wiring technique leave something to be desired? Is anyone able to confirm whether one 11 '' immersion heater situated near the bottom of the 125 litre cylinder will suffice - it is on 24/7.

If the wiring is ok is it likely that the bottom immersion heater is faulty? If not can anyone suggest anything, I'm a complete novice so any info would be great


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Hopefully someone out there will be able to check what I'm saying here but I thought that two heaters on one cylinder weren't meant to work at the same time.

One is for use over night when the leccy is cheap and the other is used if you need to heat up the water quickly. I didn't think they were meant to work at the same time. I can't remember if the top one is the quick heat one and the bottom is the cheap overnight heater or vice-versa.

Don't know if that helps or just muddies the water! :confused:
i agree with Tindog the bottom one is for overnight use, but i also replied in the origonal post in the plumbing forum
I would have thought the top heater is for when you need a small amount of hot water and the bottom heater is for when you need a large amount
If you are using one heater on a cheaper tarriff through the night then that would be the bottom one and it would be off in the day time.
You could use the top one in the day time to boost the temp.
I gather your not on a cheaper tarriff,this being the case. you need to able to switch either or both 'on' or 'off'.
Secondly your heaters may have a thermostat incorporated which is adjustable and your bottom heater stat may be set too low.
It's the wattage of the heater,not the length, that matters and I would think the bottom heater may well have a higher wattage than the top one as it has more water to heat.
These are my thoughts on the matter which may or maynot help
:rolleyes: :rolleyes: :rolleyes:
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That's great. I wondered about economy 7 and all that but wasn't quite sure if this had something to do with it. Large cylinders had up to 3 immersion heaters so that confused me slightly.

I'm 99% certain that each immersion is 3kW. The immersions came with the cylinder as part of a package but the instructions were limited to say the least.

Do any of you know why one is cutting out? Any ideas would be greatly recieved. Having people to bounce ideas off certainly helps.

Many thanks to you all,

Hi Mark,
don't quite understand when you say 'one is cutting out'. Do I take it there is a neon indicator light that goes off and on . It will have a thermostat that is supposed to cut out when specified temp. reached and come back on again when a lower temp. is reached and if a light goes off and on when the heater is off and on then theres no problem.
If a 3kw heater was to be operating without a stat 24/7 you'd have more than just hot water , you could have one hell of a leccy bill.
You also said they were both wired to one switch through a junction box, so they are both on 24/7.
Why not have them both on separate timers, it seems a waste to have them on before 5am or after 10pm
:rolleyes: :rolleyes: :rolleyes:
Sorry, probably not as clear as it could have been.

By cutting out I mean that the thermal cut off on the thermostat keeps tripping. Funnily enough the immersion heater does not have a temperature guide on it. Originally I set is quite low so I know that the temperature did not rise over 85 degrees.

Can anything other then the temperature going OTT cause the thermal cut off on the immersion heater's thermostat to trip? I was thinking that because I have wired up two immersions where there used to be one it may be doing naughty things, but then again I certainly no electrician!

Hope this gives more insight


The way your post is written, you have connected both elements to the one supply circuit - do NOT do this, the circuit cannot cope with two elements running at the same time. You MUST have two independant 15amp circuits from the consumer unit to use two elements.

Each element will be 3Kw (top one could be 2kw but this is unlikely). The circuit is designed for 15 amps - both elements together will take 25 amps, thereby overloading the switch, cable, and fuse. DO NOT switch them both on together, and modify the wiring to ensure it cannot happen (ie disconnect one or add a new circuit). Please ensure you do this, there is a potentiallly dangerous situation here.

The true use of these two-element beasties, as pointed out, is for economy 7 or the like. The whole tank can be heated overnight on cheap rate by the bottom element then the top one can be used to top up the supply if it runs low.

To be honest, I would probably just wire the bottom element in and use that. It will do the job just fine, it will just be a bit more pricey to heat than the old smaller tank. I assume you use a fair bit of hot water (hence the bigger tank) so it's probably not worth using the top one.

As for your thermostat cutting out, if that is what you are saying, it may simply be a dodgy stat. swap the top stat for the bottom one, then try them (not together!!). If the bottom one is now fine, it was a dodgy stat.- try the top one first - once the bottom one has warmed the water above it, the top stat will switch off anyway!

Finally, you say you set the thermostat low - 85 degrees is not low, the normal setting is 55-60 degrees (for hard water). 85 degrees is way to hot for a tank stat setting. It'll shorten the life of your element and the water could cause scolding.

Hope this helps - ask if you need more info..

Take care


Thanks for the detailed advice. I'll post all you guys the results of switching the immersions over (I don't think the stats are interchangable on these immersions - but I'll give it a try).

Once again, cheers everyone


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