Twin immersion heaters

Many thanks

If both are switched on at the same time the top one will turn off on its thermostat anyway once the top half of the cylinder is heated.


I assume you have baths on a regular basis and will heat the whole cylinder accordingly, but if not the whole cylinder needs heating at least once a week as only using the top heater could result in nasty bugs living in the lower part of the tank.

I think I'm being persuaded for "one at a time".  Cheers

ebee, I liked the idea of two heaters for redundancy - for when they go on the blink - and recovery from when we've had too many baths.....


I can see how one heater can cover the latter.  And with some of the great suggestions posted here, I can get some redundancy by being able to switch between the two.

Do you REALLY want what would effectively be a 6 kW immersion heater in a domestic unvented cylinder?


Then there’s the issue of manufacturers instructions, which are particularly important with unvented cylinders.

"You should not be using both immersion heaters at the same time, just one or the other. Neither of the plans appear sensible."


From a purely electrical view it`s Ok.


But as Sparkingchip has questioned why two heaters?

They are usually top and bottom but I suppose you could have them side by side bespoke. 

Do you want a load of 6KW for a long time might be a question depending upon tank size

Depends how you envisage using them

I have such an installation I installed a couple of years ago to replace a Fortic tank with a single immersion. I had only a single 2.5mm from the CU and it would have meant major surgery to get a second circuit from the front of the house to the back where the unvented tank is. I opted for the single cable to to terminate in to a 20A switch to use as an on and off switch which supplies a 2 way 20A switch which selects the upper heater or the lower heater. The upper heater  switch position is marked day/ boost and the lower is marked night to benefit from my E7 tarrriff.

What is wrong with the B32 and two 2.5s Sparkingchip? Overload is impossible as I'm sure you realise? Ring connection is not necessary Chris, although it doesn't hurt and satidfies the overload point which worries some people. I expect only 1 heater at a time, but both would work safely. However it would be easier to have a Bath/Sink switch and just use the existing 2.5 cable. Job done. Local isolation/switching on/off is not necessary, although you may find it convenient, along with a timer. Ensure no non-return valves have been fitted anywhere back to the water main.

1. Keep the existing circuit with a B16 MCB and install a bath/basin switch to control both immersion heaters.  https://www.electrical2go.co.uk/mode-20a-dp-bath-sink-switch.html


2. Keep the existing circuit with a B16 MCB and install a timer to control the bottom heater with a manual boost for the top Economy 7 Digital Timeswitch + Boost Control (tlc-direct.co.uk)


3. Keep the existing circuit with a B16 MCB and install a new circuit with another B16 MCB to supply each immersion heater through separate timers, however both heaters should not be run at the same time, so this is not a good plan.


Because it is an unvented cylinder both immersion heaters must be controlled through separate dual thermostats to avoid overheating of the cylinder. As the cylinder is pressurised the boiling point of the water within it is raised and you really don't want installation to be incorrect.

Just a straight tariff, not Economy seven or variant.