Twin immersion heaters

Well you need not have overload protection so that does away with one of the three reasons you might use a fuse or MCB of a certain value.

Second reason - short circuit, covered

Third reason - earth fault, covered.


So any will do.


Next question, which option will be best value in time and materials and if the 6.0 wins could you use 4.0 instead?


Supplementary question - might you need to consider future additions and alterations ( a note on the consumer unit might help the unwary from false assumptions)

That 2.5 mm² T&E ought to be on a 16 A or 20 A MCB, although as ebee says, with only one (3kW) immersion, the cable could not be overloaded. By the same argument, a second 2.5 mm² cable could be added to the 32 A MCB. If you take each 2.5 mm² cable to its own isolator and then connect the supply side of the two isolators, you and up with a ring final circuit.


Alternatively, change it for a 4 mm² radial. If it is all left loafing under the floorboards, that is just fine, but if it goes through any form of insulation (thermal or acoustic) it may be necessary to go up a size to 6 mm².


HTH.

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


Depending on if you have a two tariff supply or not and how the consumer unit is configured, one circuit may supply both being switched from one to the other or it may be beneficial to install two circuits if you have a peak and off peak consumer unit.


So what tariff are you on and how many consumer units are there?

Just a straight tariff, not Economy seven or variant.

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.

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.

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.

"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

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.

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.