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Twin immersion heaters

Former Community Member
Former Community Member
I am in the process of going unvented.  I have to use immersion heaters, and the new cylinder has two.  I have one feed to the existing immersion.  It is on 2.5mm2 T&E and connects to a 32A MCB on the CU.  I'm adding a DP isolating switch in the cylinder cupboard.  Will that be complaint for one of the immersion heaters?

To fully install the cylinder (ie the second immersion), I plan ask an electrician to either
(1) run an additional 2.5mm2 T&E cable back to the CU, connect it to the existing 32A immersion heater MCB at the CU. So the MCB protects two separate cables and the 2.5 T&E is within its capacities. 

Or
(2) run a new 6mm2 T&E cable back to the CU, connect to the existing MCB and in the cylinder cupboard, split the feed to two DP isolators, one for each immersion and its timer.


I'd be grateful for advice - are both approaches compliant?  Is one better than the other?  The amount of work will be similar.
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    Chris Pearson:

    Normally, one heater should do, but when you have company, you my wish to be able to heat the water quickly enough to be able to run two or more baths in succession.


    A two immersion cylinder should be big enough to fill most baths without using all the available hot water, then the cylinder will reheat whilst the bath is in use, unless you’re running a rapid production line type operation with one out, one in, in rapid succession.


    I have actually stood with a stop watch and thermometer checking how long it takes to fill an upright walk in bath at 38 degrees from a standard cylinder and the water only came to an inch above the seat.


    If you have a cylinder with two immersions I would expect it to be big enough to fill the bath to the brim, should you do desire.


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  • 0
    Chris Pearson:

    Normally, one heater should do, but when you have company, you my wish to be able to heat the water quickly enough to be able to run two or more baths in succession.


    A two immersion cylinder should be big enough to fill most baths without using all the available hot water, then the cylinder will reheat whilst the bath is in use, unless you’re running a rapid production line type operation with one out, one in, in rapid succession.


    I have actually stood with a stop watch and thermometer checking how long it takes to fill an upright walk in bath at 38 degrees from a standard cylinder and the water only came to an inch above the seat.


    If you have a cylinder with two immersions I would expect it to be big enough to fill the bath to the brim, should you do desire.


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