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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.
  • Former Community Member
    0 Former Community Member
    I think I'm being persuaded for "one at a time".  Cheers
  • 0
    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.
  • Former Community Member
    0 Former Community Member
    Many thanks
  • 0
    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.


    I trust that the work will be notified in the usual way. If you replace the 2.5 mm² cable, it will be a new circuit, so that would be two items for notification.
  • 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.


  • Former Community Member
    0 Former Community Member
    At the mo, it will be one immersion coonnected to the existing circuit, so no notification needed?.  I may have the the upper one appropriately connected later.


    The heating guys are fully qualified and taking care of building regs and the admin on that side.
  • 0
    DC:

    At the mo, it will be one immersion coonnected to the existing circuit, so no notification needed?.  I may have the the upper one appropriately connected later.


    The heating guys are fully qualified and taking care of building regs and the admin on that side.


    What is the kW rating of the elements? Some are just 2kW. Perhaps the top one can just be plugged into a 13 Amp socket.


    The most important thing is to have the pressurised bomb regularly serviced for safety reasons.

    Exploding Unvented Hot Water Cylinder.wmv - YouTube


    Z.


  • Former Community Member
    0 Former Community Member
    We have two baths - a "normal" one and a large corner bath.  With the old hot water cylinder (removed this morning) (162 litres) you could just about have one big bath and one normal bath one after another/simultaneously - the baths would be as hot as you could bear, but top ups from the taps would be lukewarm.  But if a third person - normally me, wanted a bath, you had to wait.  Not sure if we tried it, but three normal baths in a row might have been feasible.


    The new unvented one is 250l, so that should not be an occasional issue.  I'll just use the top immersion as redundancy when the lower one goes - ie wire it in as a temporary fix.


    Thanks for you comments everyone - very helpful at clarifying things in my mind.
  • Former Community Member
    0 Former Community Member
    Yes, annual service contract for sure.


    They are both 3kW each - which is 13A at 230V.  Hmm - like the idea of having a plug on it in case of need.... Cheers Zoom up.
  • 0
    Don’t fit plugs and use sockets, they will melt.

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