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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.
  • 0
    "Sparkingchip:

    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.


    Yes Manufacturers instructions must be taken into account and unvented cylinder regs must be obeyed. From a wiring point of view all things are possible  .


    What in the unvented regs dictates or errs toward 6KW being ill advised? (Not being an expert of such myself)


     


  • Former Community Member
    0 Former Community Member
    Thanks - just saw another thread on 13A capacity of today's kit versus 1960s and 1970s - the consensus seemed to be to keep 3kW rated appliances off ring mains, despite the nominal rating.  I suppose an domestic electric fire or fan heater at that rating would have a room to cool, immersions are normally in cupboards and the wiring cannot dissipate heat so well.
  • 0
    Sparkingchip:

    Don’t fit plugs and use sockets, they will melt.


    Agreed, for a 3kW immersion heater do not use 13 Amp plugs and sockets. A 20 Amp double pole switch is better.


    Service the beast every year for safety.


    Also it is wise to design the system so that items that will need to be removed or replaced, like the immersion heaters can actually be removed easily in the future. Also, expansion vessels often fail and need replacing. Any switches, valves or isolators should be accessible for repair or replacement. So boxing stuff in, or placing it too near to a wall or ceiling e.g. is a bad idea.


    Z.

    UNVENTED CYLINDERS FOR HOT WATER - Plumbing Tips - YouTube


    Z.


  • 0
    Don’t fit plugs and use sockets, they will melt.
  • 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.
  • 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.
  • 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
    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
    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.


  • 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.

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