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Electric boiler wiring

KMA1987

Hoping someone can help me with a question on the wiring requirements for a 12kw electric boiler.

For context, I’m attempting to have a botched installation rectified after the manufacturer said the warranty had been invalidated by an incorrect installation (both plumbing and wiring aspects). I’m not an electrician and I’ve had a couple of electricians look at it now and received different answers.

The boiler in question is a Trianco Aztec Classic 12kw (installation manual here: https://heatingpartswarehouse.co.uk/wp-content/uploads/2023/06/Aztec-Classic.pdf). The relevant page for the wiring instructions is 16.

My specific question is whether the boiler and the external wiring controls (we have a Reliance 8 zone wiring centre) should be on separate circuits? The wiring requirements state: “Miniature circuit breakers (MCB) MUST be fitted between RCD unit and boiler and RCD and any external controls….An additional MCB rated 6A will be required to supply the external controls.”

The current setup is the boiler and controls are on a 63A type B RCBO, with a switched fused spur on the same circuit for the controls. Whether this was ever correct/appropriate for the previous boiler I can’t say, but the installer has obviously just tried reuse what was already there on a like-for-like basis with the new boiler (different manufacturer/model).

One electrician has said the external controls should have been installed on a separate circuit with its own MCB, and the other that it can be on the same circuit and just replace the fused spur with a 6A MCB.

Happy to provide more information and grateful if there’s anyone with experience with electric boilers that may be able to help.

  • 0 in reply to Chris Pearson

    The only thing that switches off the boiler display is the rotary isolator. Both the “boiler control isolator” and “UFH isolator” turn off both the UFH wiring centre and room thermostats; they don’t do anything to the boiler or its display. 

  • 0 in reply to AJJewsbury

    Oddly I'd done a similar sum but assumed 2,5mm2 in singles  and decided it was OK .....   I guess you could use 1.5mm2 flex, or any other cable where the earth core is full size rather than ordering a reel of Irish T and E to be imported. But even so I'd try and keep that link below 3m long and inside something like conduit and not worry further. 

    Mike.

  • 0 in reply to KMA1987
    KMA1987 said:
    The only thing that switches off the boiler display is the rotary isolator. Both the “boiler control isolator” and “UFH isolator” turn off both the UFH wiring centre and room thermostats; they don’t do anything to the boiler or its display. 

    I see. The boiler display is part of the boiler.

    The UFH wiring centre and the thermostats are what the manual calls, "external controls". I cannot see any real reason why the various bits should not be isolated independently. Neither BS 7671, nor the manufacturer's instructions (as I read them) requires isolation of the neutral. If it is necessary, the main switch may be opened.

    I think that the situation could be improved, but I do not think that is necessary. A C3 rather than a C2 in the electrical world.

  • 0 in reply to Sparkingchip

    there is not electrical separation between the two circuits as they can  share a neutral

    The OP's one seems to be different - as the instructions call for a relay or contactor between the control wiring and the boiler's "volt free" terminals..

       - Andy.

  • 0 in reply to Sparkingchip

    If the boiler is not wired correctly the service engineer could be electrocuted if they are not very careful

    I'm seeing it the other way around - the boiler only has the one (63A) supply. The risk is at the wiring centre where the contactor/relay has the control circuit on its coil side and the boiler circuit on the contacts side.

    I've seen some systems where there can be more than one control circuit - especially where UFH has been added to a conventional system. Typically the UFH wiring centre provides a "volt free" contact (relay) to call for heat, which the main control wiring uses, and in this sort of situation would in tern use anther relay to switch the boiler. No shared neutrals and normally just labels on the wiring centres saying  to isolate both circuits before removing covers. Nothing too out of the ordinary there. My HP works on a similar principle but uses SELV CAN-BUS(ish) for communication rather than relays - but the circuit separation is similar,

       - Andy.

  • 0

  • 0 in reply to Sparkingchip

    That does seem rather different to the OP's version..

    Given the obvious problems of a shared N, I'd hope that the OP's is a new revised version and the one you found an outdated one?

       - Andy.

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