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No neutral wire to upgrade thermostat

Discussion in 'Plumbing and Central Heating' started by A Balchin, 11 Oct 2021.

  1. A Balchin

    A Balchin

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    I'm replacing an old Honeywell CM907 thermostat with a Drayton Wiser smart thermostat.

    [​IMG]

    The current thermostat is wired directly into the boiler (Vaillant EcoTec 831) and only uses a live a switch live. The new thermostat requires a neutral wire. I don't know how to wire in a neutral myself, is there a way around doing it?

    [​IMG]
    New thermostat wiring diagram
     
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  3. oldbutnotdead

    oldbutnotdead

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    The controller requires a neutral because its innards use power. You need to use the 3rd core in that piece of flex (sleeved appropriately).
    Nicest way to do it:
    Find the other end of that flex (might go straight to the boiler, might go to a wiring centre).
    At the boiler (or wiring centre) move the blue core to a neutral.
    Leave the brown core where it is.
    Sleeve the green and yellow core (brown or black or red) and connect it to wherever the blue core was connected.
    At the thermostat end, brown core goes to L and link to 1
    Blue core goes to N.
    Sleeved (same colour sleeving as at boiler end) green/yellow goes to 3.
    Set the heating on the boiler controller to CONSTANT.
    Program and run via your new Wiser.
    NB Isolate supplies before doing this.
     
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  4. A Balchin

    A Balchin

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    Thanks for the response. That sounds like a good plan. I've opened up the boiler and you can see the live, neutral and earth of the power supply to the boiler. The thermostat is connected to 3 and 4. Can I just connect the blue core to the neutral of the power supply or should I move the live and neutral to the same L and N of the power supply and just use the eath as the switch live on 3?

    IMG_2091.JPG IMG_2092.JPG IMG_2093.JPG
     
  5. CBW

    CBW

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    Personally I wouldn’t use the green and yellow (earth/cpc) as a live conductor, even though you plan to sleeve it. The mains in for your boiler is on twin and earth (not generally suitable for inside a boiler), but you could obtain some 5 core heat resistant flex for your thermostat, piggy back the live and neutral so they are permanent, then 3 to com and 4 to On, connect the earth and the other end in a choc block to keep continuity.

    The cover on your boiler forms part of the combustion circuit, so recommended to get it checked for safety afterwards.
     
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  6. DP

    DP

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    The red and black conductors, can they not be bigger gauge. Does that boiler need 20 amp capacity And solid conductors
     
  7. DP

    DP

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    Good idea CBW

    Or a battery operated programmable thermostat?
     
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  8. Harry Bloomfield

    Harry Bloomfield

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    20amp - Most boilers draw much less than 2amps.
     
  9. DP

    DP

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    So why use what looks like 2.5 twin and earth cable.
     
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  11. oldbutnotdead

    oldbutnotdead

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    Agreed sleeving the yellow/green core is not ideal but the flex at the stat end looks buried in plaster so replacing it may be a significant task if it is remote from the boiler.
    I'd advise leaving the brown core where it is for now-that way it is protected by any internal fuses in the boiler- move the blue core to neutral and use sleeved green/yellow to 3 at the boiler end.
    If you get problems with power being interrupted to the Wiser then you'll have to think again (5 core flex as @CBW )
     
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  12. Harry Bloomfield

    Harry Bloomfield

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    Perhaps 2.5mm was all that was available?
     
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  13. fezster

    fezster

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    Drayton Wiser uses wireless thermostats. You should locate the wiser hub close to the boiler, wired from a fused spur, and then link out the terminals where your current stat is (insulate the wires sufficiently, as they will be permanently live). You can then mount the wireless thermostat in the same location.

    Alternatively, you could link out the current thermostat wires in your wiring centre, so that the wires in the wall are not live.
     
  14. DP

    DP

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    That does not make it right though. Have seen this heavy conductors pull the termination off the pcb hence the makers stipulate the conductor size
     
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  15. DP

    DP

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    Why bother. Fit a Danfoss Tp5000. 2 wire circuit. Wires already exist.
     
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  16. DP

    DP

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    Looked at the picture again on post 1

    That is a battery powered thermostat hence neutral not required. As I have said above, keep it simple, fit a battery powered thermostat
     
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  17. A Balchin

    A Balchin

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    Thanks for the help everyone. I followed the advice to get new flex and wire into the boiler locally. Works a charm
     
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