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Replacing Honeywell CM927 with Nest Heatlink

Discussion in 'Plumbing and Central Heating' started by Johnpcvinson, 27 Nov 2017.

  1. Johnpcvinson

    Johnpcvinson

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    Hi Stem,
    I have exactly the same set up as Dwag. just after a bit of confirmation. I seem to have some linked lives in my set up, see picture. Wire at bottom of photo is my fused spur live coming into the BDR91 (NL) no earths connected, but assuming BDR91 is double insulated). There is then a link live between the two Lives in the left block and another link across to A. Top wire from boiler live is linked to A (live) N to N (left block) and grey (Connection 4 internal (room thermostat) on Vaillant ecoTec). So assume this is heat call? So on NEST, Boiler L to L in Nest, Boiler N to N in Nest and grey to 3 in Nest? Fused Spur L & N to L&N in Nest. What's confusing me is the two wiring diagrams in the Nest manual see photos, one seems clear but on the specific 230v combi boiler diagram it looks like I need a link wire from the L in the Nest to connection 2 in the Nest? And the L from the boiler to 3, but this must be the grey heat call wire, musn't it?
    Many thanks!
     

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  3. stem

    stem

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    The first diagram is generic and describes what each terminal actually does. It is not a wiring diagram.
    That is correct, it corresponds to the existing link on the BDR91 between L and A

    Terminal (3) from the Nest is the 'heating call for heat' it becomes live when heating is required and sends the live to the boiler to make it operate.

    These drawings are only schematic. There are thousands of models of boiler out there all with different wiring configuration which will also vary depending on the layout of your property, the personal preferences of the installer and cables used. So, where the wires are connected will vary from installation to installation.

    The way to look at is that you just need to swap the wires from each terminal at the existing BDR91thermostat receiver, to the corresponding terminals at the Nest that have the same function. Including the wire link.

    There are two terminals marked L at the BDR91, they are electrically the same terminal, but the manufacturer kindly provide two for ease of installation. So, you will find it difficult to get all of the wires from them into the single L terminal at the Nest Heatlink, and may have to connect them to an additional separate terminal, or a junction box first to reduce the number of wires going into the Heatlink terminal.

    All of the wires in BDR91 N = Nest 'Neutral'
    All of the wires in BDR91 L = Nest 'Live'
    All of the wires in BDR91 A = Nest 2 'Common'
    The wire in BDR91 B = Nest 3 'Heating call for heat'

    Make sure that when you make the connections that there aren't vast swathes of copper sticking out of the terminal as per your existing poor installation. The wire insulation should go just up to (but not under) the terminals.
     
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  4. Johnpcvinson

    Johnpcvinson

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    top man! thanks a lot, much appreciated!
     
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