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Replace Honeywell CM907 with CM927

Discussion in 'Plumbing and Central Heating' started by YukonKnights, 12 Nov 2017.

  1. YukonKnights


    12 Nov 2017
    Thanks Received:
    United Kingdom
    I currently have a Honeywell CM907 room thermostat and wish to replace it with a wireless Honeywell CM927 thermostat. To keep live simple, I am planning on placing the wireless receiver for the new thermostat where the existing CM907 is fixed, in order to reuse the existing wiring. Before I start though, I need some advice on the existing wiring.

    Currently the CM907 has a live and switched live supply attached to A and B on thermostat back plate. The wire colours are brown and black.

    At the boiler end (iSar HE35), the live and switched live go into terminals L1 and L2 (furthest left two cables on connector block). The wire colours are grey and black.

    The first piece of advice I need is why the wire colours on the live and switched live supply change between the boiler terminals and the room thermostat? Can I just assume that they change at some junction that I cannot see in the wall?

    Secondly, the CM927 needs a neutral supply along with the live and switched live. Looking at the wiring at the existing room thermostat, there is an used earth and unused grey cable. How can I test that this grey cable is a neutral?

    Ultimately I want to do this work myself, but if I am not confident then rest assured that I will get a professional in. Many thanks.
    Last edited: 12 Nov 2017
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  3. stem


    20 Jul 2005
    Thanks Received:
    United Kingdom
    The Honeywell CM927 usually comes with a switching relay box usually a BDR91 with wiring connections like this. (Post back if you have a different relay box)


    From your boiler manual, the connections are like this:


    When using a three core cable, they can be connected together as below, with a link added between the BDR91 L and A.


    The 'mains in' N & L supply may be connected first to the BDR91 and then to the boiler (as shown) or connected to the boiler first and then to the BDR91.
    It's likely, but never assume. (I once found a thermostat that had a Neutral connection from an adjacent wall light. Whilst wrong on several levels it had been working OK for years before it caused problems.) Normally, the cables are run directly from the boiler to the thermostat, however, it may be that your system has been altered or moved at some point and the cables extended and a junction box inserted somewhere. Unfortunately I can't advise on this only a physical inspection / tracing of the wires and an electrical test of them would confirm this or otherwise.

    An electrical test with a multimeter to determine its potential (Voltage) between a known live and a known neutral. But only do this if you know how to use a multimeter and can do this competently and safely.
    Last edited: 13 Nov 2017
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