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Replace a Honeywel CM907 with a Nest Thermostat v3

Discussion in 'Plumbing and Central Heating' started by Koen Van Looveren, 28 Nov 2019.

  1. Koen Van Looveren

    Koen Van Looveren

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    Hi,

    I have a problem figuring out how to wire up my nest thermostat. I have a Honeywell cm907. And I want to replace it with the nest thermostat v3

    This is my wiring and my heating system.

    How do I know if my nest thermostat is compatible?
     

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  3. stem

    stem

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    The two existing wires in A & B go to the two Nest Heat link terminals (2) 'heating common' and (3) 'heating call for heat'. It will work with them either way around.

    The Honeywell is battery powered, but the Nest Heat link is mains powered, so it also requires a 230V supply for it to operate. As this doesn't appear to be present, it will need to be installed. Here in the UK it should come from the same 3A fused spur that supplies the boiler, but you may have a different arrangement in Belgium. However, the idea is that when the boiler is isolated for maintenance, then the Heat link is isolated along with it.
     
  4. Koen Van Looveren

    Koen Van Looveren

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    Thanks for your comment. So I should either go with the Nest thermostat E? or add a 230v supply to power the Nest Thermostat V3? Does it realy need to be 230v? or could it be 12 or 24v as well?

    For the wiring. Blue/Black should go to `W1`. and Blue/Black should go to `C` (for the Nest Thermostat V3)

    Maybe this could work as well? And add the extra cable into the wall to the nest itself?
    https://www.amazon.com/Adapter-Transformer-Requires-Honeywell-Thermostat/dp/B07C3J6PQX
     
    Last edited: 28 Nov 2019
  5. stem

    stem

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    I suspect that you have different versions of Nest products than we do. 'W1' for example doesn't appear on Nest products here. So you would probably be better asking your question on a more local forum.

    The Nest E we get here is battery powered, and doesn't need a 230V mains supply. With the UK version the two wires would then go in 'NO' and 'COM'

    [​IMG]


    The UK Gen 3 Heat link only comes suitable for a 230V operating supply.

    [​IMG]
     
  6. Koen Van Looveren

    Koen Van Looveren

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    Oke perfect thanks for your time and information. I will contact a local plumber to help we with this.
     
  7. fezster

    fezster

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    FYI, your cm907 is the thermostat, and you most likely have a programmer near to your boiler.

    With Nest Thermostat 3rd Gen, the heatlink would be placed where your programmer is - as this will have the requisite wiring for L/N and connection to your boiler and/or motorised valves. The actual Nest thermostat has a wall plate which would be placed where your cm907 is, and the 2 wires would be repurposed to connect to the 12V connectors on the HeatLink.
     
  8. Koen Van Looveren

    Koen Van Looveren

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    Aaah yes oke I see. The heat link is included with the nest thermostat v3?
     
  9. fezster

    fezster

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    Correct. In the UK at least.
     
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  11. stem

    stem

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    The Honeywell CM907 is a programmable thermostat. It it provides both time and temperature control in the one unit.

    Capture.JPG

    The end digit '7' indicates that it has a 7 day operating programme. So it doesn't require connection to an additional programmer, although sometimes one may exist as part of an earlier installation or as an integral part of the boiler.
     
  12. fezster

    fezster

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    You are, of course, right.

    I have (or had!) that thermostat, but it was connected via a programmer near the boiler. I suppose the point I was making is that OP would be better off wiring his heatlink close to his boiler, rather than using the existing thermostat wiring which would usually be used for the 12V wiring.
     
  13. Koen Van Looveren

    Koen Van Looveren

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    The heat link will be installed next to our boiler for the nest thermostat installation. We will replace the old wires with new ones.
     
  14. stem

    stem

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    Agreed, they would be ideal for the T1 & T2 thermostat connection. Originally as the OP has a 'European' installation with flexible conduit in the wall, I had thought that it might not be too difficult for him to feed a new 230V supply inside it.

    Capture.JPG

    Or, in the event that the brown wire is actually a live, and the cut off wire might be able to provide a neutral connection, it could be even easier.
     
  15. ericmark

    ericmark

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    Depends what you want the thermostat to do, likely the Nest e is best option for DIY fitting, personally I prefer not to rely on batteries, and the comparison chart does show some advantages with Gen 3 over e. However Nest is also OpenTherm enabled (both versions) and seem to remember Valiant boilers are also OpenTherm enabled?

    Having got it wrong, and tried to get better control with wall mounted thermostats, I now realise my errors, main point is the cold wall, in theory thermostat should be on opposite wall to radiator facing radiator at 1200 mm from the floor, distance from floor is because it is both manually altered and needs one to view it's display not anything to do with where it works best it is one of the Parts of UK building regulations.

    But as well as cold wall there is the time taken for heat to reach the thermostat, using modulating (analogue) controls like OpenTherm helps, so no on/off it instead sets boiler output up/down to suit.

    But what transformed my mothers house was to actually set the thermostatic radiator valve (TRV) and not try to use a single wall mounted thermostat to control all rooms, I selected Energenie MiHome as I also wanted some wifi sockets and they use same hub, I had intended to add Nest latter and I knew Nest was suppose to work with Enegenie MiHome TRV heads, but in that house never installed Nest. Reason for not installing Nest, was the TRV heads controlled house A1 so no need for Nest.

    On moving to this house, there was not wall thermostat, and there was also some wiring faults, so since already had the TRV heads from last house, Nest Gen3 was a clear selection for a wall thermostat. However it does not work well with Energenie MiHome TRV heads, yes on the mihome app if I adjust Nest then the TRV heads follow, but with schedule changes they often failed to follow, what the should do, is not what they did do.

    Nest works well, and the MiHome TRV heads work well, but they don't work together as they should. I have also read that Hive has the same problem with their TRV heads not communicating with the wall thermostat as advertised. I suppose no perfect system, Tado will not release info on how it works, EvoHome needs an add on module to work with OpenTherm, and many ebus connected thermostats will only control one room.

    However often that is all that is needed, control one room A1 and rest of house relies on the TRV heads, as long as that one room needs more heating time than any other room, it will work, but that means in the main the house is controlled by TRV heads, not the wall thermostat, and as I say we learn from errors.

    In this house, with oil fired boiler the pre-wired point for the thermostat is not opposite the radiator but along the same wall, although Nest works out time to heat a room so shows on changing settings the time to reach new heat, in this house it is between doors to other rooms so door left open or closed alters how well it works, so in the evening drops to 17°C over night, but in morning the rise to 20.5°C is in 4 hourly stages of 0.5°C per hour to stop the hysteresis as it over shoots when doors are left in a different position to what thermostat has learnt.

    In hind sight, and hind sight is easy, wish I had fitted EvoHome, but by time I bought this house already had 4 MiHome heads, so kept to same system.
     
  16. Stuw101

    Stuw101

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    I have similar wiring in my new build. I have the cm907 thermostat and want to switch to a smart system hub. The wires are live, switched live and an earth behind the stat. There is a single spare wire not used in the look cut at both ends. Is there any system that will connect straight to this without additional wiring? If they need a 230v supply why do the smart comapanies not have a plug in adaptor like a mobile phone charger type that plug on to a socket to provide this for customers like me in new builds fitted with that stat? I would also like to use the OpenTherm feature of my ideal logic 35 boiler. Can the cm907 be made to use OpenTherm or would an outdoor kit fixed your the boiler work with the exsisting cm907? So annoyed with this wiring. Even the evo home which you would think they would of designed as an upgrade to the cm907 has not been made to run off the same wiring there earlier cm907 does. Grrrrr.
     
  17. stem

    stem

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    The Nest-e will do that, its receiver (Heat link) is battery powered and the thermostat Live connects to the Heat link 'COM' and the switched live to 'NO'
     
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