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Vaillant Ecotec Plus/BDR91 relay/Honeywell CM921 -> Nest

Discussion in 'Home Automation' started by Laura147, 28 Aug 2021.

  1. Laura147


    28 Aug 2021
    Thanks Received:
    United Kingdom
    Hi all :)

    I'm trying to figure out whether I need to call a professional to install a Nest Thermostat E:

    I currently have a Vaillant Ecotec Plus, with a BDR91 relay that talks to a (wireless) Honeywell CM921.
    The Honeywell is starting to fail (the down button is randomly triggering which means I can't turn on the heating :eek:). I could just replace it but would like to try and get something a bit smarter.

    Looking at the Nest Thermostat E installation instructions, they seem to imply installation is very simple. My main worry is this won't be the case when I get to the wiring inside the relay.
    I managed to get the cover off the BDR91 and take a picture (see attachment). Does anyone know if this wiring arrangement would be compatible with a Nest E?

    Thanks in advance :D

    Attached Files:

    Last edited by a moderator: 28 Aug 2021
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  3. CBW


    26 Sep 2019
    Thanks Received:
    United Kingdom
    It will take a Nest E, but thermostat would need to go in place of BDR91, and depending on how it’s fed, may need to alter the Live and Neutral. You could get a different wireless thermostat brand, a Nest learning thermostat (but the thermostat itself would need to be used via the usb lead), Hive would also work.
  4. ericmark


    27 Jan 2008
    Thanks Received:
    Llanfair Caereinion, Nr Welshpool
    United Kingdom
    I read this with interest
    the control in the main is a compromise, and after having a problem with mothers I did look into the options.

    So option one is keep it simple, so we select a cool room, often the hall, and use that room to control if boiler on or off, then we control all other rooms using TRV heads, all the wall thermostat is there for, is to stop the boiler cycling in the summer, so really not worth spending a lot on the wall thermostat, in fact it can actually end up doing reverse to what you want, with a modulating boiler either you want opentherm wall thermostat, or you let the TRV do the work, so although with an open plan house Nest may work well using OpenTherm, Nest does not link to TRV heads so not a very good selection if you have doors on your rooms.

    It is down to house design, there is no best wall thermostat, so my mothers house hall cooled fast, may seem odd, but cooling speed is important, so in her house we had a wall thermostat in the hall, and a TRV in the hall, so in all three controls in the hall, and for the system to work they have to be set so they work together.

    So the lock shield first, this controls the speed at which the radiator heats up, to slow and room clearly will not get hot, but too fast and it will heat up faster than the TRV can react so temperature will over shoot. The TRV needs to be set lower than the wall thermostat, how much lower depends on how the air circulates, but in winter the room temperature should not get hot enough to turn the wall thermostat off, the idea is as summer arrives then the wall thermostat will turn off to stop boiler cycling. Then all other room temperatures are controlled with the TRV heads.

    If due to way house cools this will not work, then one can get TRV heads which link to the wall thermostat, and using a programmable wall thermostat and programmable TRV head you can vary temperature through the day.

    Setting up three items to work together is not easy, in fact even two can cause problems when the TRV is marked *123456 not °C, as you want better and better control you move to TRV's with algorithms built in, and wall thermostat which are more like hubs to wall thermostats, like EvoHome. But the thing to remember is a TRV is analogue, and with central heating analogue is king.

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