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Replace a Cosy Geo Heating switch with a Nest Heat link

Discussion in 'Plumbing and Central Heating' started by Jimigills, 3 Jul 2020.

  1. Jimigills

    Jimigills

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    Hi,

    Apologies if this is in the wrong place. We’ve recently moved into a house that has a cosy geo installed which we want to replace with a nest system.

    Having had a look at the current installation I think I need to only use the two wires currently in the T1 box? If I’ve understood the wiring diagram right then the grey is the Com wire and the black is N/O?

    The mains wiring isn’t required for the Nest heat link I think?

    Has anyone replaced the cosy geo with the nest before?

    Any help is greatly appreciated, we’ve been left with no instructions on what’s currently installed so it’s a bit of a nightmare!

    Thanks
    Jim
     

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  3. Johnmdc

    Johnmdc

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    It depends which version of Nest you have got.
    If it is the Nest E, then what you have written is correct.
    If it is the Nest Gen 3 then that does require mains.
     
  4. ianmcd

    ianmcd

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    As @Johnmdc if you have a nest E then that is correct,if it is any other Nest you will need a L&N to power the nest
     
  5. ericmark

    ericmark

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    Looking at the diagram the existing is an on/off thermostat, where Nest can be used both on/off or modulating, and if the boiler can use OpenTherm then it would make sense to use the OpenTherm option.

    To be frank rather disappointed with Nest, back in around 2017 I looked at thermostats and control for mothers house, it had a Bosch boiler so no Opentherm, so decided to do in stages, TRV heads first using Energenie and then Nest which it claimed worked with Energenie heads, as it transpired the Energenie heads worked spot on, so never did get Nest in that house, but on moving replaced the old wax heads and brought the Energenie with me to new house, and in this house there was no wall thermostat, so made sense to use Nest which was designed to work with Energenie.

    However it transpired they only part worked, Nest should in theory send a signal to the TRV so the two are singing from same song sheet and change together, well on the Energenie app on the phone yes change Nest and the TRV followed, but turn the dial on Nest or set a schedule on Nest and it failed to change temperature on the Energenie. We also found the TRV needs to be around 2°C lower than Nest since Nest is mounted higher, with every other system the TRV tells the wall thermostat what to do, Nest is the odd one out where the thermostat tells the TRV what to do.

    Ringing up Energenie to complain, it seems when Nest became Google the support was removed, Nest is not the only smart thermostat that does not seem to have dedicated TRV heads, EPH seems to be the same, however EPH wall thermostats can be set as Master and Slave so you can have multi thermostats with one boiler and still use OpenTherm. It seems there is no one thermostat that has all features.

    However there is little point fitting a new wall thermostat and not use features offered, so step one is find out is your boiler Opentherm.
     
  6. Jimigills

    Jimigills

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    Thank you for the responses, it’s a nest e so will try and set it up later on today.

    @ericmark I’ve had a look at the opentherm option but from what I can tell the boiler doesn’t currently support that.

    Hopefully we get on with nest okay, one of the other reasons we are swapping what is currently there is we weren’t left with all of the cosy equipment either. Thermostat display is here but no hub!
     
  7. ericmark

    ericmark

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    Your not the only one, moved in here a year ago, found the timer which was marked CH and DHW simply switched the boiler on/off it did not matter if DHW or CH there were just two wires not three so simple on/off, there was a thermostat receiver but no sender and it seems the method was to go outside down a flight of steps and into flat under house and plug the pump in or unplug pump to change from CH to DHW only, and there were three FCU's and a socket all supplying the central heating not even all from the same CU.

    One does wonder how it ended up like that, and how the last owners managed, seems likely they didn't and used a open fire instead.

    However Nest has worked of sorts for me, Gen 3 in my case, and my boiler does not have Opentherm either, but better to be aware now, than after all wiring done.
     
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