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Urgent help please - Nest 3rd Gen Thermostat

Discussion in 'Plumbing and Central Heating' started by Surchyboy, 12 Feb 2019.

  1. Surchyboy

    Surchyboy

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    Can anyone help in how to wire the Nest Thermostat.

    I have a Baxi Megaflow combi boiler with a hot water cylinder tank on my top floor. The only connections I have on the boiler is a four core cable going to Live, Neutral, Earth and Switch Live. I then obviously have my spur output Live, Neutral and Earth. The current thermostat I don't need as I have the Nest thermostat base to make it portable.
     
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  3. stem

    stem

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    Are you sure? I have only seen Baxi Megaflo's in system boiler variant and it is a bit unusual (albeit not unknown) for a Combi boiler to be connected to a hot water cylinder. In addition systems with hot water cylinders connected will have a lot more wiring that you describe. i.e. motorised valve/s, programmer, cylinder thermostat, wiring centre etc.,
     
  4. Surchyboy

    Surchyboy

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    The only cable going to my Drayton LP522 controller for the heating and water were the power from the spur unit, the four core flex from the boiler as described and the four core cable (only three cores used) from the room thermostat.
     
  5. stem

    stem

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    Now that we know that you have a Drayton LP522. There will be a lot more wiring than you describe. But before anyone can advise you definitively you need to confirm what sort of heating system that you want to connect the Nest to. Do you have for example:

    1. A fully pumped S-Plan with 2-port motorised valves.
    2. A fully pumped Y-Plan with a single 3-port motorised valve.
    3. A system with gravity circulation of hot water and pumped central heating.

    It's not a simple swap, there is quite a bit of work to do, and you will need to have working knowledge of your heating system. From what you have posted so far, I'm not sure that you do, so you may be better advised to get a professional to install the Nest for you.

    To give you some idea of what needs to be done, the Nest Heat Link replaces the Drayton LP522 so all of the wires from here......

    LP522.JPG

    .......move to the Nest Heat Link terminals that have the same function. [Unless you have a system with gravity circulation of hot water and pumped central heating, in which case there will be some additional reconfiguration to do]

    Nest.JPG

    Then you will also have to decommission the old room thermostat. It can't just be disconnected, otherwise the heating wiring will be permanently 'open circuit' and not work. So a wiring modification will be required to bridge the existing thermostat live switching wires.
     
  6. ianmcd

    ianmcd

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    As @stem correctly says you have a Baxi Megaflow condensing boiler not a Combi boiler
     
  7. Surchyboy

    Surchyboy

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    I have to be honest I don't know heating systems but electrics yes. I have the boiler in the kitchen down stairs and a hot water cylinder tank upstairs. Both heating and water being controlled by the LP522. I currently have no heating but I can manually switch the water on by the manual override switch by the tank.

    From the picture you have shown of the controller all I have is the live and neutral going in for the supply.
    Live out from the spur went to (HW no 3) Neutral from the spur went to (CH no 4). The live from the boiler flex goes to (HW no 3), the neutral to (CH no 4), the live out of the boiler went into a connector block with a cable from the room thermostat. The live cable from the room thermostat goes to (HW no 3) and the neutral to (CH no 4).

    I am more than capable of wiring it just cant get my head round wiring currently, though I may have to submit defeat and pay for someone to come and look at it.

    Appreciate the help so far though
     
  8. ianmcd

    ianmcd

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    you are not as capable as you think, colors of wires mean absolutely nothing on heating systems and if you had a neutral in terminal 4 you would need a new controller
     
  9. stem

    stem

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    Sorry none of that makes any sense to me there can't be a neutral going to terminal 4 ch on that's a live output!! Don't assume because it has a blue wire in it that it's actually connected to neutral, it's probably been used as a live wire.

    Also terminal 3 HW on will not be a live from the spur, that's the output to control the hot water.
    Are you sure about that? Are you referring to an immersion heater? In which case that is a separate electric heating element and nothing to do with your boiler whatsoever.

    And you still haven't answered the basic questions I have posed about the type of heating system you have.
     
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  11. Surchyboy

    Surchyboy

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    It is definitely a Baxi Megaflow yes.
    Sorry yes I should be saying blue instead of neutral and brown instead of live.
    The nest works in that it turns the boiler on but it doesn't turn the heating or water on but its only wired as a 230V combi boiler.
    Appreciate the comments but you are right this is not within my realm as I said I have never touched heating systems.
     
  12. ianmcd

    ianmcd

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    the nest receiver unit does not have any wires going from it to your boiler, the receiver controls either your mid position valve (Y Plan) or your 2 port valves (S Plan) it never directly connects to your Baxi Megaflow, the receiver gets wired into your wiring centre not the boiler
     
  13. stem

    stem

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    Now you have really confused me :confused:

    So the Nest is installed already and providing time and temperature control but only of the heating. But you also have a LP522 and a room thermostat still wired in???

    ...or this is a wind up :sneaky:
     
  14. FiremanT

    FiremanT

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    I thought the Op understood electrics.

    The wiring diagrams and associated YouTube videos are always going to be better understood than coming here and giving confusing statements

    IMO
     
  15. Surchyboy

    Surchyboy

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    Stem, I took the LP522 off thinking it looked straight forward and wired the Nest thinking it was a combi boiler from the Nest manual (my mistake I know assuming it was a standard combi boiler). The Nest thermostat and heat link can see each other and when you press the button on the heat link the boiler turns on however it doesn't turn the heating or hot water on so im guessing I have either missed a link or connection. I have bitten the bullet and got a gas engineer coming in this afternoon to sort it so will see where I went wrong.

    FiremanT - yes I do understand electrics and have an 18th Edition qualification (though that's nothing difficult to obtain). I appreciate my wording has not been exactly clear though and I have watched many installation videos but none start with identifying your heating system and what cables you should have. Clearly heating systems are more than just wiring though!
     
  16. ianmcd

    ianmcd

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    has to be a wind up
     
  17. picasso

    picasso

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    Gonna be a shock when he finds out its not a combi boiler.
     
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