Nest Gen 2 install help...

Discussion in 'Plumbing and Central Heating' started by christianbeccy, 6 Apr 2020.

  1. christianbeccy

    christianbeccy

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    I have an 'old' Gen 2 Nest Thermostat which I fitted to our Combi system a few years ago. The install was a doddle.

    I've just moved and took the Thermostat with me (the new owner didn't want it!!). I'm trying to fit it to the current system, which is a conventional boiler, an Ideal Mexico 2, controlled by a Drayton SM2.

    I'm happy for the SM2 to continue taking care of the hot water, but would like the Nest to take over the heating control. However, the wiring is a bit more puzzling to me. The grey cable entering from the top right corner goes to the room stat. The bottom right had corner cable goes to the boiler, next one goes to the pump.

    [​IMG]

    The grey sleeved wire entering on the left is the mains feed in. The next one with the brown/black hanging down is what I have started to add. The next one goes to the SM2, with this on the other end...

    [​IMG]

    Finally, here is a pic of my Nest Heat Link. As I said, it's the older Gen 2, so doesn't have hot water ability.

    [​IMG]

    I would greatly appreciate any help regarding connecting the Nest up into this arrangement and how it will impact the functionality of the heating/hot water.

    Thanks in advance.
     

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  3. sxturbo

    sxturbo

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  4. picasso

    picasso

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    Disconnect 9 and 12 (and the neutral)to the room stat, connect 2 on the nest to 4 in the wiring centre, connect 3 on the nest to 13 in the wiring centre, leave the programmer c/h on constant.

    or connect the live in the nest to 2 with a small link wire and the grey you have connected to 3 in the nest to 13 in the wiring centre.
     
    Last edited: 6 Apr 2020
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  5. christianbeccy

    christianbeccy

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    I'll give this a try this afternoon. Seams logical though, thanks!
     
  6. christianbeccy

    christianbeccy

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    I haven't had a chance to do this yet, hopefully today. Would both solutions have the same effect? Although it's not the reason I'm doing it, will this enable the heating and hot water to run independently of each other? Is there a reason why they run together though?
     
  7. picasso

    picasso

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    Yes both have the same effect and both should run independently.
     
  8. christianbeccy

    christianbeccy

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    Sorry to be persistent on this.

    Here's what I have now, could you please cast your eye over this before I put the power back on. I've connected the Neutral from the Nest to the common Neutral in the middle of the junction box. I've connected #3 from the Nest to #12 instead of #13, but they are bridged so the effect is that same? The one I'm uncertain about is whether I leave the grey wire from the SM2 in #4 in the junction box along with the brown wire from Nest? I connected the earth, but of course the other end in the Nest isn't connected, so that is pointless I guess!

    [​IMG]

    [​IMG]
     

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  9. picasso

    picasso

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    All you have to now is put a small link wire between L and 2 in the nest and your done.

    20200409_184754_LI.jpg
     
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  11. christianbeccy

    christianbeccy

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    I did all that and it appears to control the heating BUT...

    The hot water side seems to be on permanently, even when there is no call for heat, the boiler fires up every few minutes.

    Despite what I originally said about wanting to control heating and hot water independently, I'd be happy for it to work like the old controller, just with the Smartphone integration as I know our system is pretty old school and not ideal.
     
  12. picasso

    picasso

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    All you have done so far is replaced the old stat with a nest, whatever you had before will remain exactly the same, if you want the heating and hot water to be independent you need motorised valves like this...

    Capture.PNG

    or this,,,

    three port valve.PNG

    do you have anything like this ?
     
  13. christianbeccy

    christianbeccy

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    No, I don't. But that's OK. I accept its shortcomings.

    However, what I don't understand why it seems that there's a call for hot water.

    So, I have set the CH to constant on the Drayton. Hot water is off completely. When the Nest calls for heat, the boiler fires. All is well. Then when the temperature is reached and the Nest ends the call for heat, the boiler continues to cycle as it does with the hot water (the radiators cool so I know the heating is off). It appears as though the hot water is on constant even though its technically turned off.
     
  14. picasso

    picasso

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    Not sure whats going on but turn off the ch on the old programmer, you don't need that on and the link in the heat link.
     
  15. CBW

    CBW

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    What’s calling for the hot water? Is it the Drayton? Could do with a photo of cylinder cupboard.
     
  16. picasso

    picasso

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    I did not realise that it was gravity hot water/pumped central heating, the drayton is set for gravity so the central heating being on will automatically bring on the hot water, I asked the op to switch the ch off on the programmer so should cure the problem,
     
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  17. picasso

    picasso

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    Ok now I know its a gravity system you will have to make a small change in the wiring, remove the link in the heatlink and turn on the central heating on the drayton, you should be back to what you had before.
     
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