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NEST Installation

Discussion in 'Plumbing and Central Heating' started by Andy Byford, 3 Dec 2019.

  1. Andy Byford

    Andy Byford

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    Hi all, first post here! Please move if it's in the wrong section.

    I'm quite into my DIY and have been upgrading parts of the house, I bought a Nest thermostat last week and just wanted to check everything checks out, if anyone could give any advice on the below it would be greatly appreciated!!

    I removed my old programmer in the Kitchen, I plan to place the nest 'Heat Link' here in it's place. There are a couple of things I wanted to check first.

    Wiring diagram I figured below:
    H21XL ----> NEST Heat Link
    N --> N
    L --> L
    1(HW ON) --> 6 (Call for Heat HW)
    4 (CH ON) --> 3 (Call for Heat CH)

    L-2-5 linked Common.

    There are currently no T1 or T2 wires behind the programmer, I am guessing these are located upstairs in the tank cupboard but I cannot see where. Meaning I will be just using USB mains power for the actual Nest stat to avoid chasing new wires around. What's the best way to locate and disconnect the old thermostat wires so they don't sit live in the walls?

    I gather I have an S plan system as there are 2 valves upstairs in the tank cupboard and there are no HW off or CH off wires to the programmer. There is a thermostat on the tank upstairs also, leading me to believe the HW is just stat satisfied? Only problem with this is I read online the Nest may require an extra wire from this stat into the HW off slot so the CH isn't turned off once the tank is at temperature, is this true or can the system run separate as it is now with just the 4 wires connected? (Along with the L-2-5 too)

    In the tank cupboard upstairs there is a blanking plate with 6, 5, 2 and 1 above it colour coded, my interpretation would be:
    6 - CH off
    5 - Common
    2 - Common
    1 - HW on

    Are these relevant for this installation or can I leave these as is?

    I can provide more info or pictures if needed, I have an ideal vogue s26 boiler.

    Thanks to anyone who can help!
     
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  3. Andy Byford

    Andy Byford

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  4. stem

    stem

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    That looks good to me. (y)

    No, they won't be there T1 & T2 are a unique Nest requirement, they provide the 12v supply for the Nest thermostat from a power supply built in to the Heat link. If you don't want to install them a separate plug in power supply is the way to go

    It's not the Nest that needs the HW off. Nest provides it for heating systems that have a 3-Port valve (Y-Plan) that need a 'HW off' signal to work. As you have 2-Port valves (S-Plan) it's not required.

    To remove the old room thermostat, trace the cable from the thermostat back to its origin. Note where the wires are connected and remove them. Then insert a link to join together the two terminals where you have just removed the wires that went to the thermostat terminals (1) and (3) so that they are joined together. The old room stat and its cable can then be removed.

    Alternatively, if you are happy to leave the old room thermostat in place, take the wires out of 1 and 3 and join them together with a new connector.
     
    Last edited: 3 Dec 2019
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  5. Andy Byford

    Andy Byford

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    Thanks for your reply, it's good to know I'm almost there with it! What is the best way to trace the cables? Will they be connected directly to the boiler connectors?

    Thanks again!
     
  6. stem

    stem

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    Unfortunately, the room stat cable could terminate in several places based on how the original installer decided how to wire the system up. The most common place is usually at the wiring centre (behind the blanking plate in your airing cupboard) in which case, the wire from thermostat terminal (3) should be connected to the brown (Live) wire of the central heating motorised valve. The second, but less common place is behind the programmer. When I think I've found it, I always check with a multimeter to make sure.

    The one place it won't be connected though is the boiler. With S-Plan, the room and hot water cylinder thermostats control the motorised valves and so are wired to them. It's the motorised valves that then control the boiler.
     
  7. Andy Byford

    Andy Byford

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    Thanks again, I'll take a look on Thursday to see if I can get this done :D
     
  8. Andy Byford

    Andy Byford

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    Ok, so I took the plate off today and trying to figure out which wire(s) lead to the old thermostat and being honest, this wiring it probably a little out of my depth to figure out without a wiring diagram.. I have posted up some pictures of the wiring.

    Which wires are most likely to be connected to the thermostat and using my multimeter, what is the best way to test this?

    Thanks again for your help and advice.
     

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  9. Andy Byford

    Andy Byford

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    Better view of top side 20191205_101733.jpg
     
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  11. stem

    stem

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    Difficult to see, there's not much room in there is there?

    Normally the switched live wire from the room thermostat terminal (1) will be connected to the brown wire of the central heating motorised valve. Once you have a 'suspect cable' and noted where the wires go they should be disconnected and then with a multimeter set to measure 'Ohms' (resistance) and connected as below, get an assistant to turn the thermostat up and down. When it's on 'max' there should be a low resistance, and when it's on 'min' a high resistance.

    photo.JPG


    1.jpg

    The wire colours above show one convention, but yours may be different, it's where they are connected that matters.

    Yours though is a difficult installation to work on, and there is a danger that you may disturb other wires whilst making the tests. It might be easier to disconnect the cable and the wires at the thermostat end and put them in a junction box that can be relocated out of the way. The live wires that were in (1) and (3) would be connected together, and the neutral (if present) isolated. Electrically it achieves exactly the same thing. Except you wouldn't be able to use the cable for connecting to T1 & T2, but if you're planning on using a separate plug in power supply for the Nest Thermostat anyway, it doesn't matter.

    2.jpg
     
  12. Andy Byford

    Andy Byford

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    I took your advice and connected the old thermostat lives wires via junction box and isolated the neutral. Fitted a blanking plate to ensure I know there are live wires and keeping the option for a future thermostat there. All fitted now, whipped out to buy the nest stand and fitted it in the living room. 20191205_123054.jpg 20191205_133435.jpg

    Edit*

    The nest has power and the heat link, but it is not calling for heat from the boiler currently even when it should be.. Any ideas for this?
     
    Last edited: 5 Dec 2019
  13. Andy Byford

    Andy Byford

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    This is what the boiler currently says:
     

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  14. Andy Byford

    Andy Byford

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  15. stem

    stem

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    Is that what you've done? Doesn't look like it to me. (5) looks to be linked to (3)

    And it looks like you have two wires in (2) already. The only wires in (2) and (5) should be the links to (L)
     
  16. Andy Byford

    Andy Byford

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    Sorry it's a bad angle, L-2-5 are linked.. 3 has the old CH on wire, 6 has the old HW on wire
     

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  17. stem

    stem

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    OK, that's fine then.

    The Heat link should be showing 3 green lights when everything is switched on. Is it?

    lights.JPG

    Is the hot water working OK?

    What happens when you press the centre button on the Heat link? It should manually switch the heating on & off.

    Check the terminals are tight in L, 2 and 3. Heat links can be fiddly even when you are used to them. Check the original thermostat wires from 1 and 3 are securely connected together too.
     
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