Help wiring 3rd gen nest heat link

Discussion in 'Electricians' Talk' started by markh1, Oct 16, 2021.

  1. markh1

    markh1 Member

    I was told it would be simple to change to the nest thermostat gen 3 so went and bought one. The house originally had a condensing boiler and the previous owner changed it to a Combi. The boiler is a bosch 27cdi. Controller is a honeywell st9400c1000, and room stat is a salus. I’ve attached photos, the thermostat just has two wires which is what the nest back box takes so hopefully that is just a straight swap. The heat sink I’m unsure as lots of wires going into the Live and Neutral and not sure if the wires going into the 3 and 4 (two end ones) are the T1 and T2 for the thermostat but oddly on the diagram 3 and 4 are for ch on and hot water on, but it’s a Combi so doesn’t need control for hot water.

    I’m hoping someone on here has had or seen the same and knows if it’s simply case of squeezing all the live and neutral into the nest live and neutral and the two wires in 3 and 4 on the Honeywell just go into t1 and t2 in the nest.

    Any help would be greatly appreciated
     

    Attached Files:

  2. bright_Spark

    bright_Spark Screwfix Select

    Just ignore the water control side, the two wires in your existing stat go into the new stat base, any order it doesnt matter BUT do not turn it on at this stage until you have wired it into the new nest box as it is presently will be 240v and the nest base delivers 12V to the new stat. These stat wires go to terminals T1 and T2 in the new nest box. is the clock remote to the boiler or is it next to it?
     
  3. markh1

    markh1 Member

    If by clock you mean Honeywell controller yes it’s about 1m from the boiler.
     
  4. PandA3

    PandA3 Member

    If you are putting the heat link in place of the Honeywell controller then yes keep the live and neutral cables the same as currently wired. you’ll need to put a live feed into terminal 2 of the heat link and the ‘CH on’ cable goes to terminal 3.
    The issue that you have already stated is why you’ve still got a wire in the HW On terminal but we will have to assume the previous owner disconnected this at the other end (usually at the zone valves) so make it safe and tape it up.
    The wires at your existing room stat will be redundant (last time I did a nest system the stat was wireless and you could position it anywhere) but they will need permanently connecting together if you are not doing any other changes to the wiring.
     
  5. bright_Spark

    bright_Spark Screwfix Select

    Ok so the boiler L&N will be in with those on the clock then, so you need to put all of the L&N that are currently in your clock into a choc bloc obviously all the Lives and all the neutrals in two seperate choc blocs then take out of the choc bloc a L&N into the L&N terms of your nest.
     
  6. markh1

    markh1 Member

    Ok that makes sense, easier than what I would have tried squeezing them all into the nest connector. So what about the 3 and 4 will this be the t1 and t2 as just seems odd they have been wired into the Honeywell clock hot water and heating on rather than heating on and off connections. I have a multimeter if that helps but not sure how I can test the connection on one room is same as wither coming from the clock or how to confirm the stat will be getting 12v not 230 once all wired so I don’t damage the nest.
     
  7. bright_Spark

    bright_Spark Screwfix Select

    You connect the two wires that are comming into your timeclock into terms T1 and T2 of the nest box, this will deliver 12v to your stat, just use your multimeter to ensure the two wires that you put into T1 and T2 are the two wres going into your stat at the wall
     
  8. markh1

    markh1 Member

    Ok so do I need to run a cable from the live choc block into number 2 as well as live on the heatlink as PandA3 you have suggested. And PandA3 are you saying if I want to have the stat remote (plugged in) then the thermostat wires just need joining to close the connection, or are you saying this is my only choice as the existing clock doesn’t have any ch off connection.

    Bright spark would the two wires coming into the clock in connection ch on hw on be the two stat cables or is this a workaround. Anyway if confirming the stat is only getting 12v once done?
     
  9. bright_Spark

    bright_Spark Screwfix Select

    No dont join your two stat wires together they are needed to power the nest
     
  10. PandA3

    PandA3 Member

    I’m curious why the thinking is that the stat wires are the ones in the Honeywell. The 2 wires in the Honeywell are the old CH and HW In demand signals that would have been wired to the zone valves. And the existing stat wires are prob also wired to that location. just saying, not trolling.
     
  11. bright_Spark

    bright_Spark Screwfix Select

    I am assuming that you used to have a fully pumped system before you got the combi boiler and if this is the case then you will probably still have a wiring center in your old cylinder cupboard. you need to find the two stat wires at this point to put them to the new stat
     
  12. PandA3

    PandA3 Member

    They can be adapted to power the Nest if the Next stays in same place. But the Nest is designed to be portable hence don’t need as battery powered.
     
  13. PandA3

    PandA3 Member

    Exactly my point.
     
  14. markh1

    markh1 Member

    I bought the house after, and found out the previous owner changed to a Combi and removed the tank etc I do have some choc blocks in a box with wiring in but nothings labelled. I’m not that clued up on multimeter use so how do I use it to check the t1 and t2 wires are the same as ones into stat?
     
  15. markh1

    markh1 Member

    I understand the nest is just battery backup and either needs to go on wall replace old stat or connect to mains on a stand (optional extra)
     
  16. bright_Spark

    bright_Spark Screwfix Select

    No I mean the two stat wires in the wall are needed to power the nest stat if he is hanging it on the wall, the two stat wires are used to deliver the 12v for the nest. the two stat wires will come from the old wiring center in the immersion cupboard
     
  17. bright_Spark

    bright_Spark Screwfix Select

    OK well it will become a little complicated as your system will need to have the wiring jigged about a bit to get the stat wires joined into the wall where it being sited, your easiest option would be to purchase the nest stand and plug it in if you want to do it yourself. It is quite simple to find the stat wires but you do need to know how to identify these wires with your multimeter
     
  18. PandA3

    PandA3 Member

    No they are not the 2 stat wires.
    And in my opinion you dont need to make wiring mods which could end up complicating and damaging what should be a very simple swap.
     
  19. markh1

    markh1 Member

    So if they are not the two stat wires in the existing clock is there a simple way to connect it up so I can have the nest on the wall using the existing stat wires for power? Otherwise it looks like will need to pay for installation
     
  20. bright_Spark

    bright_Spark Screwfix Select

    Mate not been funny here but I am a registered nest pro and it is a simple job to do for a spark. you mention these are battery powered?
     

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