Help wiring 3rd gen nest heat link

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

  1. bright_Spark

    bright_Spark Screwfix Select

    Right sorry mate it is me confusing you
     
  2. bright_Spark

    bright_Spark Screwfix Select

    You will need to identify the wiring by switching the full power off to your CH as you will be using one of those lives that are currently live now and seperating it in both the wiring center and your time clock, so it will no longer be a live wire the other wire will be your hot water on.
     
  3. bright_Spark

    bright_Spark Screwfix Select

    You should have the wiring diagram with the nest as they are pretty comprehensive with all basis covered. You just find the combi boiler diagram and follow it.
     
  4. markh1

    markh1 Member

    Confusing me now, and likely getting more complicated and beyond my ability.
    Would I use the existing wires in 3 and 4 on the Honeywell (ch on and hw on) if so which do I trace and connect to one of the stat wires. And then I find a spare live wire and take this out of the live and use it as the wire to connect to the second stat wire and then into the other nest 12v connection
     
  5. bright_Spark

    bright_Spark Screwfix Select

    To be honest it is more complicated to try to explain how to do than to actually just do it. It is a really simple job but it may be out of your scope to actually do it. Ypu should look at the wiring diagram and just follow it, basically you need the two wires that are at your stat now and identify these upstairs in the wiring center then you need these two wires back at your time clock via the wiring center. You would have to split the lives as you no longer need a feed back up to the wiring center or at your stat. So you need to identify these cables that is the hardest part of the job. Once you have got this then the rest is as easy as paint by numbers
     
  6. PandA3

    PandA3 Member

    You’ve now told him to use the 2 wires in the existing Honeywell controller for the 12v Nest stat power. so what wire is now going to be the Boiler Ch On Demand signal from the Heat Link? He will have to wire a new cable from the heat link to the boiler and then adapt the wiring in there. Or install a 3rd wire from heat link to the wiring centre to pick up the Old Boiler demand from the old zone valve wiring. Jeez that’s way above a novice ability. Just use a separate Nest power supply and stand (yes I realise it’s not batteries) and forget about the old stat position (hang a picture over it :)) and follow my original post. (Or of course pay for a pro install:rolleyes:)
    By the way.. if you’d bought a TADO stat then it would have been a 5 min swap over of the existing wall stat and you’d be fully functioning without any other wiring changes and it does the same as the Nest. Good luck:)
     
  7. bright_Spark

    bright_Spark Screwfix Select

    He just needs to identify the cables that he has, there is plenty of cables in the time clock, he doesnt have to use the exact same ones that has been used at the moment. he has bought the nest now so your silly advice of hanging a picture over the original stat and buying a cheap thermostat isnt a nest is it, says a lot about how you probably. These are not hard to fit but you seem to have more of a problem with it than the OP. If you havent got anything constructive to add may 'i suggest you stop assuming that the OP is some sort of idiot. I fit loads of nest products as a nest pro and on average an instal like this takes around half an hour to an hour. He just needs to strip the wires out of the time clock, identify what cables he has there and wire it up, he doesnt need to add any more cables than what is already there. Yes it needs an element of identifying cables and seeing what goes where but that is the most difficult part. You are really making more out of this than the poor OP needs to. He does not have to run any further cables.
     
  8. bright_Spark

    bright_Spark Screwfix Select

    Just to add with the exception of the call for heat wire that exists yes you are correct in picking that up but we will get to that as soon as the cables are identified as to what goes where, at this stage the cables to use are not important as he will realise that the call for heat goes to his boiler as well as a permanant feed, the rest of the cables are then easier to identify in the wiring center. Let the poor bloke at least identify his cables before he starts on the connections. Whatever cables he has left over from his previous install will be redundant and able to use as stat wires or whatever is needed. They will be different now using them on his new nest. He needs a permanat 240v at the nest heat link and two stat wires and one call for heat wire. These at the moment are at his original time clock position other than the two stat wires which just need altering in his wiring cnter.
     
  9. ScottyP

    ScottyP New Member

    To Mark (OP) here is a simple way to visualise how heating circuits work. Daisy chain, and feedback loop.

    The main live from the fuse spur, powers the controller (now receiver if using hive / nest / tado etc) and loops over next, to the output relays "common" terminal for ch and hw. (HW + CH on/off) That provides a live for the relays in the controller to power the next thing in the chain, the zone valve motor(s). "Ch/hw Off" isn't usually used, so is normally not wired up. The zone valve motor is spring-return. Power to open it, no power, it closes by itself. Between the zone valve motor and the controller relay output (the ON terminal) is the stat...room or cylinder when it's a split system (non combi) - room stat to CH zone valve motor and cylinder stat to HW zone valve motor.

    Power daisy chain goes:
    Spur > Controller > stat > zone valve motor.


    Next is the feedback loop, so there is a switch inside the zone valve that closes when the valve is fully open, which also gets a live from the spur at one terminal, the output wire from these switches (other terminal) is always orange, that comes back to start the boiler and pump via the wiring centre.

    Power feedback:
    Fuse spur > zone valve switch > boiler + pump


    So the controller CH ON terminal gets energised by the controller switching the heating on, which then passes through the standard room stat, and goes on to power the zone valve motor to open the circulation valve. The valve signals when open, via it's internal switch, to start the boiler and pump.

    If your nest stat now needs 12v from the new controller, you need to identify a pair of wires in one cable that are no longer needed and use those. With all power off and verified dead, you can link the red/black or blue/brown at the end of a cable and at the other end, should measure a short circuit with the meter set to ohms or the diode symbol. On that last setting, it will beep when a short is detected. Open the pair again that you joined at one end, and test again. The short should now be gone. Cable identified, mark it at both ends. That would be a cable you can use for the new stat. Mind you, the stat wiring might only go to the wiring centre - not all the way from the stat position, to the old controller. Maybe you can use the old HW ON wiring as the combi doesn't use it.

    With a smart system, you only need power to go from the spur to the controller live and relay "common", then from CH ON to the zone valve motor. The feedback loop needs to stay as it is. Normally you connect both wires the old stat was using to each other, (COM and SL or OUT) and if it was 3 wire with a neutral, cap that off on its own. This passes power to the zone valve and only the controller and remote stat can now turn it on. If yours is like that, you won't easily be able to reuse the stat wires to carry 12v without rewiring some of that circuit.

    Hope this makes sense and helps.
     
    Last edited: Oct 17, 2021
  10. PandA3

    PandA3 Member

    Mate. I’m sure we’ve both got better things to do on a Sunday morning.
    I only continued on this thread because I saw you suggest some things that could confuse the OP.
    1. You initially suggested The stat wires are the ones in the Honeywell box (which they are not)
    2. You said use the HW Off wire? There isn’t one!
    3. You are using the 2 wires in the Honeywell box for 12v power which leaves nothing for a Heat Demand signal. Yes there are probably enough other wires to reconfigure one that is redundant but that has now taken the OP down the road of a total disconnect, test, rewire. Which in my opinion, I don’t think he was expecting, and in my experience most people would then take the option to use the Nest stat on the remote stand and plug it in a socket on the wall. (Curious why they sell such a thing)


    And for your ref.. TADO is a Nest equivalent costs about £200 (not cheap) which doesn’t require wiring mods. By far the simplest solution for anyone wanting Smart control of a combi by just swapping the stat and connecting 2 wires.

    I’ll leave the rest of this thread in your capable hands (assuming we haven’t bored the OP already)
    Have a nice Sunday…
     
  11. bright_Spark

    bright_Spark Screwfix Select

    ha ha dont worry buddy I come accross as arrogant but really not, if I can help someone then I will, I accept I may have confused the OP a bit with my instruction attempt but he really needs to identify his wires as a first priority rather than putting them into the terminals first. It is like trying to tell someone how to make a cup of tea, more complicated than actually just doing it. I tend to think everyone on here is a spark so that is where it falls short. I would have told him to go to his boiler and identify the mains and the call for heat wire before telling him to put certain cables into terminals. I find it easier to explain bit by bit than the whole process.
     
  12. adgjl

    adgjl Active Member

    At the moment the OP has a working heating / hot water system, albeit not controlled by a Nest. Once the first wire is disconnected, he will not have a working system until:
    - he manages to connect and configure the Nest.
    - he manages to put the wiring back to how it was.
    - he gets a heating engineer round to install the Nest for him.

    My advice considering his very basic knowledge of wiring is to pay for the installation.
     
  13. bright_Spark

    bright_Spark Screwfix Select

    In some cases you are correct, but it does depend on the OP and his abilities to work through it, also depends on his finances if he cannot afford to have someone do it for him
     
  14. markh1

    markh1 Member

    Thanks everyone for the advice given, I must admit I’ve hit a wall with it as was expecting it to be simple, I have reasonable electrical ability for a novice, but the previous homeowner who wasn’t a plumber or electrician converted the old standard system to a Combi, I had to get the whole system certified before buying the house by another gas fitter. To give you an idea of his incompetence when I moved in I found he had wired a double socket from another which he just added the cable into the existing fuse box downstairs ring and used lighting flex! I guess he’s just left cables from the old set up in place and capped them off as the clock and stat are the original ones. I shouldn’t have been surprised things would have been straightforward given the previous owners cowboy methods. I hadn’t even looked at the Tado to be honest, although I did like the look of the nest and have nest smoke alarms and several google smart devices. (Appreciate they all work with google/Alexa now)

    I may attempt to identify and label all the wires by turning off the power and one by one continuity tests, labelling them all up, but I need to go and get about 10-15m of cable so I can create the loop from the clock to the wiring box upstairs first. Even if I do this I guess it will save an installer the job should I manage to get one. I will be looking to see if any pro installers local to me as well but from experience getting trades in is not easy as always very busy around here and booked up for weeks in which time I may have figured it out myself!
     
  15. bright_Spark

    bright_Spark Screwfix Select

    where abouts are you located buddy?
     
  16. markh1

    markh1 Member

    Droitwich, near Worcester
     
  17. markh1

    markh1 Member

    I thought of a possible other solution, if I identify the two stat wires into the airing cupboard I have a plug socket in the airing cupboard and I could take these two stat wires put them into a 12v transformer and just plug it into the socket so the stat gets power from a socket rather than the nest heat link. In a way this is like using the stand but utilise the wires for existing stat.

    Not sure what 12v transformer in screwfix would be suitable? and what if any difference it makes not using the heat link 12v connections (assume nothing as it’s like using the stand).

    The 2nd generation nest connected via a plug in method and they removed the plug option from 3rd gen, not sure why other than to charge extra for a stand, not sure what the new 4th gen one has.
     
  18. Mike83

    Mike83 Screwfix Select

    The easiest way to do this would be to fit the heatlink at the old wiring centre.
    The wiring centre is usually at the same location as the pump and zone valves. This is usually at the old cylinder location but are sometimes fitted under the floor.
    If at the old cylinder location you may be able to see the heating systems wiring.
    If you do locate it upload a picture or two.
    It could be simple to identify the wires.
    If you can then identify the 2 stat wires then you’ll also identify the wire from programmer terminal 4 and the switched live to the boiler as these wires are both connected to the stat wires.
     
  19. markh1

    markh1 Member

    That’s another option, will have a look at sorting a photo. If I did this how would I deal with the existing programmer next to the boiler. Could I just transfer what is in the programmer to choc blocks all live in one, neutral in another etc and box in wall with a box plate cover?
     
    Last edited: Oct 17, 2021
  20. Mike83

    Mike83 Screwfix Select

    Depends on how it’s wired.
    If you can find the thermostat cable and there’s a neutral wire available at the wiring centre location then yes the programmer can be covered over.
    If you can find the thermostat cable then it should be straightforward as you’ll have a live from the programmer and a live to the boiler connecting on to this cable. That just leaves needing a neutral. I’m guessing there will be one as there’s 4 neutrals in that programmer.
     

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