1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Upgrade to Nest 3rd Gen

Discussion in 'Plumbing and Central Heating' started by Mart Andrews, 1 Nov 2018.

  1. Mart Andrews

    Mart Andrews

    30 Oct 2018
    Thanks Received:
    United Kingdom

    I currently have a Worchester Bosch green star 30 Ri compact boiler system. It's a open vented system with a hot water tank in the loft with the pump and valves etc.

    I would like to upgrade the system to use the Nest 3rd Gen but I'm a little unsure of the wiring and what to do with the existing 3 cables at the current wall stat? I know the Nest smart stat requires only 12v supply and 2 connections on T1 & T2

    This was the thread I was reading:


    I have attached pictures of my current wiring set up, any help would be greatly appreciated.

    My only concern is the 3 wire existing room stat, at the moment on the C27 back plate it looks like the Red & Yellow from the stat are on the L connection and the Blue is on the N.
    I know I will only be using 2 wires (Red & Blue) on the Nest in T1 & T2 what happens with the spare wire (Yellow)? Do I just need to place it into a connection block as its not needed at both ends?
    With the Live and Neturals they just swap into the same on the Nest heat link as well as the heating on and hot water on from terminals 3 Blue, and 4 Red on existing back plate.

    Do I then just need to link the two common terminals 2 and 5 to the L terminal?

    Any help would be greatly appreciated.

    I have been reading other people's post and have decided to make a post myself for help.

    Read more: https://www.diynot.com/diy/conversations/hortsmann-c27-change-to-nest-3rd-gen.33873/#ixzz5Vd2VRF2h

  2. Sponsored Links
  3. stem


    20 Jul 2005
    Thanks Received:
    United Kingdom
    The wires at the C27 can be identified from the diagram below:

    They are moved from the C27 to the Heatlink terminals that have the same function:
    ['CH On' is the equivalent of 'Heating call for heat' and 'HW On' is the equivalent of 'Hot Water call for heat']
    As you suspect, you will need to link the Heatlink 'L' to the two 'common' terminals. (The C27 has this link already made internally)

    Space is tight at the Heatlink terminals. So, where you have 3 wires in one terminal now, you may have to terminate them first elsewhere and then run a single wire to the appropriate Heatlink terminal.

    The existing thermostat, will need to be decommissioned properly, it can't just be removed or the heating wiring will be 'open circuit' and the heating will not operate. You need to trace the existing thermostat cable back to its origin. Once you have discovered that, note where the red and yellow wires are presently connected, and disconnect them, along with the blue neutral in the same cable. Then the two points where the red and yellow cables have just been removed from should be connected together. The old thermostat should then be disconnected and removed.

    You will now have a three core cable that is no longer connected to anything at all at either end, if required, you can now use two of the wires to connect T1 at the Heatlink to T1 at the Nest thermostat, and to connect T2 at the Heatlink to T2 and the Nest thermostat. The third unused wire is simply insulated and tucked out of the way.

    It is imperative that the old thermostat cable is completely disconnected before using it to connect T1 and T2, they 12v only. Connecting them to 230V will likely cause some serious damage. If you or someone you know has a multimeter and knows how to use it it is possible to test the cable to be sure.
    Last edited: 5 Nov 2018
  4. DIYnot Local

    DIYnot Local

    3 Sep 2019
    United Kingdom

    If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

    Select the supplier or trade you require, enter your location to begin your search.

    Are you a trade or supplier? You can create your listing free at DIYnot Local


Share This Page