Replacing control panel with Nest Link

Discussion in 'Electricians' Talk' started by Murrell, May 4, 2021.

  1. Murrell

    Murrell New Member

    Hello,

    I am looking to replace my Myson 2c dual channel programmer with the Nest thermostat. The boiler is a Ideal Logic Heat H15 with an immersion heater upstairs.

    I just wanted to run the wiring past this community to make sure I understand it correctly.

    Removing the Myson panel I can see the wiring behind (https://i.imgur.com/d0An7Iz.jpg).
    Terminal 1 is Hot Water Off (No wires going to it)
    Terminal 2 is Central Heating Off (No wires going to it)
    Terminal 3 is Hot Water On (1 Wire)
    Terminal 4 is Central Heating On (1 Wire)

    I assume in this Nest Heat Link diagram below;
    Lives go to L,
    Neutrals go to N,
    Myson Terminal 3 goes to 6 (Call for heat HW)
    Myson Terminal 4 goes to 3 (Call for heat CH)

    Is there anything else to it?
    Do I need a link from Live to Commons 2 and 5?

    We have a bog standard Myson MRT1 Thermostat in the living room which I can just connect to the new Nest. How does the Thermostat tell the boiler to turn on? I can't work out how that is wired.
    [​IMG]

    Thanks in advance!
    Ross
     
    Last edited: May 4, 2021
  2. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Yes, as far as I remember I linked the L to the two commons.

    Your old thermostat wiring from the programmer will become 12v to the Nest thermostat. The rest is done via WiFi direct to the Heat Link
     
  3. Murrell

    Murrell New Member

    This is the wiring inside the boiler
    [​IMG]
     
  4. MGW

    MGW Screwfix Select

    It depends on what you want. You can use Opentherm but to be frank, I am unsure about Opentherm and I am not happy with my Nest Gen 3.

    The problem with Nest is it does not connect to TRV's so only works with one room.
     
  5. Mike83

    Mike83 Screwfix Select

    The only way to utilise the existing thermostat cables for the nest stat would be to fit the nest at the wiring centre.
    If you fit the nest at the existing programmer location you will need an alternative power supply for the nest stat.
     
  6. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    When I did mine I found a spare cable that I could repurpose to get the Nest in the same place as the original stat, but it's hard to tell whether the OP will have this option
     
  7. Mike83

    Mike83 Screwfix Select

    Even with 1 spare wire from the programmer to the wiring centre could make it possible.

    Theres definitely other wires in behind that programmer and maybe the current thermostat cable could be behind it.

    More pictures of the wiring at the wiring centre and behind programmer would help though.
     
  8. MGW

    MGW Screwfix Select

    When I fitted mine it was to a oil boiler so not possible to use the OpenTherm option, how it is connected depends on the system in use, mine was a modified C Plan, but I would guess yours is a combi boiler so you do not need any control for DHW so two connections one is power we call that low voltage i.e. 230 volt and the other is classed as extra low voltage which is under 50 volt and better if not the same cable, you don't want inductive or capacitive linking between low and extra low voltage supplies, so better is not same cable, since the heat link is not the bit that measures temperature it can be placed close to the boiler.

    From the heat link to the thermostat can be either hard wired or wireless, I prefer hard wired, as it carries both the 12 volt supply and data, so less likely to have interference problems.

    [​IMG] Looking at the picture I wonder if you have not got it set up as a combi boiler, and you have some motorised valves, if so the normal method is to have the thermostat work the valve, and the valve works the boiler, EPH make a thermostat that will work with OpenTherm and the valves and can be set as master/slave. And there are other systems which will link to TRV heads, so since we don't know your system we can't really advise.
     
    Last edited: May 5, 2021
  9. Murrell

    Murrell New Member

    Sorry for the late reply, I didn't see anyone had replied!

    It's a heat-only boiler but I think the HW on terminal goes to the immersion heater upstairs.

    I have fitted the nest thermostat and the control is working but as previous replies have said, I now have no power to the existing thermostat. I've been looking at the wires behind the control but can't work out which is which.

    Behind the stat, I have a brown wire, black wire with brown sleeve and grey wire with blue sleeve.

    Here is a pic behind the control. I appreciate it is hard to see so any questions let me know and I can look or get a better picture.
    [​IMG]

    The white cable to the right goes to the boiler.
    The only grey wire coming into this box is coming from the second grey cable which is grey wire sleeved blue. This wire going into a terminal block with other neutral wires and then connects to the N of the Nest
    The live browns go into a terminal block and connects to the L of the Nest
    The black wire into the terminal block goes to call for HW
    The brown wire into the terminal block goes to call for CH

    [​IMG]

    Can anyone work out from this wiring mess how I can get power to the old stat?!

    Really appreciate the help! :)
     
    Last edited: May 10, 2021 at 9:32 AM
  10. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Where is the wiring centre for the system? Usually found in the airing cupboard.

    The chances are you'll find the cable that feeds the existing stat in there and be able to disconnect it.

    Quite how you send it to your Heat Link it entirely up to you though, I can't advise without knowing the layout of your house
     
  11. Bazza

    Bazza Screwfix Select

    I rather think that the "wiring centre" is that mass of choc blocks behind the existing programmer, combined with all of the connections to the L&N terminals!
    Dont ya love plumbers?
     
  12. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    I was REALLY hoping that wasn't going to be the case!

    I will never understand why people find it so difficult to pick up a permanent pen and label their cables!
     
  13. Bazza

    Bazza Screwfix Select

    Because its often done by other trades. Mind you, I rarely mark cables if I'm connecting as I go along. Only need to mark if running in a bunch of cables to identify before second fix, and the markings get chopped off when connecting up.

    PS I had a plumber friend who could wire an S Plan in one of those round junction boxes. It was a work of art. nobody else could understand how it worked, but it did.
     
  14. Murrell

    Murrell New Member

    This is a new home done by Bellway - They knock them up cheap and I'm pretty sure that is the "wiring centre". I can't see a pretty, well labelled box in the airing cupboard :(
     
  15. Murrell

    Murrell New Member

    I hate to assume by colour, but there is only one grey sleeved wire in the box, and I assume it goes to the stat...? Those wires are plumbed into a live N and L feed already

    I also assume that there are 4 cables.
    1. Live feed from the house
    2. Going to the boiler
    3. Going to the stat
    4. Going to the upstairs immersion heater for HW
    To add complexity, the house is dual zoned heating. Upstairs has its own stat and timer in the master bedroom. God, I wish there was a wiring diagram...
     
    Last edited: May 10, 2021 at 10:29 AM
  16. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Where do the cables feeding the cylinder stat, motorised valves etc come from? Just from holes in the wall?
     
  17. Murrell

    Murrell New Member

    I guess the box on the floor in the upstairs airing cupboard

    [​IMG]
     
  18. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    That's the wiring centre
     
  19. Murrell

    Murrell New Member

    Ohhhh...sorry! I'll isolate the power and crack it open
     
  20. Murrell

    Murrell New Member

    Here's the inside of it. The Nest says I need no more than 12v going to the thermostat...

    [​IMG]
     

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