Using a Nest thermostat with a Junkers boiler (and OpenTherm)

I’ve been hesitating to buy a Nest thermostat for a long time since I wasn’t sure it would work with my already installed Junkers boiler.

The Junkers boiler is equipped with modulation so it would be unwise to simply use an ON/OFF temperature control. Unfortunatly Junkers doesn’t support OpenTherm out of the box. And that’s exactly the reason why I’ve been hesitating to by a Nest thermostat whitout doing a lot of research.

For those few people who want to install a Nest thermostat on a modulating Junkers boiler: it IS possible!

What you need

  • A Junkers boiler with a Heatronic 3 or newer
  • A Nest thermostat (3rd generation) with OpenTherm support
  • An ‘EMS-OT’ converter (use Google to find a supplier that fits your needs)

Wiring things up

The EMS-OT converter is the connection between the Junkers boiler and the Nest. Remove the orange connector of the converter to make it possible to connect the wires on the two “B” connectors on the boiler. There’s no polarity to take into account.

On the other end make the connection between the Nest heatlink OT1/OT2 terminals and the green terminals of the EMS-OT converter.

Plug in the boiler and the nest heatlink. If everything goes well you can configure the nest thermostat now. Ofcrourse you don’t select on/off but modulating boiler.

After one week of using the thermostat I’m pretty happy with it. The house is warm when we want it and the temperature of the boiler is modulated according to the required heat.


  1. Philippe on December 2, 2017 at 9:23 pm

    Hi Peter,
    Great post, thank you very much for that !
    Just a quick question, can you tell me from whom you bought your ems to ot converter ?
    Thanks in advance,

  2. Andrej on December 13, 2017 at 6:20 am

    Hello Peter,

    I have Junkers ZSBR 16-3A23. What do you think. Will it works also with my model?
    It means, with this converter I will plug in just 230V and Opentherm 1 and 2 connectors, right?

    Thanks a lot


    • peter on December 27, 2017 at 7:53 pm

      Hi Andrej,
      I cannot tell for sure if it will work. If it is a recent boiler with bus connection (‘B’ terminals on circuit board) it will probably work.
      The converter is between te openterm terminals of the nest and the bus terminals of the boiler.
      Good luck with your project, and let us know if it works with your model!

  3. Didac on March 11, 2018 at 8:22 pm

    Given your diagram, it seems you have been setting the Nest Heat Link.
    I bought the Best thermostat 3 with 10 wire connection.
    Would the ems-ot work with that device?
    It has pins for w1, w2, c, *, Rh, y1, y2, g, ob, Rc

  4. Xenseifir on October 30, 2018 at 10:52 am

    Good morning, I wanted to know if after this time the installation works correctly, or has given problems, I was interested in doing it and wanted to make sure. Thank you

    • peter on October 30, 2018 at 11:58 am

      I’m still using the setup as described in this post and didn’t notice any issues.

      • xenseifir on October 30, 2018 at 12:05 pm

        perfect, thank you very much, I will do it myself and I will comment my results

  5. Carlos on February 6, 2019 at 8:23 am

    Hello! I have a question: do you keep your boiler heating dial off? or on? and in that case, to which value? Thanks!!

  6. blahusf on March 12, 2019 at 11:32 am

    Hello everybody.
    Thank you for your post, it helped me a lot.
    If anybody interested: My old Buderus Logamax U124 (iRT protocol) is now working with Google Nest with modulation. Details here:

  7. Paul Moore on October 19, 2019 at 11:01 am

    Thanks for the post, I’ve installed the OT to EMS converter using your instructions. I finished it this morning, so it’s too early to say if it works.

    I have some questions. How do you know if the boiler is modulating? Are the scheduled on/off times on the Nest now ignored by the boiler? If so, does this mean that the boiler runs 24/7?


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