Connect Nest 3rd generation to Bosch 26 HRC

Connecting a 3rd generation Nest Learning Thermostat is pretty simple if you know where to connect the wires. In this blog I have some instructions to connect to the Bosch 26 HRC furnace. The first part is about the old connection and the second part is about connecting the Nest.

First I didn’t get it to work, this was because the OpenTherm module was disconnect for the previous installation. The red circle shows the disconnected plug.

Bosch furnace connections 2

Blue square = Bosch connection, red square = OpenTherm module connections, red circle = Opentherm connector.

This OpenTherm connector was unplugged/disconnected because the previous thermostat was a simple on/off (non-modulating) thermostat which was connected like this schema on the Bosch furnace:

non-modulating-thermostat-bosch 26 hrc

Connect a non modulating thermostat to Bosch 26 HRC (image source: Bosch 26 HRC manual)

So the very sloppy wires in the blue square pins 1 and 2 are connected to the old thermostat in the living room. The resistor is connected between 2 and 4 and there is a bridge between 8 and 9.

Bosch furnace connections 1

Blue square = Bosch connection, red square = OpenTherm module connections

So this was the old working connection with an old non modulating thermostat. The Red square in the picture above shows the pins connected to the OpenTherm module. Bosch calls this the OTM1 in their manual. OpenTherm Module

OpenTherm Module (OTM1)

I removed to old wires (in the blue square) and placed the resistor as a loop on pin 7 (as the original state) and then plugged in the OpenTherm plug onto the board just as the picture below:


Connect the OpenTherm Module (OTM1) to the furnace (image source: Bosch 26 HRC maunal)

Now connecting the Nest is simple. Nest comes with a Heat Link unit which you have to connect to the furnace and a thermostat which you probably place in your living room.

The 3rd generation Heat link looks like this:

IMG_3392 edited

Nest Heat Link

The three circles mean;

  • Red; 230V to power the Heat Link unit
  • Blue; connections to furnace OpenTherm adapter pins O and T
  • Green; connection to the Nest Thermostat

In the Installation Guide you can find this image for this 3 connection pairs.

Nest Opentherm connectionsNest OpenTherm connection schema (
image source (PDF)

That’s all. the most time I spend figuring out how to the connect the Bosch OTM1 module to the furnace and remove the old wires. good luck!

Posted in Domotica Tagged with: , , ,
5 comments on “Connect Nest 3rd generation to Bosch 26 HRC
  1. Yke says:

    Hi Peter, thanks so much for the clear instructions. My time-consumer was finding the place where i should put the OTM1 connector :)

    I disconnected all the old on/off connections and connected the OT1 and OT2
    Can you please tell me why i should reconnect the resistor as a loop on pin 7?

    It this moment there is no wires and resistor in the blue-square part, all seems to work.

    Thanks for your reply

  2. Wout Donker says:

    Ha Peter,

    Thanks to you I’ve got it all working. A Nest 3rd gen and a Bosch 35 HRC. With the loop on nr 7….

    Many thanks,

    Wout Donker
    The Netherlands

  3. Krystian says:

    Is it important how to connect OT1 and OT2 on Bosch O and T?
    How to check that my bosch is modulating?

    What are the yours pro-settings on the nest thermostat for bosch?

  4. ik heb verkend voor een beetje voor elke hoogwaardige artikelen of blog posts op deze soort gebied. Verkennen in Yahoo ik eindelijk struikelde over deze site . Het lezen van dit informatie Dus ik ben over te brengen dat ik heb een zeer goed griezelige gevoel dat ik ontdekte net wat ik nodig had. Ik zal zeker maken zeker te niet website en geef het een kijkje op een constante basis .

Leave a Reply

Your email address will not be published. Required fields are marked *