Help Needed: OpenTherm Activation and X21 Terminal on Viessmann Vitodens 100-W B1KF with Nest Thermo

[Elsconde]

Hello everyone,
I am using a Viessmann Vitodens 100-W B1KF boiler with a Nest Learning Thermostat 3rd Generation and trying to connect them via OpenTherm for modulation control. I have some questions and would appreciate any insights:
1. On the boiler’s board, I found a connector marked “21” with terminals “L” and “N”. According to the wiring diagram and manual, this should be the X21 terminal for OpenTherm communication. Can anyone confirm if this is correct for this model?
2. To activate OpenTherm, I set the boiler parameter C.7 to 14. However, after doing this, the boiler showed a generic error symbol without a code and the Nest thermostat did not connect. When I reset C.7 to 1 (On/Off mode), everything worked fine. Is it necessary to restart the boiler after changing C.7 to 14 in order to enable OpenTherm correctly?
3. In On/Off mode, the Nest works perfectly as a basic thermostat, controlling the boiler without errors. But with OpenTherm, I don’t see modulation (no green leaf icon on Nest) and get errors on the boiler. Are there any additional settings or prep work needed in the boiler or Heat Link to ensure OpenTherm functions properly?
4. Has anyone successfully paired a Nest 3rd Gen with a Viessmann Vitodens 100-W B1KF in OpenTherm mode? Were there any firmware updates or special configurations required?
Thank you very much for any help or shared experience!

[GabrielaG]

Hi @Elsconde,

Thanks for reaching out to the community. I understand you're experiencing an issue with your Nest Learning Thermostat (3rd Gen) and Viessmann boiler. I appreciate the detailed information you've provided. I'm glad to share more information.

It sounds like you've correctly identified the on/off mode as a temporary solution and are now looking to utilize the full efficiency of OpenTherm. You can find instructions on how to activate OpenTherm configurations in this article: Separate hot water tanks (On/Off or OpenTherm).

For your safety and to ensure the system is properly configured, I highly recommend having a certified technician inspect your wiring. They can make sure everything is set up correctly and provide specialized advice for your specific boiler and heating system.

Hope this helps, and please let us know if you have any other questions.

Regards,

Gabriela