Nest 3rd Generation thermostat, the installation guide specifies 42v DC MAX

[mark1963]

Hello, my installer wired my new heat link to my new viessmann vitodens 100-w combi boiler on open therm and my nest thermostat read an error of h73 saying no open therm detected, my question is the installation guide specifies 42 DC MAX for the OT1 and OT2 wires, is there a minimum DC Voltage? on testing the boiler is supplying 2.8 volts will the nest system work with 2.8 volts supply on OT1 and OT2, the T1 and T2 output is 12v DC.

I am currently running the boiler with the nest/ heat link wired 230 volt switching on/off to the boiler and every thing is working great, but the point is I bought the Nest to run on open therm.

[moralesvicente]

Hi mark1963, 

Thank you so much for posting. I understand you’re experiencing troubles with a Nest Learning Thermostat 3rd gen new installation regarding the error message H73. No worries; it's a pleasure to be able to assist you.

An H73 error occurs when a wireless connection between the display and Heat Link is established, but OpenTherm is not detected.

To provide you with further assistance, check out this article: Troubleshoot Nest thermostat T error codes during a system test. It'll walk you through some troubleshooting steps. If you still get this error code after trying those steps, I recommend contacting a technician to get professional assistance. 

If you have more questions, feel free to write in this post.

Best regards,

Daniel

[Rixington43]

I have a Vitodens 100w system running OpenTherm and my partner runs a Vitodens 50 combi in OpenTherm, both on nest 3rd gen.

I may be wrong but I would suggest the boiler has been commissioned for 230v control rather than OpenTherm control. Were the boiler and the nest both installed together and was the nest the first controller installed as part of the setup of the boiler? I would speak to your heating engineer and ask them to check the commissioning menu. I can't remember which setting it is off the top of my head but it's a 2min job to access the commissioning menu and check the control method setting.

You may even be able to speak to Viessmann and ask them to check/change remotely but I'd start with your engineer.

[Rixington43]

P.s If you do get it running ok on OpenTherm then I'd recommend you enable eco mode for hot water as the preheat can mess things up.

Also, changing the hot water temperature from the nest app doesn't seem to override so I had to set it on the boiler (or boiler app) for my partner's combi.

Happy hunting, OpenTherm is very worthwhile.