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Nest Learning Thermostat H73 Error

AndyB151
Community Member

Hi,

     I have a Viessmann Vitodens 100-W boiler has been working fine with Nest Learning thermostat for eighteen months using the open-therm. A few months ago I had an H73 error on the thermostat suggesting unable to detect the open-therm connection and the boiler just ran uncontrolled (all power supplies were good and the heat link had a green light on). I switched off the power to the system and checked connections etc, all were good. I switched back on and it still had the fault. After several hours of checking and reading it suddenly cleared of it's own accord and all back to normal. 

Fast forward to yesterday and it did the same and cleared itself after a few hours of power off. The software is up to date and the boiler is working correctly; it appears to be a problem with the thermostat or heat link although the heat link appears to be functioning correctly. I am at a loss as to what to do next? the thermostat is under warranty until February 2024. If this happens when were out then the boiler just runs uncontrolled as it's lost the open therm control.

Any advice would be appreciated.

Kind Regards

Andy B

8 REPLIES 8

Vladut
Community Specialist
Community Specialist

Hi there, thank you so much for reaching out. Can you please tell me if you've tried to restart the thermostat as a first step?

AndyB151
Community Member

Hi,

     thanks for the reply. I have only de-powered the mains supply to the system and have not restarted the thermostat; what would be the procedure for that?

Kind Regards

Andy

Vladut
Community Specialist
Community Specialist

Can you please go to Settings>Reset>Restart?

AndyB151
Community Member

Hi, 

     Thanks for the reply. I have reset the thermostat and am waiting to see if it throws up the H73 error again. 
Kind regards 

Andy

AlexD
Community Specialist
Community Specialist

Thanks for checking. Keep us posted on how things go here. 

AlexD
Community Specialist
Community Specialist

Hi, I wanted to check in. How has it been going so far? 

AndyB151
Community Member

Hi,

     it hasn't repeated the fault again but last time it was a few months before it recurred.

Kind Regards

Andy

AlexD
Community Specialist
Community Specialist

Understood. In that case, if you do see that occurring, let us know here and we'll see what we can do. For now, just keep an eye on it.