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
