My 2 year experience of my Viessman 100w Combi (B1KF) with Evohome on opentherm has been good. The boiler modulates down to 8% output (as shown by Vicare APP) when the evohome setpoint temperature for each individual room has been reached . Its a 35 kW boiler for hot water function and supposed to be 32 kW for heating . The 8% output , I suspect refers to 100% of 35 kW , I am not certain, still this would be a turn down to 2.8 kW , lower than the data sheet 3.2 kW . It is condensing reasonably well most of the time , the flow temperature drops to about 48 deg C when output is 8%. I have limited the cold start flow temperature to 70 degC, I have not optimised the temperature drop across the radiators of the house , so I suspect the condensing can be improved further.
I have the boiler located back of my loft space . The vicare app is very helpful as I can do an occasional check on the system pressure without visiting the boiler .
I have had 2 boiler services done now , with no issues . Everything was spotless when I watched the service guy open up the heat exchanger.
My only negative is the boiler heating comfort mode , which keeps a small volume of hot water stored at a target temperature to reduce time for heat to taps, this cannot be switched off when opentherm is connected (this is dumb Viessmann) . I would have liked to switch this comfort mode off to save a tiny bit of gas . I am able to adjust the hot water target temperature from the Vicare app. As a partial work around , when I am away from house on holidays or work travel, I just set target temp Hot water temp to 25 deg.
I did lots reach on combi boilers before I selected this one as replacement in 2021 for the Worseter Bosch greenstar CD 35II from 2007. The WB was reliable and I used with with the Evohome with on/off tpi control , for control of the WB it was all OK but little condensing .
I would have selected the Viessman 200w , as it had turn down to 1.9 KW , if it allowed opentherm which it does not.
I would probably select the same 100-W on turndown, but there is another good choices using opentherm, with stainless steel Heat exchange and combustion control , i.e Vokera
