Viessmann installation manuals do not require the use of inhibitor to be added to filling water . If you use inhibitor they blame it for causing blockages hot plate heat exchanger. Viessmann installation manuals also allow potable water to be used for filling . The exception would be if you are using your own well water for supply. I suggest down load the manual for your model and check the wording on water quality .
I'm pretty sure the installer didn't use well water lol