We replaced a boiler in our last house a few years back. We got a Vaillant, at least in part because I found that there was a Vaillant add-on board available which could do OpenTherm, and the nest controller could do that in theory too.

When it came to the install I was told in no uncertain terms that even though they produced the board and it was compatible with my boiler, any attempt at installation by myself or their approved installation engineer would immediately void the warranty.

So that was the end of my OpenTherm journey. Thanks Vaillant!