HVAC help - Nest & Viessmann

[rr_coyote]

Might be a problem with the Nest itself. Mine has an issue where, randomly, it'll say that there's no power connected to it, so I have to disconnect the wire that runs the A/C.

But, your system is totally different than mine. Too bad getting ahold of Nest isn't super easy, since they're based in the UK.