I've not seen one, and been looking a good while actually. I use a pair of
Ecobees, I like them, and they provide a decent external API to other
ecosystems that is well supported. They're probably yet another crapgadget
iot things infected with junk corporate or chinese code, who knows where
they stop or begin. They use some remote external wireless sensors that
are cool for various rooms that really I don't see ever happening, so while
not ideal, ideal doesn't exist today.
Sadly I think just no one has gotten around to this, or scared to take
responsibility to do so. I use OpenSprinkler boards for, well sprinklers,
and works well enough as an attachment to a RPi - been waiting for like for
thermostat control, but nada. I'd imagine you need some 24vdc conversion
and relays to handle your hvac, rest is all logic to not blow up a house
accidentally, freeze, or bake to death in extreme climates. Who's open!
Otherwise, just buy an Ecobee or Nest, talk to an API, and hope they aren't
selling you out to anyone by design or ignorance with shoddy dev's.