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.
-mb