Why does it need to be complex? It is just managing temperature same as the analog one. Same wires to the furnace and ac. Setting a schedule shouldn't require a heavy OS on top of this. One way I came up with in 5 seconds, just have it sit on the network and take in emails with your schedule changes and you're done. Or just ping it, don't even need the mail setup. Should be pretty light and performant with not very many lines of code.