I looked into X.10 stuff a few years back. Seemed interesting, but a bit cumbersome. There was a newer software/wireless protocol (not sure it was zwave) that I looked into briefly when I was considering upgrading my irrigation system to a computer-controlled smart system.
The other stuff, HVAC, etc. seems also highly custom-build and subject to rapid obsolescence of the components. Some years back (>10?) Even IBM had some obscure line of home controllers. Most of these systems used a central cabinet with fixed-specification computer to manage stuff and almost all of them needed a lot of wiring that was non-existent in older homes.
There are however, increasing opportunities to at least collect/report on data such as HVAC, Pool, etc.
A buddy of mine with an aptitude for tinkering had his irrigation system under Internet control since the late '90s. He also had an x.10 interface, but I don't know if he still has it, since at that time the web page he controlled it from was practically unsecured.
Keep us informed, I'd love to hear what can be achieved.