I currently work on embedded industrial control systems, I get enough of a fix there. :-)

The HVAC application is so niche that it could only be a hobby project. Basically, I don't want to dogs to fry in the RV in the summer. Some temp sensors, a few relays, and the GPIO pins on the Pi, and when it gets too hot it can kick on the generator and turn the A/C on for happy dogs. While I'm there, I might as well program in the algorithm I do in my head to tell the wife whether or not she needs the generator to make supper. "Lessee, if we've got two hours of daylight for solar that is currently putting out 221W, and the battery is at 74%, using a 1200W griddle...". Let the Pi do that math, and turn the generator on or not.