I didn't see the UPS he needs to keep the Pi up and running, nor did I see anything about a watchdog to reboot the thing in case of lockups. (Although someone in the comments did mention that.) There's also the problem of overshoot and actually controlling the the devices - you can't just turn them on and off, you have to cycle and anticipate the need. (There's a circuit called an anticipator in a thermostat, it waits for a certain differential before activating.) I've had various Pis running since 2015, constantly. I've found that even if they aren't accessing the uSD card, the card can die. If you're lucky, you just get a zombie process list and the machine keeps running until it needs something then it crashes, or it just immediately crashes when the card becomes inaccessible. Those shitty little uSD card sockets can also get wonky after the card just sits there for a long time, I have one that you have to eject and re-insert if you power down and reboot because the contacts are just not meant for how they are used. There's a reason stuff like this runs on microcontrollers that don't need to boot like a traditional computer, and using a hobby/education computer board that's morphed into a real computer (while still keeping the bad parts of the hobby computer) is not the best of ideas. Hope it doesn't lock up with a power flicker in the middle of winter! (6 individual heating zones sounds like a nightmare to start with. Friend of mine has hot water heat, and that house just has one Honeywell Roundie that works a treat. I use an ancient LUX-TX programmable that I bought in 1998, and it's been working fine since then. I'm sure if I didn't have a couple in storage NIB it would die, but it is what it is.)