Pick one up and see for yourself. I think they still make them. It’s the basic round dial type thermostat from Honeywell. It uses an at328p MCU and it even has a programming header. The debug was open on the ones I played with, so I downloaded the code, but I never got around to (or had the tools to) disassemble it. I was going to just write an arduino sketch to replace the factory code, but we figured out the “cut out the battery and jumper to 3v “ solution so I never did. Saved about $4000 in thermostats replacements though.