Ended up ordering a new furnace. The old one was about 26 years old so I couldn't really see limping it along, plus the new one will be considerably more energy efficient.

That said, any coffee roasters out there looking to support free useful tools, I wrote https://crucs.net/ for designing coffee roasting plans and am working on adding some neat analysis stuff. There's a support link available to toss some money at me if you think this is useful and want to see more like that.