When I was building Pearl, back in the ‘90’s, one of my neighbors upgraded his forced hot air furnace to a bigger, newer one and I got his old one - an 85,000 BTU unit - for nothing....Just get it out of his yard. It ran on oil. Parked it in one corner of my extended, one-car bay, ran the flue through a “Powervent” and out the back wall, put a sheetmetal 90° outlet on the top to direct the hot air into the shop space and ran the fuel oil from a tank sitting just outside the back of the bay. It could go from 20° to 70° In about 30 minutes. I went through 300 gallons of oil in four years, running it a lot of the time at 55° just to keep tools warm-ish. I really miss that furnace.....