Shop is 30' X 32' X12'. Insulated OH door, and walk in door. 1-1/2" Styrofoam between purlins, and between trusses. Then covered it with white cover sheet metal barn siding. I have 2 sources of heat. #1, is a Reznor 110.000 btu. waste oil furnace, I leave on all through the winter, set at 55º. #2, Is a 1939, Florence Hot Blast wood/coal stove I fire up when I'm working out there, to conserve oil. I had the oil furnace in the other shop, at the other place, and brought it with me. The shop there was 30' X 36' X 12', and I would go through approx. 400 gallons of oil an average winter. Myself, friends and neighbors pretty well keep me in oil. I saw an ad on Craigslist 2 years ago for free coal. It's industrial steam coal, 2" down to bug dust. I ended up getting around 12-14 tons, and barely put a dent in the pile, but, that's all the room I had at the time to store it. I get a good wood fire going, then keep adding, maybe a #2 flat shovel of coal every 1/2 hour or so, then add more, and a bit of wood. It's pretty well banked up when I'm ready to quit, so I set the damper just so it's cracked, enough to keep it going. It will settle down to where the flue temp is around 300º. Worked out there until about 7:00 p.m. last evening, when I shut it down. Didn't get back out there until around 1:30 today, and still had a good bit of hot coals left, to start it back up with a little fine kindling, after shaking it down. A few bigger pieces of wood, then went to coal. Kept it pretty well simmered down, and as still 70º in there.
Last week when we had sub zero temps at night, then barely into double digits, the old stove kept it at 68º, once I warmed it up to 70º, with the oil furnace. It took the oil furnace 6 minutes to take the temp. from 51º, to 73º, until I got the stove going good, and kept it fired it pretty decent all day. At most, I used 1-1/2, 5 gallon buckets of coal, and a several big arm loads of wood. There is some maintenance to the furnace once a year, and I have to clean the ashes out of the stove, and stock wood & coal, but, I like the idea of nearly free fuel, and a warm shop.