In the old grinding room, which was a small and unheated shed right off the open smithy, I would set a propane torpedo heater outside in the smithy and point it toward the open door. It would warm the place up a lot in about 20-30 minutes. I would back it down and grind away. When forging in the smithy, I would place it about 8 feet away from me and it would keep me toasty while I worked on the cold days below 35°.
They come in kerosene, diesel, propane, and multi-fuel types. I like the propane ones because they are safer, don't have any smell, and can turn on/off instantly. They are very affordable new at placed like HF, and often found for low prices at pawn shops and yard sales.
In the new shop there are four "areas". One is the 20X13 clean shop, which as the name implies, is not for machining and grinding or hot work. The main area is the "machine and hot shop" and has workbenches, machine tools, and welding equipment. The stairwell going upstairs in it. The 10X12 grinding room is enclosed by half-walls with glass windows above them and has its own in and out air supplies as well as dust collection and air filtering. The 20X40 upstairs is open and has the big building vent fan.
I used two propane catalytic heaters while building things. I have only used them to warm the hot shop up a few times since construction. Since the machine shop work areas are ventilated to the second floor, there is no worry about O2, CO2, and CO levels, but I have detectors anyway. The torpedo heater was overkill in the machine shop except one very cold period of 20° weather.
The clean shop portion of the new shop has an AC/heat mini-split system that keeps it between 65° and 70° 24-7. I insulated that portion of the shop very well and it only sips electricity. I doubt it averages $10 a month in electricity. I highly recommend them for enclosed and insulated shops.
