I have a 45K Mr Heater natural gas forced air heater in my garage and it works great. Quiet, and the output air is fairly low velocity, so it doesn't blow papers off the tables, etc. It only costs me about $100 a season to heat my garage a season - I live in So Michigan. Garage is 26x30 with 10' walls and 11'6 at the peaks and 2 - 9x8 insulated doors. There is 4" of fiberglass batt on the walls and 6" in the ceiling, with everything sheeted over with OSB. Outside is vinyl siding with a vapor barrier. I just leave the heat set at 55* - I don't mess with changing it, since it is cheap enough to heat it to not worry about it. Even when it's 20* outside the heater only runs about 5 mins every hour. IMO the key is being able to keep the heat in, not so much how it's made. The first year I could heat it comfortably with a 35K torpedo propane heater - it was just noisy and annoying getting bottles filled.