Mark...I've used a 85K btu kerosene heater in my two-car garage for over 20 years now without an issue. Mine is a "Kerosun" unit, about 3 feet tall, with a 16" diameter on the cylindrical part. The bottom of the unit is about 24" square, and it has a catch-tray on the bottom that's slightly larger. On a typical 8-10 hour day, it uses about 1.5 gals of kerosene and puts out enough heat to let you work in your shirt sleeves after about an hour of warm-up time. Granted, we're in Memphis and our low temps are generally no lower than the high teens at night, so I'm sure there would be some adjustment for your location.
I have a larger than normal 2 car garage, with 14 foot ceilings...it's partially insulated, although the biggest difference in mine and other garages is two insulated garage doors with top and side air seals. The doors alone made a huge difference in being able to heat/cool the garage. Although the heater is a convection design, I also use a 20" box fan with it set up so the fan blows just over the top of the heater about 4"....just enough to not let the air flow disturb the kerosene flame. That way the fan distributes the heat around the garage really quickly...and it seems to work well. I've used this same set up in two different garages since 1985 without any incidents.
Initially, I was worried about carbon monoxide, but even with all the doors closed, I've never had any issues with CO. Incidentally, I've used the same mantel in the kerosene heater since new...it's never needed a replacement. I clean the heater every spring, and drain the kerosene from the tank before I put it away dry...seems to be in fine condition and has worked well all these years.
Good Luck...and stay warm.
