I use a kerosene/diesel heater That I put outside near my air intake fans/filters so all of the air that comes in is heated. I can adjust the temp in the shop by the speed of the intake fans or the distance the heater is from the fans. I can keep the shop at 75*-78*F all day and night long when it's 20*F outside. I use diesel in the heater and have never had contamination issues.