As I'm spraying, the furnace gets shut down to 1. prevent an explosion and 2. to keep dust out of the paint. My HVAC freind said not to worry about the explosion part considering the amount of fuel that needs to be in the air for that to happen ( his opinion ) but I'll take the precautions just in case. By the time I have to crank the furnace on again, the paint is dry and most of the fumes are gone. If I get the garage to 80 deg.,it takes about an hour and a half to go down to 65-70 deg. I'm sure that depends on the outside temp as well, but it was relatively cold last sat ( around freezing ). When I had the woodburner, I couldn't paint in the winter because I couldn't "shut it off".