Just some info....I had a friend who bought my 1970 Z/28. It was sitting in his garage when, at night, the garage exploded and blew the garage door out (it hit his neighbor's house), moved the garage side wall 6" outward, cracked the concrete slab in the garage but did ZERO damage to the Camaro except for minor scorching underneath. Turns out, their lawn mower (Briggs & Stratton) had a leaking gas tank and the fumes filled the floor of the garage until they got thick enough to reach the water heater riser (about 24" high) to set it off. Scary shit. For every flammable, there is an optimum air/fuel ratio that can cause the excitement. A pressure rise of perhaps 1/4 of a PSI doesn't sound like much but a 20' X 8' garage door represents 23,040 square inches. That 1/4PSI rise is really fast and generates almost 6000 lbs on the door.....maybe more if the rise is higher pressure and faster. Blowing up the cheap box fan is not your worry, blowing up the BIG volume of gas inside your garage is the worry. I doesn't take much of a mix if the mix is concentrated in an area. ONE measly box fan isn't enough....look at a professional paint booth, do they use a small box fan? Be safe and make sure your fire insurance is paid up.