Backdrafting happens when combustion gasses are sucked back into the house and can harm or even kill building occupants. This is due to low indoor air pressure, bad flue configuration or flue blockage. Inspectors should try to spot evidence of backdrafting by, releasing smoke or powder into the draft diverter to see whether it gets sucked into the duct or if it spills back into the room, keeping an eye out for soot and, using a carbon monoxide detector.