Dryer fires usually start beneath the dryer when the motor overheats. Overheating is caused by a build-up of lint in the duct that increases the drying time and blocks the flow of air, just like cholesterol in your arteries can build up and block the flow of blood to your heart. Naturally, any lint that has collected under the dryer will burn and the draft from the dryer will pull that fire up into the duct. Since the duct is coated or even blocked with lint, many times a house fire results. Other contributing conditions may include failure of the thermostat and limit switches in the dryer, lint inside the dryer, a missing or damaged lint screen, a crushed hose behind the dryer, or a bird’s nest or other debris blocking the vent.