Over time, lint from your clothes slips through the filter in your clothes dryer and gets into the pipe that leads out of the house. This lint then builds up on the walls of this duct until it literally blocks the air from getting through. When this duct gets blocked, the air can no longer circulate freely among the clothes you are trying to dry. This causes the drying cycle to take much longer than it should. If your dryer takes longer than 40 minutes to dry your clothes, you need your dryer duct cleaned today!