A dryer duct is a 4-inch-wide rigid or semirigid tube leading from the dryer to the outside of the house, allowing moist air and lint to leave the dryer. Flexible tubes are not recommended for use as dryer ducts because they have ridges that can trap lint.
The maximum recommended length of a dryer duct is 25 feet, provided that the tube is laid out straight from the dryer to the exterior. Every time the duct bends at a 90-degree angle, the maximum length decreases by 5 feet. Dryer ducts should be cleaned twice a year to maintain energy efficiency and reduce fire hazards.