Inline duct fans install into existing ductwork and move air through the ventilation system. General purpose units are designed for general ventilation. They boost airflow to spaces that are hard to heat or cool and increase efficiency. Some units include a built-in heater to boost heating efficiency while others are designed for continuous duty to help remove radon gas from a structure. Other units are designed to connect in line with the dryer exhaust and activate when they sense the dryer is turned on.