The 6” Booster In-Line Duct Fan is durable, quiet, easy to install, and will also fit into metal or flex ducts. Featuring fans that increase airflow in hard-to-ventilate rooms, their strong motors, excellent. The 6” Booster In-Line Duct Fan is durable, quiet, easy to install, and will also fit into metal or flex ducts. Featuring fans that increase airflow in hard-to-ventilate rooms, their strong motors, excellent performance, and higher temperature ratings set these booster fans apart. Rated for both heated and cooled air and perfect for outdoor AND indoor use. Also available in 8" model.