Fantech DBF 4XL Dryer Booster Kit uses a 170 CFM fan to support up to 130 feet of dryer duct run. It is specifically designed with backward inclined blades to allow lint to easily pass through the fan. The DBF 4XL features an automatic pressure sensing switch which automatically activates the fan when the dryer comes on. Dryer booster fans allow you to exhaust dryer fumes through extended ventilation.