ACTIVE AIR ACSC Duct Fan Speed Adjuster It will correctly and precisely adjust the speed of centrifugal fans These controllers are more sensitive than any on the market, and are compact and built to last. Meant for use with brushless motors, and all Hydrofarm inline fans, not booster fans Not recommended for use with "squirrel cage" type fans or the 12" Max Fan. For the 12" Max Fan, use speed controller CFH0350290.