Field Controls 46120002 AB-8 8" Air Booster For 8" Round Metal Or Flex Duct (425 CFM Max). The distribution of conditioned air to "problem" rooms in residences and businesses is improved using field air boosters. They are installed in ductwork and increase air flow by up to 1400 cfm, increasing the effectiveness of the heating and cooling system and conserving energy. Nine variants are available for square or circular ducts up to 14" in diameter.