The Faber Mudamper8 8-In. Make-Up Air Damper Brings Fresh Air Back Into The Home And Improves Hood Performance. With Higher Cfm Hoods More Air Is Exhausted In Tighter Homes With Less Air Coming Back In. To "Make Up" For Lost Air, The Universal Make Up Air Kit Brings Fresh Outside Air Into The Home Through A Separate Fresh Air Duct. The Automatic Damper Opens Inside The Duct To Bring In Air When The Hood Is Operating Above 400 Cfm And Closes Tightly When The Hood Is Off. The Kit Includes 8-In. Switch Assembly, 8-In. Automatic Return Air Damper, And 24-Volt Plug In Transformer. The Air Switch Assembly Attaches On Top Of The Hood In The Duct Work And Via Low Voltage Wire "Talks" To The Make-Up Air Damper. The Air Switch At The Hood Duct Exit, Activated By Air Flow, Opens The Damper Outside To Let In Fresh Air. The Air Switch Then Lowers When Hood Is Off & Tightly Closes The Outside Damper. It Opens When The Hood Is On, And Closes When The Hood Is Off.