Broan-Nutone Steel Wall Cap For 3-Inch And 4-Inch Round Duct, Black Select The Broan-Nutone 885Bl Wall Cap To Provide Reliable Ventilation. Features A Black Enamel Finish And Is Designed For Use With 3 In. Or 4 In. Round Ducts. Includes A Built-In Back-Draft Damper, And Protective Bird Screen To Prevent Small Pests From Entering Duct.