Therwen 2 Pack Fits 8 Inch Neck Air Duct Steel Butterfly Damper for Round Vent, Air Supply Black Ceiling Diffuser with Plastic Key Handle, Outer Dimension: 9-1/2"

This item is a butterfly damper used for regulating airflow in round ventilation ducts. [1, 2] Function: It acts as a one-way valve, often found in kitchen exhaust systems (like over-the-range microwaves) to direct air outside. Design: The spring-loaded butterfly flaps open when air is pushed through in the direction of the "OUT" arrow and close when the air stops to prevent backdrafts. Installation: The attached chain and springs help secure the unit within the ductwork. [1, 2]