Dampers For Round Ceiling DiffusersRound air dampers are made for use with Round Ceiling Diffusers (sold separately). These HVAC dampers feature knob-operated worm gear that adjusts the patented butterfly valve using only 2-1/2 turns for full open or close. Ceiling duct dampers control the air volume, not air pattern, to provide minimum air restriction. Damper is made with unpainted, galvanized steel. Steel ring has a brown finish and includes adjusting handle.