Snap-on Duct Dampers are made to be used with Tab Collars (sold separately). HVAC dampers feature knob-operated worm gear that adjusts the patented butterfly valve with only 2-1/2 turns for full open or close positions. Ceiling duct dampers control the air volume, not air pattern, to provide minimum air restriction. Damper is made with unpainted, galvanized steel. Includes adjusting handle.