4" Start Collar with Built-in Damper Control.
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4" Start Collar with Built-in Damper made of galvanized sheet metal - 30 gauge - completely assembled, locked seams enclosing a sheet metal damper to adjust air flow internally.
The damper extends thru the collar with adjustment handle.
Sizes range from 4" to 20" diameter.
Start Collars with Dampers are used to attach the ductwork to the main source of air supply - usually the plenum box.
The damper in the start collar is the correct way to balance the air flow in Residential Homes and Commercial Buildings.