23 In.H-W 6 In.L Aluminum Wall Backdraft Dampr Damper Application Backdraft For Nominal Duct Diameter Not Applicable Frame Material Galvanized Steel Number of Panels 1 Maximum Air Velocity 2000 fpm Overall Height 23 in Rough Opening Diameter 20 1/2 in Rough Opening Shape Square Fits Fan Diameter 20 in For Duct Shape Not Applicable Free Area 2.3 sq ft Overall Width 23 in Frame Material Thickness 20 ga Overall Length 6 in Connection Type Not Applicable Frame Depth 2 1/4 in