Model OBD is used on registers and grilles. OBDDM is used for duct mounting. Opposed-blade dampers have vertical blades that move in opposite directions to control air volume over the entire face of a register or grille.
Interwoven tension spring on blade pivot points permits positive setting from full-open to full-closed. Blades are screwdriver operated. Extremely shallow depth allows damper use in shallow stackheads.