Abstract: At an outlet of a heating, ventilating or air conditioning system air passes from the ducting through a plenum chamber, a damper provided with a set of blades which are movable between open and closed positions and a diffuser which directs the air in a desired direction. The plenum chamber is provided with internal vanes which act to guide a flow of air through the chamber between its inlet and outlet. The damper defines a plurality of predetermined blade positions between the open and closed positions and a resilient finger and teeth for retaining the blades in a selected one of said predetermined positions. The damper has a frame comprising a plurality of discrete frame elements interconnected at each corner joint of the frame by a friction fit pin passing through interdigitated projections on the frame elements.