At an outlet of a heating, ventilating or air conditioning system air passes from the ducting through a plenum chamber (1), a damper (2) provided with a set of blades (19) which are movable between open and closed positions and a diffuser (4) which directs the air in a desired direction. The plenum chamber (1) has an inlet (7) and an outlet (8) which extend in different directions and the chamber (1) is provided with a plurality of substantially planar, parallel, internal vanes (10) disposed on one side of the chamber (1). The vanes (10) act to guide a flow of air through the chamber between the inlet (7) and the outlet (8). The plenum chamber (1) may be integrally moulded from a plastics material.