RIGID Variable Air Volume Boxes are designed to achieve variable air volume supply of conditioned air to a room in single duct constant volume air distribution systems. Variable Air Volume control is achieved by directing air ow either to a room or to a bypass plenum. Air ow direction is controlled by opening and closing of damper blade with direct response to signals from the thermostat in conditioned space. In RIGID VAV box the primary air damper works in connection with bypass port damper. As the primary damper blade closes, bypass port damper blade opens and vice versa so that constant volume air is supplied by air terminal and varying amounts of air is delivered to the room and bypass plenum.