Abstract: A unit for heating or cooling multiple rooms including a plenum diffuser mounted parallel within a ceiling runner and supported by hangars within the ceiling plenum. Each unit includes a round inlet collar, an elongated plenum to form a "Venturi," hinged volume dampers, secondary air openings, a bottom plenum chamber, and a pattern controller. The volume dampers housed within the Venturi work in unison. When the volume dampers are open, the unit functions similar to any conventional heating or cooling plenum diffuser. Primary air (heating air or cooling air) will enter the unit and leave through a discharge opening with no change in the air quantity or temperature. As space cooling or heating requirements are met, the volume dampers close at a preset distance, the primary air is reduced (variable volume), the velocity is increased and induces (induction) warm return air through the secondary air openings from the ceiling plenum, or ducted room return, into the primary air stream.