and squeezes the points of the rods 40 which are part of the longer and gasketed hinges 23 and protrude through the plenum chamber casing 40 a slotted opening is provided not shown in the plenum casing to allow these rods to move back and forth this position is held fig6 until the overheating or overcooling condition is corrected and satisfies the thermostat when the thermostat is satisfied the damper controller 38 is once again energized and moves upward to allow the triangular shaped control linkage 39 to return to its original position not shown when the triangularshaped control linkage 39 is in the original position the spring loaded hinge which is housed within its short side 47 forces the longer gasketed hinge 23 to rest against and seal the secondary air openings 24 in this position as shown in fig2 full heating or cooling is achieved fig7 shows an induced air 34 venturi 34 plenum diffuser 10 with the dampers in the semi closed position of fig1 and modified with a ducted room return as described in the preceeding units mentioned both in plenum diffusers 10 and terminal boxes 43 had warm air returned into the secondary air openings 24.