air from the plenum 12 further includes aligned cutoff plates 16 and 17 which are provided with a curved surface for coacting with inflatable bellows 14 and 15 by varying the inflation of the bellows the area between each of the bellows and the cutoff plates may be varied to regulate the quantity of conditioned air discharged into the area or space being conditioned the manner in which inflation of the bellows is controlled shall be explained in detail hereinafter the operation of the air distribution unit and the control system related thereto shall now be more fully explained the adjustment of the tension of springs 32 and 62 of regulators 30 and 60 respectively will ordinarily be factory set and will be determinative of the seating bias of ball valves 31 and 61 respectively with spring 62 providing a greater biasing force than spring 32 alternatively the regulators 30 and 60 can be set upon installation since they will not be readily accessible thermostat 50 will be set at an adjustable set point assuming that the area to be cooled is at a temperature substantially above the set point pressurized control air will serially pass through the filter 20 and line 22 to pressure regulator 30 where it will cause ball valve 31 to open against the bias of spring 32 pressure inside regulator 30 is communicated via line 38 to distributor 40 thence via line 41 to bellows 14 and via line 42 to bellows 15 the bellows 14 and 15 will be inflated to a degree dictated by the pressure in regulator 30 and the degree of inflation of the bellows 14 and 15 will dictate the amount of conditioned.