12 into a distribution chamber 40 defined by the top and side walls of distribution plate 38 a damper arrangement illustrated as an inflatable bellows 30 regulates the flow of air from the distribution chamber 40 to outlet means 34 the manner in which bellows 30 operates shall be more fully explained hereinafter the conditioned air flows from outlet means 34 to the area or space being conditioned details of the air distribution unit shall be more fully described hereinafter referring now to both fig1 and 2 there is schematically illustrated a control arrangement for air distribution unit 10 a portion of the conditioned air furnished to plenum chamber 12 flows to a filter 14 whereat any foreign bodies entrained therein will be removed the conditioned air passing through filter 14 is used for controlling the operation of unit 10 the conditioned air thereafter passes through at least one of the conduits 16 18 or 32 to several other elements of the control system for the unit the conditioned air flowing from conduit 18 is delivered to a lower chamber 41 of a pressure responsive valve 42 the air passing through conduit 16 flows through a pressure regulator device 20 conduit 24 and thence to parallel conduits 25 and 27 conduit 25 delivers the air stream to one chamber 50 of a three way valve 28 conduit 27 delivers the conditioned air stream to upper chamber 43 of differential valve 42 conditioned air passing through conduit 32 flows through a second regulator device 44 a conduit 46 and thence to a second chamber 48 of three way valve 28 depending upon the position of three way valve 28 either the pressure signal delivered through conduit 25 or the pressure signal delivered through conduit 46.