Abstract: An air duct has a rectilinear face plate with an internal face and an external face. The face plate has a rectangular, central opening with a height and width less than the lengths of the edges to form a solid periphery of between about 1.0 and 2.0 inches around the edges of the face plate. The central opening has a series of air directing grates creating a plurality of rectangular shaped slots. A rectilinear housing has two parallel long side walls and two parallel short end walls oriented perpendicular to the internal face and have external edges coupled to the interior face of the face plate adjacent to the opening. Each end plate contains two large bores and one small bore. A pair of similarly configured rectangular flappers, slightly wider than 50 percent of the height of the opening and slightly shorter than the width of the opening, have a formed cylindrical extension rotatably received within the large bores.