Abstract: An economizer has a single sliding door which leads to the indoor section from both the outdoor air inlet and the return air inlet, with the two inlets being separated by a divider baffle. The sliding plate is movable from its fully closed position, wherein the indoor section is shut off from the outdoor air inlet, to a fully opened position, wherein all of the air passing to the indoor section is coming from the outdoor air inlet and the opening between the indoor section and the return air inlet is closed. The sliding plate can also be placed in intermediate positions wherein a mixture of return air and outdoor air is provided to the indoor section. The economizer is so designed that it may be applied to either a down discharge or a side discharge unit without any change to the economizer structure itself.