A direct evaporative air handler for conditioning an enclosed space with supply air, the air handler comprising: a direct evaporative cooler; an outdoor air port positioned upstream of the evaporative cooler in an air flow direction, the outdoor air port including a damper that is configured to selectively allow outdoor air to enter the air handler; a return air port positioned upstream of the evaporative cooler in an air flow direction, the return air port including a damper that is configured to selectively allow recirculated air from the enclosed space to enter the air handler; wherein the air handler is configured so that outdoor air entering the air handler through the outdoor air port passes through the evaporative cooler, and the recirculated air entering the air handler through the return air port passes through the evaporative cooler, characterized in that the air handler furthermore comprises: an outdoor air bypass port including a damper that is configured to selectively allow outdoor air to enter the air handler at a position where the outdoor air entering the bypass port will not pass through the evaporative cooler; and a supply fan positioned downstream of the evaporative cooler, wherein the air handler is configured to: (i) operate in a mode in which the supply fan mixes air that has been drawn through the outdoor air bypass port with recirculated air that has been drawn through the return air port and passed through the evaporative cooler, and (ii) operate in another mode in which the supply fan mixes air that has been drawn through the outdoor air bypass port with outdoor air that has been drawn through the outdoor air port and passed through the evaporative cooler. a direct evaporative cooler; an outdoor air port positioned upstream of the evaporative cooler in an air flow direction, the outdoor air port including a damper that is configured to selectively allow outdoor air to enter the air handler; a return air port positioned upstream of the evaporative cooler in an air flow direction, the return air port including a damper that is configured to selectively allow recirculated air from the enclosed space to enter the air handler; wherein the air handler is configured so that outdoor air entering the air handler through the outdoor air port passes through the evaporative cooler, and the recirculated air entering the air handler through the return air port passes through the evaporative cooler, characterized in that the air handler furthermore comprises: an outdoor air bypass port including a damper that is configured to selectively allow outdoor air to enter the air handler at a position where the outdoor air entering the bypass port will not pass through the evaporative cooler; and a supply fan positioned downstream of the evaporative cooler, an outdoor air bypass port including a damper that is configured to selectively allow outdoor air to enter the air handler at a position where the outdoor air entering the bypass port will not pass through the evaporative cooler; and a supply fan positioned downstream of the evaporative cooler, wherein the air handler is configured to: (i) operate in a mode in which the supply fan mixes air that has been drawn through the outdoor air bypass port with recirculated air that has been drawn through the return air port and passed through the evaporative cooler, and (ii) operate in another mode in which the supply fan mixes air that has been drawn through the outdoor air bypass port with outdoor air that has been drawn through the outdoor air port and passed through the evaporative cooler. (i) operate in a mode in which the supply fan mixes air that has been drawn through the outdoor air bypass port with recirculated air that has been drawn through the return air port and passed through the evaporative cooler, and (ii) operate in another mode in which the supply fan mixes air that has been drawn through the outdoor air bypass port with outdoor air that has been drawn through the outdoor air port and passed through the evaporative cooler.