Abstract: A low profile, high volume window fan has a large air filter mounted horizontally therein. The fan is enclosed in a rectangular box-like frame having a sloped top cover. The front section includes a squirrel cage blower, a front louvered section for directing the flow of air into the room, and a horizontally opening door for controlling the flow of air through the fan from either the outside air or the inside air, and for the insertion of the filter therein. The bottom surface includes an external opening for outside air. A double plenum is provided within the frame with the filter dividing the two plenums. When the door is closed, outside air is forced to flow into the second plenum, being in the bottom of the frame and under the filter and through the filter. When the door is opened, interior air is then filtered and circulated.