A filter assembly for mounting over an opening at an end of a forced air system duct, such as a cold air return. The filter assembly is designed to replace an existing louvered grill covering. The filter assembly is dimensioned which substantially retains the shape and appearance of the conventional duct grill being replaced. The filter assembly includes a filter frame having a perforated back wall, horizontal and vertical walls extending forwardly from the periphery of the perforated back wall defining an open front end and a filter receiving space between the perforated back wall and the open end. A rectangular grill having a peripheral flange extending rearwardly therefrom. The rectangular filter frame being received interiorly of the peripheral flange. A filter element being removably seated within the filter receiving space and being sandwiched between the perforated back wall and the grill causing the entire surface of the filter element to substantially contact the perforated back wall.