Abstract: A filter assembly includes a main body configured to attach to an air duct and having an opening defining an air flow area dimensioned to match dimensions of the air duct. A track is disposed outside of the air flow area; and a filter is disposed in the track. A method of filtering air with a filter including a filter medium and a filter frame surrounding the filter medium includes installing the filter along a duct which defines an air flow area so that the filter frame is disposed entirely outside of the air flow area; and moving air through the duct along the air flow area and through the filter medium.