Abstract: A nonwoven needlepunched polyester batt of single monolayer construction is cut to a dimension slightly larger or smaller than the dimension of an interior warm air register, and fitted into the plenum of the heating system by compression or by stretching, as the case may be. The batt is integral, having no pockets or chambers therein. The filter has no frame, as the nonwoven batt has sufficient rigidity in relation to its weight to remain in place in the plenum, and is kept against the air register by the flow of air through the duct system, thereby filtering air flow.