An air filtering device for indicating when to replace the internal filter of an HVAC system. The air filtering device includes a frame having a plurality of sidewalls each having an upper end and a lower end. Each sidewall contains a plurality of slits that cycle air therethrough. A filter member is affixed to the upper end of the plurality of sidewalls, wherein the upper end containing the filter member is closed and the lower end is opened. Magnetic fasteners are affixed to the lower end of the plurality of sidewalls, wherein the magnetic fasteners removably secure to an outer perimeter of a return vent, such that the filter member overlays the grill of the return vent. In an alternate embodiment, a magnetic support member is affixed to a lower surface of the filter member at an equidistance from opposing sidewalls to prevent the filter member from touching the grill.