fibrous material the filtration panel 26 is held in place in the frame 12 by four elastic retention straps 31 32 33 34 extending diagonally across the four comers of the frame 12 with ends secured as by fasteners 36 to the side walls 16 17 18 19 of the frame 12 the frame 12 may be made of a aluminum cardboard stiff fibrous material or plastic and the grid 21 is preferable made of a relatively stiff plastic fig2 shows the filtration panel 26 held in place in the frame 12 against the grid 21 by the straps 31 32 33 34 the grid 21 may also be held in place in the frame 12 by the straps or it may be fastened to the rim 13 by a suitable adhesive the frame 12 and the grid 21 may be made of a plastic and the grid 21 may be secured to the frame 12 by fusion when positioned the filter element 11 is installed in the inlet of the air return duct not shown of a hvac system the filtration panel 26 is on the air intake side of the grid 21 and the return air tends to hold the filtration panel 26 against the grid 21 thus the upstream side of the filtration panel 26 requires no additional support other than the corner straps 31 32 33 34 which releasably hold the filtration panel.