An electrically conductive flat filament arrangement for a fluid permeable heater assembly for aerosol-generating systems includes a center portion and two side portions. The two side portions are on opposite sides of the center portion. The center portion defines a heating region of the filament arrangement and the side portions define electrical contact regions of the filament arrangement. The center portion and the two side portions each include a plurality of openings. Each plurality of openings defines an open area of the center portion and an open area of each of the two side portions. A percentage of the total area of the center portion including the open area of the center portion is greater than the percentage of the total area of one of the side portions including the open area of the side portion.