Honeywell CF2200A1005 4-Inch High-Efficiency Collapsible Air Cleaner Filter MERV 10 rating-up to 20x more efficient than typical 1-Inch fiber glass filter Fits Aprilaire/Space-gard model 2200 Installation is simple with custom pleat holding string 20 x 24 x 4.5-Inch ONLY works with Aprilaire or Space Guard 2200/2250 (CF2200)