Conversion Rails For 16” x 25” and 20” x 25” Cabinets. This frame kit converts NON-Aprilaire units that use either 16 x 25 or 20 x 25 cardboard boxed filters from using the cardboard box filter to using an Aprilaire 613 (for 16 x 25 units) or 813 (for 20 x 25 units) filter. Just snap the frame together and install it in the place of the old cardboard box filter. Now you can use the Aprilaire collapsible filter media inside your NON-Aprilaire unit. Some people install screws or pop rivets through the existing metal cabinet into the new Aprilaire frame to hold it perfectly centered in their old cabinet. This kit ONLY includes the metal conversion frame. You will need to buy the corresponding air filter media, which is shown below as a related product. For converting air cleaners by Carrier, General, Honeywell, Lennox, Trion, and many others.