Replacement filter box, part number UFB908. The Universal Filter Box is designed to be installed on any add on furnace. Specifically, it covers the blowers and allows for easy connection to the existing HVAC return duct work or to newly installed ducting. It comes with an integrated slot for a 16-Inch by 20-Inch air filter. Made of durable Galvanized steel, the UFB908 will provide years of trouble free service. For units: 1300, 1537G, 1602, 1802, 24AF, 1400, 1500, 1557M, 1602M, 24A, and 30A.