This return air support box and plenum kit is for use with systems where the return air must come in underneath the furnace or air handler. The kit includes 8 feet of 24 inch by 8 inch sheet metal ductwork that can be custom trimmed to the correct height for most applications, and a support box that can hold a furnace or air handler up to 22 inches wide. The support box features an interior support bracket for maximum strength, and a 20 by 25 inch filter slot that can accommodate a one, four, or five inch thick media filter. This kit does not include a filter. Please click here to see our recommended filter. For return air box only, please see the Snappy SBD30 .