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 25 inch by 10 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 26 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.