This return air support box is for use with systems where the return air must come in underneath the furnace or air handler. The support box 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. The filter rack can be positioned on either the front or the back of the unit. This gives you the flexibility of having your return plenum on either the right or left hand side. This kit does not include a filter.