Rheem/Protech RXHF-21 - External Filter Rack - 21 in. - Rheem/Ruud/Protech - A complete line of OEM and factory replacement parts for your furnace; air conditoner; water heater; air handler; boiler; rooftop unit and more. Technical Data Typical Application: Upflow - Electric Insulation: Yes Duct Configuration: Through Air Opening Inlet Dimensions Width (in.): 19-1/4 Air Opening Inlet Dimensions Length (in.): 19-1/2 Air Opening Outlet Dimensions Width (in.): 19-1/4 Air Opening Outlet Dimensions Length (in.): 19-1/2 Housing Overall (Outside) Dimensions Width (in.): 21 Housing Overall (Outside) Dimensions Length (in.): 21 Housing Overall (Outside) Dimensions Height (in.): 3-5/8 Door Type: Hinged Door Opening On: 21 Side Number of Filters per Unit: 1 Replacement Media Thickness (in.): 1 or 2 Replacement Media Dimensions - Nominal (in.): 20 x 20 Replacement Media Dimensions - Actual (in.): 19-9/16 x 19-9/16