Suburban Mfg Universal Furnace Duct Collar. It is compatible with all Suburban furnaces except GT and DD models. It features a 4" opening. Features of Suburban Mfg 50715 Universal Furnace Duct Collar Designed for use with RV furnaces Compatible with all Suburban furnaces except GT and DD models 4" opening Includes one collar per package Package Contents: Suburban Mfg 50715 Universal Furnace Duct Collar Suburban Mfg 50715: Universal Furnace Duct Collar