The Ubbink 224078PP is a 90-degree elbow designed for Rinnai condensing tankless water heaters (specifically RUC and RU models) with a 3-inch inner/5-inch outer (3"/5") concentric venting system. This component is designed for high-efficiency units that produce corrosive condensate, featuring a polypropylene inner pipe for exhaust and a white plastic outer pipe for air intake