This is a 3" stainless steel category II/III condensation drain pipe. It is used for vertical runs and can also replace a 90 degree elbow in some instances. It has gasketed connections for a seamless and easy installation and is UL approved.
With a properly sized Eccotemp tankless water heater, you will never run out of hot water and significantly reduce your energy consumption. Eccotemp tankless water heaters are the future of efficiency.