Condensing models feature the highest efficiency and utilize PVC venting. All residential models are Energy Star qualified and eligible for a $1,500 federal tax credit. The primary heat exchanger is constructed of HRS 35 copper, which is stronger than standard copper and more resilient against erosion. Also providing protection against corrosion is a secondary heat exchanger made of type 316L stainless steel. The heater has low nox emissions and complies with the lead plumbing law. The unit’s safety features include an air-fuel ration sensor, exhaust and water temperature safety control and an overheat cutoff fuse. When it comes to intake and exhaust from the unit, the ATI-520H-N uses inexpensive PVC, CPVC or ABS pipe, with a 4-inch pipe venting up to 50 equivalent feet and a three-inch pipe venting up to 25 equivalent feet.