A.O. Smith continues to lead the way in residential and commercial water heating with close to 70 years of innovative hot water solutions. The ATI-540HX3-P condensing gas tankless water heater features X3 Scale Prevention Technology to last three times longer than other tankless models without X3 Technology. Complies with Ultra-Low NOx regulations and utilizes commercial-grade copper alloy for the heat exchanger tubing and are approved to work with an external recirculation pump and cross-over valve. Making this one of the most efficient heaters on the market. X3 Scale Prevention Technology With the patent pending technology, no annual descaling is required. With no scale buildup means the heater maintains like-new performance longer. Heavy Duty Heat Exchanger The primary heat exchanger is built out of copper, offering better heat transfer than stainless steel and designed to be more resilient against corrosion. A.O. Smith water heaters are built to last and save you money in the process. Multiple Safety Features A.O. Smith water heaters come with a long list of safety features standard. These include built-in freeze protection, overheat cutoff fuse, exhaust & water temperature safety control and an Air-Fuel Ratio (AFR) Sensor. Recirc Capable Tankless water heaters with X3 Technology are approved to work with an external recirculation pump and cross over valve. Ultra Low-NOx Emissions This A.O. Smith tankless water heater is designed to qualify for the most stringent Low-NOx standards. These standards coincide with SCAQMD rules and other air quality management districts. Product Includes Installation manual and owner's guide Bird screen X3 cartridge