The GT-540P series integrates a recirculation pump to provide instant hot water in residential applications with dedicated recirculation lines. The pump is easily controlled with a simple user interface and multiple pump settings allow you to customize pump operation to meet your needs while maximizing heater efficiency. ENERGY STAR® qualified 15-year limited Heat Exchanger Warranty, 5-year parts warranty, 1-year labor warranty for Residential applications 5-year limited Heat Exchanger Warranty, 5-year parts warranty, 1-year labor warranty for Commercial applications Integrated Recirculation Pump - 1/2" pipe - 100'; 3/4" pipe - 500' Primary heat exchanger is constructed of a commercial gradecopper that is more resilient to erosion. Secondary heat exchanger is made of type 316L stainless steel to protect against corrosion. Complies with SCAQMD 1146.2 Ultra-Low NOx regulations of less than 14 ng/J (<20ppm) Easy-Link with up to three other 540H units to increase flow capacity Common Vent up to 8 units Condensing Indoor model Safety features include Air-Fuel ratio (AFR) sensor, freeze protection, overheat cut-off fuse, exhaust & water temperature safety control, and troubleshooting diagnostic codes