Rheem IKONIC super high efficiency condensing gas tankless water heater delivers a 0.96 UEF, resulting in up to a 36% reduction in energy use and carbon footprint and up-to 65% reduction in NOx emissions, the company says. Also featuring a built-in recirculation pump (SR models), Rheem IKONIC offers homeowners no wait access to continuous hot water, preventing up-to 12,000 gallons of water from being wasted down the drain each year.