The ETi 400 ASME Natural Gas High Efficiency Heater features the TitanTough™ Direct-Fire Pure Titanium Heat Exchanger, the longest-lasting heat exchanger ever built. And it delivers 96% thermal efficiency, the highest of any pool heater.The ETi 400 heater provides excellent flexibility for installation, with fully sealed Category IV direct air venting that does not require large room openings for combustion air.