RBI’s FlexCore 1.5 is built for performance and longevity with symmetrical fire tube technology. It achieves 95.1% AHRI-certified efficiency and uses a precisely temperature-balanced flex core heat exchanger. It essentially scrubs every last bit of heat out of the combustion gases, resulting in a compact footprint while absorbing all expansion and contraction, with no stress on the tubes. Inside, each tube has been formed in a patented shape that is compressed and dimpled, such that very high gas side film coefficients are created.