The new K2 fire-tube boiler from U.S. Boiler has a 95% AFUE and features a stainless-steel vertical firetube heat exchanger. The K2 is available in six sizes ranging from 85-270 MBH. A combi version also is available (155 MBH) and all models can be wall-hung and have a 5:1 modulation. The boiler’s dimpled, firetube heat exchanger provides lower pressure drop compared to a water-tube heat exchanger of similar capacity, U.S. Boiler states