KN-Series Condensing Cast Iron Boilers from Advanced Thermal Hydronics (Hydrotherm) are the one and only high efficiency cast iron boiler ever made and offer an industry leading 25 year heat exchanger warranty. KN-Series Boilers combine the efficiency of condensing, gas-fired technology with the strength, durability and heat retention of time-tested cast iron construction. From raw materials to the state-of-the-art digital control system, the complete line of KN-Series products is proudly manufactured at the ATH facility in Boyertown, Pennsylvania. Utilizing decades of foundry and manufacturing expertise, KN-Series boilers are cast, machined, wired and assembled to the tightest standards possible resulting in a finished product that is all-American in terms of innovation, reliability and craftsmanship. Models available from 200,000 - 4,000,000 BTU/h.