KN Series by Advanced Thermal Hydronics are the only high-efficiency, cast iron boilers available for the commercial marketplace that combine the efficiency of condensing, gas-fired technology with the strength, durability and heat retention of cast iron construction. Our Story The legend behind the KN Series boilers is a simple one: our engineers were challenged to design a product that reflected the requirements of today's commercial boiler industry high efficiency, full-modulation and a small footprint. Add to that the desire to reduce the common problems associated with boiler installation and maintenance, as well as the ambition to tackle the challenges of environmental variability, and the KN Series line of condensing gas boilers was born. The twist? KN Series boilers aren't your conventional commercial boiler. While they have all the benefits of modern technology, our boilers are made with a historical favorite cast iron. Durable, rugged and long-lasting, cast iron is the ideal material for condensing boiler systems that are meant to last the long haul.