Mighty Therm, Tube Assembly, Indoor Boiler (8 Tubes), 715, Laars Code Compliant ASME Tube Assemblies, Built in accordance w/the requirements of the ASME Boiler and Pressure Vessel Code, Copper tubes w/bronze tubs, a non-ferrous solution for systems w/higher oxygen content. Especially suited for volume water applications w/normal water hardness (7.5 to 17 grains hardness).