Mighty Therm, Tube Assembly, Outdoor Boiler (10 Tubes), 1010, Laars Code Compliant ASME Tube Assemblies, Built in accordance w/the requirements of the ASME Boiler and Pressure Vessel Code, Copper tubes and glass-lined tubs, ideal for hydronic or other closed loop, low oxygen content systems. Performs well w/normal water (7.5 to 17 grains hardness) in volume water applications.