Double Heat Exchanger Solar Water Heater Tank with Double Copper Coil, Relief Valve, Electrical Heater and Drain Valve Inner tank material: porcelain enamel or SUS304-2B stainless steel Outer tank: 0.5mm Thickness colored steel With double copper coil (Ø16mm) Insulation: polyurethane foam Relief valve Electrical heater with 1,500 or 2,500W