... stainless steel, *Outer tank: Color steel or stainless steel *Copper coil: diameter 12mm,thickness 1mm length 30-45m *Heat Preservation: ... . Working principle of pre-heated solar water heaterUse the copper coil as the heat exchanger, according to international fashion trend, ... Water The water come from the copper coil, so it is clean and drinkable.One copper coil is installed inside the tank ...