Use copper coil, the water can be heated rapidly. It can produce hot water with constant temperature, more comfortable to use. Pre-heated solar water heaterDetails of material1. Vacuum tube: OB-AL-N/AL-58mm*2000mm*58mm*1800mm2. Inner tank: stainless steel SUS304-23. Outer tank: Zinc plated board4. Bracket: coated galvanized steel sheet (2 Types Types 1 as per picture below, type 2: to fit on a ...