... 4.Professional OEM Vacuum tube solar water heaterPrinciple: Compact Non-pressure Solar Water Heater relies on desity of the water, cold water down, hot water up, a cosmical physics ... .6mm, 304-2B(food-level stainless steel)Material of outer water tank: 0.31mm, 0.4mm, 0.5mm stainless steel,or colorful armor plate, or ...