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Non-pressure solar water heater is a kind of compact system of solar collector and water storage tank. The whole system relies on the different density of cold water and hot water, thus Non-pressure solar water heater also called thermal- siphon solar water heater. The system can work without the circulation pump and no heat pipe connection. The cold water directly filled into the vacuum tubes, the vacuum tubes gather energy and heat the cold water, the lighter hot water rise up into the water tank and cold water come down then be reheated, that how the water do circulation by itself
