... work on principal of difference between hot water and cold water. When water in vacuum tubes is heating, it become ... Optional: heat exchanger (red copper), electric heater (1.5kw, 2kw, 2.5kw, 3kw), assistant tank( 5L, 8L, 12L, 15L), magnesium ... water automatic, and ensure the outlet water as stable temperature.3. Simple system, easy installation and low price. compact non-pressure solar water heater ...