Heating Solar water heaters use the sun to heat either water or a heat-transfer fluid in collectors. That water is then stored for use as needed, with a conventional system providing any necessary additional heating. A typical system will reduce the need for conventional water heating by about two-thirds, minimizing the cost of electricity or fossil fuel and the environmental impacts associated with their use. Systems Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors.
Direct systems use pumps to circulate water through the collectors.
These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water. Indirect systems pump heat-transfer fluids such as a mixture of glycol and water antifreeze through collectors. Heat exchangers transfer the heat from the fluid to the water stored in the tanks. Because the water in the collector loop drains into a reservoir tank when the pumps stop, this is still a good system for colder climates. Passive Systems Systems that move water or heat-transfer fluid through the system without pumps are called passive systems.