Early application is the most widely used solar water heating, millions around the world now have solar water heating devices. The main components of solar hot water collector systems, storage devices and the circulation line of three parts. In addition, there may be auxiliary energy devices (such as electric heaters, etc.) to supply without the use of sunlight, the other still may have forced circulation of water, or control of the power to control most of the water level or temperature control devices and receiving the load Road. By circulating solar water heating systems can be divided into two ways:
     1, natural cycle: this type of storage tank placed above the collector. Receiving water in the collector heating solar radiation, temperature rise, causing the collector and storage tank water temperature difference in the density difference, thus causing buoyancy, thermal siphon this is like, make the water tank and the collector in addition to the natural flow . Relationship with the density difference by the water flow in the collector is proportional to absorption of solar energy. Because without this type of recycled water to maintain very simple, it has been widely adopted.
      2, forced circulation type: hot water system with water so that the water in the collector and storage tank to cycle. When the collector temperature higher than the top of the bottom of the tank water temperature several degrees, the control unit will start the water make the water flow. Check valve to prevent the entrance of water at night with water from the collector currents, and cause heat loss. Hot water system by such types of flow can be learned (because we can see from the flow of water), predictable performance, can also be calculated in a number of hours of heating water. If the same design conditions, the more natural way to have access to high circulating water temperature strengths, but because the water must be used, so there is water, electricity, maintenance (such as water leakage, etc.) and stop-action control device, easy to damage the water and other problems. Therefore, in addition to large-scale high temperature hot water system or in need of the situation before choosing forced circulation type, generally most of the water heater with natural circulation.