Like solar collectors, solar water heaters can be either passive or active. Passive systems have no moving parts, no external energy source except the sun, and rely on the basic principle of physics that hot water rises and cold water falls. Active systems utilize a mechanical circulating pump and some type of temperature control.
A simple passive solar water heater consists of a water tank that has been painted black and placed in a well-insulated box that has glass or plastic on one side. The sun’s rays penetrate the glass and heat the tank. This type of system is often called a “bread box” or batch heater. It allows cold water to flow in at the bottom and hot water to flow out from the top. The system operates on water pressure from the water supply. Water from the system is then transferred to a conventional gas or electric water heater. A thermostat on the water heater determines whether the water is hot enough to be used. If it is not, the conventional water heater adds the required heat.