Typical controllers have sensors for the temperatures in the collector and in the solar storage tank. Notice that the circulation loop through the pump and collector does not directly affect flow into and out of the solar storage tank through the other two pipes. Whether the pump is running or not, the water in the tank will be replenished by fresh cold water, whenever hot water is being used. Extra hot water may be needed during some evenings, and the sun doesn't shine every day, so most households will choose to have a backup gas water heater shown, in case the water flowing from the solar tank to the shower is not hot enough. A properly designed solar hot water system will greatly reduce your energy bills because the backup heater will only run occasionally.
Water expands when heated, so a gallon of cold water is heavier than a gallon of hot water.
Thus, cold water enters at the bottom of the tank and stays there, while the shower receives hot water from the top of the tank. During the day, the pump takes cold water from the bottom of the tank and returns solar hot water to the top, where it remains until needed. The circulation loop can also take advantage of the fact that cold water sinks below hot water, and hot water floats on top of cold water. The illustration of the thermosiphon system on the right shows how the controller and pump can both be eliminated, by placing the storage tank in the attic or on the roof. When the tank is above the level of the solar collector, the density difference between hot and cold water forces water to flow through the pipes and the collector.