Solar hot water systems are commonly used for domestic hot water heating. Some systems are used for hydronic space heating.
Their are two types or systems drain back, and non drain back systems. Drain back systems
How it works. A solar collector panel panel has a solar collector plate with a metal tub attached to the backside of the plate. Fluid is pumped threw this tub extracting heat from collector plate. Drain back systems allow the water to drain back to the hot water tank at night. The water drains from the panel at night to prevent freezing damage from occurring. Drinking water can be pumped threw the panel. So it can be directly heated. Drain back solar hot water should not be used in very cold climate because of possible freeze damage.
Non drain back systems typically use antifreeze to help prevent freeze damage. But antifreeze is toxic. So the antifreeze is pumped threw the solar panel then passes threw a double wall heat exchanger. The reason for the double wall heat exchanger is to prevent the antifreeze from contaminating the drinking water. Water passes threw the other side of the heat exchanger. The heated water is then pumped into a storage tank or the hot water heater. The storage tank can act as a pre heater for the hot water heater. The pump that circulates the antifreeze threw the solar panel is controlled by a differential temperature controller. The controller uses two temperature sensors, one in the solar panel, and the second in the water tank. When the solar panels temperature has risen above the water tanks temperature the circulation pump is then turned on. The pump is turned off when temperature of the solar panel has only slightly warmer than the storage tanks temperature