The colder water in the storage tank drops down to enter the base of the solar collector and is heated by the sun. As the heavier colder water in the storage tank continues to try to get to the lowest point of the system, the lighter hotter water is pushed up out of the solar collector and back into the top of the water storage tank replacing exactly the water leaving the bottom.
This circulation of water results in stratification of the water in the storage tank which means to say that there is a layer of hot water sitting on top of a layer of colder water – see the image above for typical convection currents in water in an insulated hot water cylinder. When the hot tap is turned on, preheated water from the top of the storage tank feeds through the boiler (where it is further heated or mixed with cold water if necessary) and comes out of the hot water tap. At the same time cold mains water enters the top of the water storage tank sinking quickly to the bottom and completing the cycle.