A system for heating liquid using solar radiation, includes a plurality of solar panels (1, 2, 3, 4, 5), at least one reservoir for heated liquid and pipes for circulating liquid between the respective solar panel and the at least one liquid reservoir, the liquid circulating by gravity circulation. The present system is characterized in that a non-return valve (17) for controlling the flow of heated liquid from the respective solar panel is placed in a portion of the circulation pipe between the upper end of the solar panel and the at least one liquid reservoir, the non-return valve (17) being adapted to open and close at a predetermined pressure in the liquid flow from the solar panel.