The present invention relates to a solar water heating system operating in a closed circuit and making use of a change of phase in a working fluid as a heat transfer medium, comprising: at least one collector to be exposed to sunlight for transmitting radiant solar energy to a working fluid and thus converting the fluid into a liquid-vapor mixture; a hot water storage reservoir connected to a heat exchanger; a pump for returning the working fluid to the collector; and a pipe means connecting all of the above mentioned components. The system according to the subject invention is characterized in that it comprises a separator connected to both ends of the collector; a pipe for returning the liquid coming from the liquid-vapor mixture to the lower end of the absorber of the collector in such a manner that it is constantly wet; and two pipes connecting the separator to the exchanger, one for circulating the vapor to the exchanger where it condenses and the other for allowing return of the condensed liquid to the separator, by means of the pump; the exchanger being of the type that allows free gravity flowing of the condensed liquid to the pump; and the system is further characterized in that the pump is placed below the mean level of the liquid in the exchanger and in that it ensures the return of the condensed liquid accumulated at the bottom of the exchanger to the separator.