Abstract: A method and apparatus is described to prevent freezing in the piping and tank of a solar water heating system mounted above a heated space. The supply and return piping to the solar water heater is used in conjunction with a heat exchanger mounted within the heated space below, to create a mechanism for the gentle circulation of water by natural convection, sufficient to prevent freezing in the tank and piping. Bypass pipes that incorporate flow restrictions are introduced between the supply and return piping both in the building and just below the solar tank. This creates a loop in the piping which allows natural convection to occur. The flow restrictions are provided so as not to short circuit the normal water flow under conditions of hot water demand.