Abstract: A heating apparatus which uses solar energy to heat a solar energy working fluid moving through a solar collector is provided. The heating apparatus includes an outer tank and an inner tank completely contained within the outer tank. The inner tank is surrounded by the solar energy working fluid held by the outer tank. The wall of the inner tank acts as a heat exchanger wherein fluid from an outside source is contained in the inner tank and heated by the solar energy working fluid. The outer tank includes a resilient lid to enable the volume of the outer tank to be varied. When the temperature of the solar energy working fluid increases, the lid expands to prevent pressure increases within the outer tank. A transport line carries the solar energy working fluid to and from the outer tank. When portions of the transport line are to be drained, a section of the transport line provides an air path to assure proper drainage thereof.