Abstract: A nonpressurized solar heating system includes a solar collector array through which a heat transfer fluid flows, a conduit for delivering fluid from the solar collector array to a heat exchanger, another conduit for carrying the fluid from the heat exchanger to a fluid expansion tank, and a pump for pumping fluid from the tank to the solar collector array. Fluid from the heat exchanger is received by the tank through an inlet located near the bottom thereof, and fluid is delivered to the pump through an outlet also located near the bottom of the tank. A closable opening is located near the top of the tank for introducing fluid thereinto. The heat transfer fluid is placed in the tank at substantially zero pressure and is then pumped to the other parts of the system. The fluid has a boiling point of about 550.degree. F. at atmospheric pressure, a boiling point exceeding about 400.degree. F. at about 30 feet of liquid vacuum, and a freezing point below about -100.degree. F.