Abstract: A solar water heater comprises a unitary housing structure providing a thermally insulated storage compartment and a heating, or collector, compartment of substantially smaller volume than the storage compartment. The two compartments are interconnected by first and second liquid flow passages extending through an insulating wall portion, forming a thermosiphon responsive to absorption of solar radiation by the collector; one of the liquid flow passages extends between an inlet communicating with the storage compartment in the lower portion thereof and an outlet communicating with the heating compartment in the lower portion thereof, the passage providing a first duct portion extending upwardly within the insulation of the frontal wall portion from the inlet, a second duct portion extending upwardly within the insulation of the frontal wall portion from the outlet, and an intermediate duct portion interconnecting the first and second duct portions.