Abstract: The heat exchanger is formed by wrapping a metal sheet about a water storage tank to define an intermediate cylinder and positioning an outer cylinder over the intermediate cylinder in sealed relation therewith to provide a relatively wide annular space about the water tank. Fluid heated by solar energy is circulated through this annular space to heat water in the tank by heat exchange through the intermediate cylinder and tank itself. A very narrow annular space is defined between the intermediate cylinder and exterior surface of the tank such that any leaks that may develop in the intermediate cylinder will result in the heated fluid passing down this annular space to the exterior so that the risk of contamination of the heated fluid with water in the tank is minimized.