Abstract: A double wall heat exchanger for use in solar heaters and the like. An outer drawn copper tube of predetermined thickness and inside diameter is slid over a second drawn copper tube also of a predetermined thickness and outside diameter. A small axial groove, which is parallel to the longitudinal axis of the second or inner tube, is located in the surface of the inner tube and extends the entire length of the inner tube. The tubes are then placed in a furnace and annealed for a specified time and temperature. The double wall tubes are placed in a finning machine. Under a specified pressure and at a specified feed rate, integral fins are formed on the outside tube. While the fins are being formed on the outside tube, internal pressure is being applied forcing the inner tube to expand and conform to the inside diameter of the outer tube. The mating surfaces form a helical passageway which serves as part of the path of leakage.