A process for forming a double wall heat exchanger. A first ductile tube is placed inside of a second ductile tube. The first tube tightly fitting inside of the second tube. The first tube is the inner tube and the second tube is the outer tube. The combination of the outer tube and the inner tube is finned in a finning apparatus. A helical fin is pressure formed on the outer surface of the outer tube and simultaneously a small helical groove is formed on the inside surface of the outer tube which follows the path of the helical path of the helical fin. The internal pressure being applied to the inner tube causes the inner tube to expand and conform to the inside surface and diameter of the outer tube, with a continuous helical protrusion forming which mates with the internal helical groove of the outer tube, but not entirely filling the internal groove. A helical passageway between the inner and outer tubes is thereby formed. Preferably the inner and outer tubes are made of copper. Also preparably the inner and outer tubes, after being combined and before the rolling step, are annealed in a furnace. The production process provides reduced cost of manufacture, improved heat transfer and the safety feature required by the various state and local codes.