A special heat transfer tube and a heat exchanger constructed of tubes have been developed. The heat exchanger is a 30000 kcal/h (34900 W) rating water heater for residential heating and hot water supply. The tube is a concentric double tube constructed from an outer tube, whose diameter is 30 mm, and an inner tube, which is thermally contacted with the inner surface of the outer tube and has an elliptical cross-section shape. Water for hot water supply and for residential heating flows in two paths, respectively constituted from an inner tube and an outer tube. The water is heated by combustion gases of kerosene. The heat exchanger core is 300 mm long, 300 mm wide and 100 mm high. The following results were obtained:

(1) Overall heat exchanger thermal efficiency was 85%.
(2) Stagnant water temperature in one of the two paths is low.