A high efficiency heat exchanger is shown which can be incorporated into a finned tube water heater. The heat exchanger has a pair of flow manifolds each having a water inlet and a water outlet and a series of connecting openings. Circular flow tubes have connecting ends which fit within the connecting openings of the manifold so that the tubes are arranged in a stacked fashion to form a tube bundle. When incorporated into a water heater, a burner communicates with the interior space within the stacked tube bundle for producing products of combustion for heating water flowing in the flow tubes. The flow tubes have external fins which are crushed to form upper and lower flat stacking surfaces for stacking the tubes to form the tube bundle. The fins are also crushed to form angled baffled surfaces about a external periphery of the tubes. The baffle surfaces serve to retain heat from the products of combustion of the burner which are released into the interior space within the stacked tube bundle. A method of corrosion protecting the heat exchanger is also shown.