Single pass high-efficiency condensing water heater

A water heater comprising:

• a water storage tank;
• a combustion chamber;
• and
• a heat exchanger configured to receive combustion gases from the combustion chamber, such that heat is transferred from the combustion gases to water stored in the water storage tank, the heat exchanger comprising:
• an upstream section that includes a flue tube;
• and
• a medial section that includes an inner wall and an outer wall that opposes the inner wall, the outer wall defining an upper heat exchange surface of the medial section and an underside opposite the upper heat exchange surface, wherein a plurality of frames extend from the underside of the outer wall toward the inner wall and form concentric rings, the plurality of frames comprising:
• a plurality of upper members, each extending from the underside of the outer wall and arranged concentrically relative to each other;
• and
• a lower member extending from the underside of the outer wall and arranged as the outermost concentric frame with respect to the plurality of upper members, the lower member including a bottom flange, wherein the bottom flange of the lower member is welded to the inner wall of the medial section.