High efficiency, wet-base, downfired multi-pass water heater

A fuel-fired water heater comprising:

• a tank for holding water to be heated, said tank having upper and lower end walls;
• an upper plenum structure positioned above said upper end wall;
• a lower plenum structure positioned below said lower end wall;
• a heat exchanger disposed within said tank, said heat exchanger including:
• a vertical first pass flue pipe centrally extending through the interior of said tank between said upper and lower end walls, said first pass flue pipe having a dividing structure therein which separates it into upper and lower longitudinal portions, said first pass flue pipe further having a lower end communicated with the interior of said lower plenum structure;
• a circumferentially spaced plurality of vertical second pass flue pipes outwardly circumscribing said first pass flue pipe and intercommunicating the interiors of said upper plenum structure and said upper longitudinal portion of said first pass flue pipe;
• and
• a circumferentially spaced plurality of vertical third pass flue pipes outwardly circumscribing said first pass flue pipe and intercommunicating the interiors of said upper plenum structure and said lower longitudinal portion of said first pass flue pipe;
• and
• a power burner operative to force hot combustion gas sequentially through said upper longitudinal portion of said first pass flue pipe, said second pass flue pipes, said upper plenum structure, said third pass flue pipes, said lower longitudinal portion of said first pass flue pipe, and then into said lower plenum structure.