Downfired high efficiency gas-fired water heater

A fuel-fired liquid heating apparatus comprising:

• a tank adapted to hold a quantity of liquid to be heated, said tank having upper and lower ends;
• a combustion products transfer chamber structure underlying said lower end of said tank, said combustion products transfer chamber structure being a single chamber having a top wall defined by and sized to match said lower end of said tank, said top wall being convexly shaped to be higher in a central region of said tank than at sides of said tank, said combustion products transfer chamber structure having a first sidewall;
• a combustion product discharge conduit disposed externally of said tank and having an inlet end communicating directly with an interior of said combustion products transfer chamber structure through an opening in said first sidewall, said combustion product discharge conduit having a vertically extending portion adjacent said tank;
• a combustion chamber extending downwardly into an interior of said tank through said upper end thereof, said combustion chamber being spaced upwardly apart from said lower end of said tank, said combustion chamber having a bottom end wall, at least one second sidewall, and a top end that define an interior space of said combustion chamber, said top end being disposed at substantially the same elevation as said upper end of the tank, and said combustion chamber disposed in said interior of said tank such that an exterior surface of said bottom end wall and at least a portion of said at least one second sidewall of said combustion chamber are in direct contact with said quantity of liquid to be heated;
• a single pass heat exchanger structure disposed in said interior of said tank and being defined by a plurality of flue pipes intercommunicating said combustion chamber and said combustion products transfer chamber structure, said flue pipes having inlets and outlets respectively connected directly to said bottom end wall of said combustion chamber and said combustion products transfer chamber structure, respectively, wherein said single pass heat exchanger is a condensing heat exchanger structure;
• a condensate drain fitting disposed on said combustion products discharge conduit at a location adjacent a bottom of said vertically extending portion of said combustion product discharge conduit at a location laterally displaced from said tank;
• and
• a fuel burner disposed at said top end of said combustion chamber and comprising air and fuel inlets at substantially the same elevation as the upper end of said tank, said fuel burner being operative to create hot combustion products within said interior space of said combustion chamber for transfer therefrom sequentially through said flue pipes, into said combustion products transfer chamber structure, and then outwardly from said combustion products chamber structure into said combustion products discharge conduit.