Down-fired U-tube water heater

A down-fired water heater has a top storage tank head portion with a central opening bordered by an upstanding annular flange. Welded within this flanged opening is a generally pan-shaped tube sheet structure having an open top side, and a bottom side wall in which a spaced pair of openings are formed. The open top ends of a U-tube immersion heating member, formed from a single length of metal tubing, are upwardly received and welded within these openings. During water heater operation, a top mounted shot-type fuel burner downwardly injects a flame, and resulting hot combustion gases, into one of the open upper ends of the U-tube. A top mounted draft inducer fan, with an inlet communicated with the interior of the tube sheet structure, draws the combustion gases through the U-tube and into the tube sheet interior via the other open tube end, and then discharges the combustion gases outwardly away from the water heater. The induced two-pass flow of combustion products through the tank interior very efficiently heats water stored therein and evenly distributes combustion heat along the height of the stored water. In an alternate embodiment thereof, the top head portion is upwardly domed, and its flanged opening and the tube sheet structure are eliminated. Openings are extended through a spaced pair of flattened areas on the top head portion, and the upper ends of the U-tube are inserted into and welded within such openings.