Abstract: There is disclosed a gas-fired water heater/boiler apparatus with a unique burner assembly that provides high levels of BTU/hour input making it suitable for commercial installations. The gas burner includes a pair of superimposed tubes, each having evenly distributed perforations of a different uniform size, that are rolled flush together and provide a thick walled burner with greatly increased strength and resistance to premature failure while furnishing an optimum flame pattern. The gas burner projects into the interior of a vertical, cylindrical array of finned heat enchanger tubes through which the fluid to be heated is circulated. The water heater/boiler apparatus is compact and thermally insulated by a ressurized forehearth and may be installed on combustible floors or in closets with zero clearance.