An inwardly firing burner design is shown which can be used in a variety of heating applications. A water heater apparatus is also shown which includes a water heating tank having generally cylindrical sidewalls with upper and lower ends, each of which is closed by a transverse wall section to define a closed tank interior. The tank also has a water inlet and a water outlet. A plurality of vertically arranged fire tubes are connected between the upper and lower transverse wall sections, each fire tube having an open interior for conducting products of combustion. A burner assembly is mounted on the tank which features the inwardly firing construction that eliminates the need for an external combustion chamber.