Abstract: A heating boiler having a vertical, completely premixing burner tube arranged in a space shut off at the top includes concentrically relative to the burner tube a helical water tube surrounded by an upwardly extending flue gas discharge jacket. The burner tube comprises a lower circumferentially closed tubular mixing portion and an upper portion having burner ports formed by a plurality of stacked alternately flat and radially corrugated annular discs held together by a flat top plate connected through a bolt or the like to a cross piece or the like wedged in an internally tapered upper end of the circumferentially closed tubular mixing portion.