Abstract: The nozzle comprises an axial chamber (23) provided in a first part (21) and into which the fuel gas is fed, and a chamber (24) which surrounds the chamber (23) and into which air is fed; the two chambers (23) and (24) are separated from the combustion chamber (4) by a front plate (5) in which three series of apertures are provided, namely a series of peripheral apertures (31) arranged to pass secondary air with a velocity having an axial and tangential component, a second series of apertures (32) disposed on a more inner circumference and arranged to pass primary air and also having an axial and possibly also tangential component, and a third series of more central apertures arranged to pass gas; by virtue of said apertures the burner thermal output is adjustable to the extent of enabling it to be varied within a very wide range of values.