Abstract: A method and apparatus for modulating the surface area of a radiant infrared burner over which combustion occurs as a function of burner heat output rate. This modulation serves to maintain the heat flux density on the surface of the burner at or near an optimum value over a range of heat output rates. The modulation is effected by means of a piston disposed in the internal cavity of a burner which blocks the passage of combustible gas to areas of the burner downstream of the piston from the gas inlet to the burner. The piston is displaced further downstream with increased gas flow rate resulting in combustion occuring over an increased area of the burner surface as burner heat output rate is increased. In its simplest embodiment, the invention comprises a burner oriented vertically with the combustible gas entering the bottom of the burner and a free piston which floats on the incoming gas stream at a level within the burner which is proportional to the gas flow rate.