A method directs fuel-oxidant premix into a reaction zone through a first total premix inlet flow area, and causes the premix to combust and form a stable flame projecting into a process chamber through an outlet from the reaction zone. At a time when the process chamber has a temperature at or above an auto-ignition temperature of the fuel, the flame is blown off to initiate diffuse combustion in the process chamber without a stable flame. The flame is blown off by directing the premix into the reaction zone through a second total premix inlet flow area greater than the first total premix inlet flow area.