For all firing processes, the energy supplied by the burners must match the requirements at any given moment so that the required temperature can be achieved for the process. The capacity changes depending on the furnace chamber and material temperature parameters.

The air control valve is actuated when capacity is required. This results in an increase in the air pressure on the burner. The air/gas ratio control regulates the gas pressure proportionate to the air pressure so that the gas/air mixture always remains at a constant ratio. With modulating control, the burner is operated at any capacity point between low-fire capacity and rated capacity.

The low-fire and rated capacity operating points are defined by the end positions of the air control valve.