a method for optimizing combustion air flow to a furnace . as the air / fuel ratio changes or is periodically perturbated , the corresponding change in heat loss to the stack due to the change in the amount of combustibles in the flue gases is maintained equal to the change in heat loss to the stack due to changes in the amount of excess air in the flue gases . the control is carried out by continuously modifying the air / fuel ratio in the appropriate direction to maintain that equality .