If the furnace burners operate with a deficiency of air, or the fuel and the air are mixed improperly, then the fuel supplied to the burners will be not burned completely, and in result combustibles such as CO will appear in the products of combustion. To ensure that the fuel is burned with little or no combustibles, some amount of excess air must be provided. To maintain the highest combustion efficiency level with minimum unburned fuel loss and minimum flue gas heat loss on a continuous basis, both oxygen and CO must be measured. From these measurements the supply of excess air can be controlled on a continuous basis, minimizing heat loss and unburned fuel loss, and therefore ensuring safe, optimal and efficient operation of the furnace, whilst minimizing consumption of the fuel and CO2/NOx emission.
In general, furnace operation at 2 % of excess oxygen in the flue gas higher than optimal results in 1-2 % fuel loss.