To ensure complete combustion of the fuel used, combustion chambers are supplied with excess air. Excess air increase the amount of oxygen and the probability of combustion of all fuel.

• when fuel and oxygen in the air are in perfectly balance - the combustion is said to be stoichiometric

The combustion efficiency will increase with increased excess air, until the heat loss in the excess air is larger than than the heat provided by more efficient combustion.
Typical excess air to achieve highest efficiency for different fuels are

• 5 - 10% for natural gas
• 5 - 20% for fuel oil
• 15 - 60% for coal

Carbon dioxide - CO₂ - is a product of the combustion and the content in the flue gas is an important indication of the combustion efficiency.
An optimal content of carbon dioxide - CO₂ - after combustion is approximately 10% for natural gas and approximately 13% for lighter oils.
Normal combustion efficiencies for natural gas at different amounts of excess air and flue gas temperatures are indicated  below:
¹⁾ The "net stack temperature" is the difference between the flue gas inside the chimney and the room temperature outside the burner.

Flue Gas Loss Combustion Oil

The relationship between temperature difference flue gas and supply air, CO₂ concentration in the flue gas, and the efficiency loss in the flue gas combustion oil, is expressed in the diagram below.

Example - Heat Loss Flue Gas

If the temperature difference between the flue gas leaving a boiler and the ambient supply temperature is 300 ^oC and the carbon dioxide measured in the flue gas is 10% - then, from the diagram above, the flue gas loss can be estimated to approximately 16%.