Excess air in the boiler combustion leads to losses. Flue gas temperatures are a good indicator of excess air and should be monitored.
During combustion heat is released when carbon combines with oxygen to form carbon dioxide. Theoretically if the right amount of air is supplied, fuel will consume the entire oxygen.
In oil and gas fired boilers, due to practical limitations of the burner, some excess air needs to be supplied to ensure complete combustion. If this excess air is higher than required, it carries away useful heat to the atmosphere via the chimney.
By rule of thumb, the maximum acceptable stack gas temperature is 30°C over the steam temperature. Also every 20°C deviation from optimum flue gas temperatures increases the fuel consumption by 1%.
Fig: Boiler loss can be plotted as a function of excess air. (Top) The minimum point of the total loss curve of a boiler is where optimized operation is maintained. (Bottom) Most efficient operation of the boiler occurs when the amount of excess air in the stack balances the loss in unburned fuel.