Why is excess air so important?

High excess air causes lost efficiency, slag deposits and corrosion. 

As combustion air passes through a boiler, it carries a great deal of heat out of the stack. Thus the higher the excess air, the lower the efficiency and the higher the fuel costs. 

A rule of thumb governing excess air versus fuel costs is that for every 1% reduction in excess air a saving of 0,25 to 1% of fuel could be achieved.