Perfect combustion

Perfect balanced mixing of all components is referred to as stoichiometric combustion.

Complete combustion of a fuel is possible only in the presence of an adequate supply of oxygen. Oxygen (O₂) is one of the most common elements on earth making up 20.9% of our air.

Most of the 79% of air (that is not oxygen) is nitrogen, with traces of other elements.  Nitrogen is considered to be a temperature reducing dilutant that must be present to obtain the oxygen required for combustion.

Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the flue gases.  This reduces the heat available for transfer through the heat exchange surfaces.  It also increases the volume of combustion by-products, which then have to travel through the heat exchanger and up the stack faster to allow the introduction of additional fuel air mixture.

This nitrogen also can combine with oxygen (particularly at high flame temperatures) to produce oxides of nitrogen (NOx), which are toxic pollutants

Unfortunately combustion is not perfect so we get carbon monoxide (CO) and oxides of nitrogen (NOx). Notice that water (H2O) is a by-product of combustion.

There is almost a pound of water created for every pound of fuel
oil burned. This water can lead to significant problems if it
condenses anywhere in the system.

Sulphur is a component of most fuel oil and it is oxidized to sulphur dioxide (SO2) or sulphur trioxide (SO3). These sulphur products can combine with water to produce sulphuric acid.

Measurement of CO2 is the normal method to determine the efficiency of combustion. Perfect combustion of number 2 fuel oil produces 15.3% CO2 by volume of dry flue gasses.

The actual CO2 percentage will be somewhat less than this number. We recommend 10½% to 11½% for warm air furnaces and 11½% to 12½% for boilers. Instruments must be used for set up of all modern appliances.