Flue Gases and the Serviceable Flue Gas Analyser

Flue gases are individual gases that are emitted to the atmosphere through specially manufactured metal, concrete, plastic or clay pipes or ducts, called a flue, which are specifically designed to allow the gases to safely pass from heating appliances such as a boilers, fireplaces, stoves, ovens and furnaces etc.

The specific content of flue gases depend on what particular type of fuel is used and how it is combusted will determine the constituents of the flue gases released. For example, a poor combustion process running at low temperatures with a poor air supply will invariably produce more pollutants than a combustion process with the correct combustion control factors of fuel delivery, ignition and air supplies etc. These are several reasons why a serviceable flue gas analyser is essential in realising the composition of flues gases within the combustion process of a heating appliance.

In essence, flue gases are the by-products of combustion and the combustion process determines their ultimate make up.

That said, flue gas or gases will primarily consist of uncombusted nitrogen taken from primary and/or secondary combustion air supplies, water vapour (H2O) created by the combustion of hydrogen in the fuel with oxygen (O2), also taken from primary and/or secondary air supplies, excess oxygen, again also taken from primary and/or secondary air supplies and carbon dioxide (CO2) which is the result of carbon from the fuel combined with oxygen.

Water Vapour

People often mistake the white plumes seen coming from flue terminals, as ‘smoke’ but is in fact water vapour condensing, as it comes into contact with cool air. These plumes are also more evident from heating appliances that use condensing heat exchangers, which recover more heat from the flue gases that would otherwise escape into the atmosphere. As of result of this action, the flue gases become a lot cooler and appear as water vapour in higher ambient temperatures when exiting the flue.

Flue gases will also contain smaller amounts of pollutants such as nitrogen oxides (NOx), sulphur oxides (SOx), carbon monoxide (CO) and particular matter derived from the substances contained in the fuel itself.

Nitrogen Oxides

Nitrogen oxides are highly toxic and are obtained from the nitrogen within the primary and/or secondary air supplies and from materials contained within the fuel. Heating appliances emitting significant amounts of nitrogen oxides will often cause irritation to the eyes.

Sulphur Oxides

The main source of sulphur oxides is the burning of fossil fuels. Sulphur oxides dissolves within water vapour in the air to form acids, and interacts with other gases and particles in the air to form particles known as sulphates and other products that can be harmful to humans and the environment.

Carbon Monoxide

Carbon monoxide is a tasteless, odourless, colourless and poisonous gas caused by incomplete burning of carbon-based fuels during a poor combustion process. Carbon monoxide accounts for most fatalities associated with flue gases from heating appliances.

The constituents of flue gases are within the control of the heating professional. As one of his or her primary responsibilities, the heating professional must be fully conversant with principles of the combustion process to overcome any potential problem associated with poor combustion performance of a heating appliance. Consequently, the use of a serviceable flue gas analyser is a fundamental measuring instrument for all who service, maintain and install fossil fuel heating appliances.