What do flue gases consist of?

Flue gases are primarily the result of the combustion process in a heating system. They consist of the products formed when fuel is burned, typically including carbon dioxide, water vapor, and various other components like nitrogen oxides and particulate matter. Additionally, during combustion, not all air used is completely consumed; therefore, some excess air that was not reacted also enters the flue gas stream.

Combining these two aspects, flue gases include both the products of combustion and any excess air that may have been present during the reaction. This understanding is crucial in contexts such as efficiency of combustion systems, emissions monitoring, and ensuring proper venting to prevent hazardous situations.

The other options do not capture the full scope of what constitutes flue gases. Focusing solely on products of combustion disregards the important role that any excess air plays in the combustion process. Similarly, limiting flue gases to only excess air or gases from ventilation fails to recognize their primary composition.