When a fire is lit in a fireplace, oxygen from the air in the room reacts with the fuel and the fire burns. In the process, carbon dioxide and water are produced, together with carbon and volatile compounds of unburned or partially burnt fuel, which rise because they are lighter than the surrounding air.

These gases then rise up the chimney and are released into the atmosphere. As these substances pass through the chimney, a material, commonly referred to as soot, begins to accumulate or condense on the colder walls. Soot thus includes flammable substances such as creosotes and carbon.

These flammable deposits have the potential to ignite and are inevitably the cause of chimney fires. In the event of a chimney fire, it is possible for the chimney breasts to become sufficiently hot as to scorch wallpaper or, in the worst cases, to set fire to flooring attached to the structure.

Over time, these deposits may also obstruct, and possibly eventually completely block, the chimney.Such an obstruction can lead to an inefficient, and potentially unsafe, fireplace. This can be explained by the fact that a well-functioning chimney is necessary not only to allow potentially harmful gases to escape from the room, but also to ensure that the fire burns efficiently. As hot gases ascend in the chimney, the fire draws in more air from the room through the grate, in order to fill the space that has been left.

Without this continuous and copious supply of air the fire may produce carbon monoxide, a poisonous gas, and in extreme circumstances, may go out. Thus, a clean chimney is also a safer, more efficient chimney.