Unburned carbon and sulfur in the fuel burned in oil furnaces, often in the form of black smoke filled with particles, is called soot. Accumulating in venting flues and chimneys
If you see soot on top of your furnace or black stains on the walls of ceilings around the furnace, and on the roof, a more serious soot problem may be developing.
Sulfur residues can cause cracking and eroding in interior metal venting flue and its welded joints, resulting in soot leaking into your facility and blowing in the environment.
Yearly flue cleaning in order to prevent the interior steel flue from not wearing down , & blowing soot away in the environment
Soot layers too thick in a boiler or furnace could be unsafe, even a fire hazard , waster energy , and increase the temperature