Soot
Soot is a fine powder consisting primarily of carbon produced by the combustion process. Because of its extreme light weight, it frequently rises with the smoke from the fire and coats the interior walls of the chimney and flue. Although the heat loss from the insulating effect of a soot layer is small (generally under 6 percent), it can cause a considerable rise in the stack temperature. Soot accumulation can also clog the flues, thereby reducing the draft and resulting in improper combustion. Soot may be blasted loose from the walls of the chimney or flue with a jet of compressed air, or it may be sucked out with a vacuum cleaner. Another method is to use a brush to remove the accumulated soot layer from the walls.
Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers