Flue buildup is a common problem in chimneys and other heating systems. It is caused by the accumulation of SOOT, which is a black or dark grey powdery or flaky substance made up of carbon particles. SOOT is created when fuels such as wood, coal, oil, and gas are burned incompletely. Over time, the SOOT builds up in the flue, reducing the efficiency of the heating system and potentially leading to dangerous levels of carbon monoxide.