The term 'Flue Residue' refers to the byproducts of combustion that are left behind in a chimney or flue. These residues include SOOT (4 letters), SMUT (4 letters), ASH (3 letters) and creosote (8 letters). SOOT is a black powdery or flaky substance made up of carbon particles, while SMUT is a black or gray powdery substance made up of fungal spores. ASH is the solid remains of combustion, and creosote is a tar-like substance that can form on the inside of a chimney or flue. All of these substances are considered flue residue and can be hazardous if not properly removed.