A definition applied to liquid fuels such as home heating oil and diesel fuel, to distinguish them from more volatile fuels defined as flammable.
The characteristics which identify flammable and combustible liquids are spelled out in national fire codes. The difference between flammable and combustible liquids is largely established by flash point – the propensity of the liquids to give off ignitible vapors at varying temperatures. Combustible liquids have higher flash points than flammable liquids and, thus, are not as likely to emit ignitible vapors. Code requirements for handling combustible liquids are less stringent than those for flammable liquids.
National fire codes classify combustible liquids as Class II, Class IIIA, or Class IIIB liquids. By code definition, Class II liquids are those liquids that have a flash point at, or above, 100ºF and below 140ºF. Diesel fuel and kerosene are included in this group. Class IIIA liquids are those liquids that have a flash point at, or above, 140ºF and below 200ºF. Class IIIB liquids, such as waste oils, have flash points at, or above, 200ºF.
See also Flammable liquid.