Flashover is a phenomenon that occurs in an enclosed area where the majority of the exposed surfaces are heated to their autoignition temperature and emit flammable gases, leading to the near-simultaneous ignition of most of the directly exposed combustible material in the area. Flashover normally occurs at 500 °C (932 °F) or 590 °C (1,100 °F) for ordinary combustibles, and an incident heat flux at floor level of 20 kilowatts per square meter (2.5 hp/sq ft) . Flashover is a thermally-driven event during which every combustible surface exposed to thermal radiation in a compartment or enclosed space rapidly and simultaneously ignites.
There are different types of flashover, including hot rich flashover, backdraft, and delayed flashover. Hot rich flashover is the rapid ignition of heated fire gases and smoke that have built up in a burning room. Once the upper area of the room is saturated with heat, it radiates downward to all combustibles in the space, causing the room’s contents to simultaneously reach the ignition point. Backdraft is the least common type of flashover that firefighters encounter, and it occurs when a fire is starved of oxygen and then suddenly re-introduced to it, causing a rapid increase in fire growth and development. Delayed flashover occurs when a fire starts in a back portion of a structure, such as a rear bedroom in a house, and the smoke and energy travel throughout the home, filling it with superheated gases. When firefighters enter the building, a hot rich flashover occurs in the rear bedroom, igniting the gases throughout the entire house.
Recognizing flashover conditions is critical for firefighters and can save lives. Factors that influence a flashover event include the location of the fire, size, volume, and shape of the compartment, fire growth rate, contents and their exposed surfaces, and compartment ventilation characteristics. Signs of an impending flashover include ambient temperatures quickly doubling and tripling as hoselines are advanced, large volumes of heavy dark smoke, rollover (active flame circulation in the thermal layer), and free-burning fire in a ventilation-deficient area.
