Dunn’s Dispatch: Flashover Defined

A flashover is the most dangerous time of a fire, the point at which a room bursts into flame. Technically, a flashover is caused by the radiation feedback of heat. Heat from the growing fire is absorbed by the upper walls and contents of the room, heating up the combustible gases and furnishings to their individual auto-ignition temperature. This buildup of heat in the room triggers a flashover.

Flashover signals several major changes in a fire: It is the end of an effective search-and-rescue in a room, and means the death of any person trapped in the blazing room. It signals the end of using a portable extinguisher to extinguish the fire. After flashover, an attack hoseline is required. It signals the end of the fire’s growth stage and that the fire is fully developed, the second stage of combustion. Finally, flashover signals the change from content to a structure fire and the beginning of the collapse danger.

When operating at a fire, firefighters want to delay flashover inside a burning room to buy several minutes which may prove critical to the success of rescue operations and the firefight. For example, you may want to delay flashover to make a search-and-rescue of the burning room, or allow a firefighter to go above a fire to rescue a trapped victim. Or, you may want to delay flashover to gain several minutes when there is a delay in the placement of the first attack hoseline.

There are three ways to delay flashover. Venting windows of a burning room releases the build-up of heat in the room, slowing down flashover and improving visibility in a smoke-filled room. In certain cases, not venting the room is required; instead, closing the door to the burning room and not venting the room starves the fire of oxygen, slowing down the combustion rate and the build up of heat in the room. This may be done when there is a delay in stretching a hoseline and all persons are out of the burning room. The third possible way of delaying flashover involves the discharge of a portable extinguisher, which can reduce the heat in a burning room temporarily and delay flashover.

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Deputy Chief Dunn (Ret., Fire Department of New York) is the author of a number of textbooks, including the new Strategy of Firefighting (Fire Engineering, 2007), Collapse of Burning Buildings (Fire Engineering, 1988), Safety and Survival on the Fireground (Fire Engineering, 1992), and Command and Control of Fires and Emergencies (Fire Engineering, 1999).