Flashover

By Vincent Dunn

Flashover! It is the most dangerous time of a fire. When the room bursts into flame, flashover has occurred. The scientific definition of flashover states it is caused by the radiation feedback of heat. Heat from the growing fire is absorbed into the upper walls and contents of the room, heating up the combustible gases and furnishings to their auto-ignition temperature. This build up of heat in the room triggers flashover.

Flashover Time-Temperature Curve

Temperature of Fire
	Time of Fire

Flashover signals several major changes in a fire:

It is the end of an effective search and rescue in a room; it means the death of any person trapped in the blazing room — either civilians or firefighters.
It signals the end of using a portable extinguisher to extinguish the fire; an attack hoseline is required after flashover occurs.
It signals the end of the growth stage and that the fire is in the second stage of combustion — the fully developed stage.
Finally, flashover signals the change from contents to a structure fire. This is the beginning of the collapse danger. Structural collapse potential starts in the fully developed stage and becomes the greatest in the decay stage of a fire — after the fire has been extinguished.

Flashover does not occur at every fire. Sometimes flashover does not take place. At some fires flashover does not occur for a long period of time. However, flashover may occur suddenly, without warning and just when firefighters arrive on the scene. Firefighting is dangerous enough; we do not want a surprise at a fire operation.

When operating at a fire, chiefs and firefighters may want to delay flashover inside a burning room. By delaying flashover, you can "buy" several minutes which may be critical. 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 make a rescue of a trapped victim. Or, you may want to delay flashover to gain several minutes when there is a delay in the placement and operation of the first attack hoseline.

Three Ways to Prevent Flashover

Venting: By venting doors and windows of a burning room you release the build up of heat in the room. This is most effective during the early part of the growth stage when a room is filled with smoke and not too much heat. This slows down flashover by releasing heat from the room delaying the radiation feedback effect. This venting also improves visibility for a search in a smoke-filled room.
Not venting: At some fires, the exact opposite may delay flashover. By not venting and, instead, closing the doors or windows to the burning room, you may delay flashover. This is most effective when the room is full of smoke and large amounts of heat, and the fire is at the end of the growth stage, shown in the above time temperature curve, just before flashover is about to occur. The logic behind the tactic of not venting is flashover is delayed in a superheated room by starving the fire of oxygen, which slows down the combustion rate, which, in turn, slows down the build-up of heat in the room. This slows down the radiation feedback effect. The combustion rate a fire depends on many factors, but the greatest effect on a fire is the amount of oxygen being supplied to the blaze. An example of when you might close a door and not vent a fire to delay flashover would be when there is a delay in stretching a hoseline, all persons are out of the burning room, and it is too hot to enter for a search and rescue.
Portable extinguisher: The discharge of a portable extinguisher can cool the heat down in a burning room temporarily and delay flashover.

To avoid getting trapped by flashover, firefighters must know the warning signs of flashover.

Warning Signs of Flashover

There are two warning signs which may signal the danger of flashover: heat mixed with smoke and "rollover."

Heat: When heat mixes with smoke, it forces a firefighter to crouch down on hands and knees to enter a room to perform search and rescue. This must be considered a warning sign that flashover may occur. Heat buildup in a smoke-filled, burning room is a triggering event for flashover. If the heat in the smoke-filled room causes us to crouch down near the floor, we must consider the danger of flashover. If there is little or no heat mixed with smoke then the danger of flashover is less severe.
Rollover: Rollover is fire-department jargon for the appearance of sporadic flashes of flame mixed with smoke at ceiling level. It is usually seen outside a burning room when the first attack-hose team waits for water to be supplied to the hoseline. When the door to the burning room is partially opened and smoke is flowing out into the hallway, the smoke may ignite into sporadic flame. Rollover is caused by heated combustible gases in smoke, which ignites into flashes of flame when mixed with oxygen in the air. Rollover precedes flashover. Rollover is another warning sign of flashover, which may be seen in the smoke coming out of the tops of doorways or window openings of burning rooms before flashover occurs.

Whenever one of these warning signs is seen and a flashover danger exists, defensive search procedures must be used by firefighters. Standard tactics and procedures must be curtailed and defensive search-and-rescue procedures substituted when there is a danger of flashover.

Defensive Search Procedures

There are two defensive search procedures that can reduce the risk of death and injury from flashover:

At a doorway: A firefighter should check behind the door for the victim, then enter the hallway or room not more than five feet, sweep the floor, look for unconscious persons, call out and listen for a response. If no response is forthcoming, close the door and wait for the hoseline. As the attack hoseline advances, conduct a search and rescue behind the line, searching room and space outward from the hoseline.
At a window: When a window breaks from either the heat of the fire or because it is opened by the firefighters and superheated smoke and rollover are seen in the smoke, preventing a firefighter from entering, the firefighter should crouch down below the heat and sweep the area below the windowsill with a tool. In some instances a person may collapse at the window and fall right below the sill. If a victim is found, a firefighter might be able to crouch below the heated smoke and flashes of flames coming out the window and pull the victim out of the window to safety.

After a flashover occurs, the point of no return is reached, a point beyond which a trapped firefighter will not survive and will not reach the door or window he or she entered. How far inside a burning room can a firefighter be and still escape back out the door alive — and not suffer serious bums after a flashover occurs? The million-dollar question is: how far into a superheated burning room that appears about to flashover should a firefighter enter?

Beyond five feet is the point of no return. We can figure this distance out by putting together several facts. For example, tests conducted in 1960 in California discovered that fire temperatures of 280º-320º F cause intense pain and damage to exposed skin. Also the average temperature in a room that flashes over is 1000º-1500º F. And, time and motion tests in the Handbook of Fire Protection reveal that the average person moves 2½ feet per second when walking. Now, the question is: how long can a firefighter’s protective clothing take 1000º-1500º temperatures before the firefighter receives serious burns. If there is 1000º flame in a burning room that has just flashed-over and a firefighter is five feet inside the room, and crawls back to the doorway at 2½ feet per second, he will feel 1000º-1500º heat on exposed portions of skin or through the fire gear for two seconds. The firefighter may not receive any burns. If you say you can enter 10 feet into a room about to flashover and it does, and you try to escape you will experience 1000º-1500º for four seconds. Fifteen feet inside a room that flashes-over, and you must crawl back to the door for six seconds. Think about it.

Lessons Learned

Firefighters should know the definition of flashover — a room bursting into flames. They should know the warning signs of flashover. Also, firefighters must know how to delay it. And most importantly for firefighters' safety and survival, they must know defensive firefighting procedures — how to search and stay alive.

The following books written by Chief Dunn are available through Firefighters Bookstore: Safety and Survival on the Fireground, Collapse of Burning Buildings, and his newest book, Command and Control of Fire and Emergencies.

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