FLASHOVER AND BACKDRAFT: A Primer

BY CHRISTOPHER FLATLEY

Flashover and backdraft have been confused for years. Part of the reason for the confusion is that they produce a very similar result-a large fire that envelops the whole room or area. They are, however, very different in how and why they occur.

Misconceptions have also evolved around these phenomena. Among these erroneous beliefs are that a flashover will occur within four minutes of first flame and that a backdraft will occur only in tightly sealed buildings. Both are dangerous misconceptions. There is NO time frame for flashover, and a backdraft can occur in almost any enclosed space given the proper conditions.

FLASHOVER

Flashover by definition is “the sudden involvement of a room or an area in flames from floor to ceiling caused by thermal radiation feedback.”1 Thermal radiation feedback is the energy of the fire being radiated back to the contents of the room from the walls, floor, and ceiling. This radiation of energy to the contents of the room will raise ALL the contents to their ignition temperature. When the contents of the room suddenly and simultaneously ignite, this is flashover. This simply means that flashover is a temperature-driven event. It requires that the fire’s energy be radiated back to the contents to produce a rapid rise in temperature and simultaneous ignition. Flashover indicates that the fire has grown to the fully developed stage (Figure 1).

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Another important concept to understand is the physics of the flashover. Several factors will affect whether or not a room will flash over. The size of the room, the combustible contents, the air supply, and the insulation of the room all combine to determine a room’s flashover potential.

Smaller rooms will flash over faster. A small room enhances the thermal radiation feedback faster because of the room’s volume. In large rooms with high ceilings, it takes longer to heat the combustibles below. When radiated heat travels longer distances, it loses energy. The close proximity of the contents in a small room increases the absorption of thermally radiated energy.

The contents of the room affect the flashover potential. A room loaded with combustible furnishings will produce more fire, thus more heat, more radiated energy, and more flashover potential.

The air supply is also critical to creating the fire growth to produce flashover. Most fires are air regulated, not fuel regulated.2 The average room has enough contents (fuel load) to produce a large fire. What is needed is the air to “fan” the flames. The air-regulated fire is included in the backdraft discussion below.

Lastly, the room’s insulation affects how efficient the thermal radiation feedback will be. Insulation of walls and ceilings prevents heat from escaping to other areas, thereby increasing the room’s flashover potential.

These factors are not going to be known to the firefighters advancing on a fire. The warning signs, therefore, must be understood to provide for the members’ safety.

The warning signs of flashover include high heat; this is from the compounded effect of the fire and the radiated heat from the room. This heat will be intense. For flashover to occur, all the contents of the room must be raised to ignition temperature, including the firefighters in the area! Any sudden rise in temperature that can be felt through bunker gear should be considered a sign that flashover is imminent.

Rollover is the fire that rolls in the black smoke venting from the fire area. It may also be seen as “snakes” of fire that dart out from the smoke. Rollover is an early sign that flashover conditions are developing.

Black smoke is a sign of flashover. This may sound ridiculous, but I’m talking about black smoke so black that no other color can be seen in the smoke-black smoke like the type you would see if tires were burning. This black-black smoke contains so much unburned fuel that it can be called “black fire.” This “black fire” needs only the right mixture of air to ignite. This black smoke will also have a tremendous amount of energy and will roll violently out of the fire area and push heat into voids and concealed spaces. This “black fire” will increase the possibility of fire extension and smoke explosions in the cockloft.

Preventing Flashovers

Flashovers may be prevented in two ways. Proper ventilation can prevent a flashover. Venting allows superheated air and fuel-loaded fire gases to escape the room or area. This can be done by horizontal or vertical ventilation. Reducing the heated ceiling layer reduces the thermal radiation feedback and the possibility of flashover. It is important to note that venting the fire gases should not expose firefighters to injury or spread heat and smoke to uninvolved areas of the building. This could increase the fire problem if those gases were to ignite.

The second way to reduce the possibility of flashover is to cool the fire area with a hose stream. Most of us have been taught never to open the hoseline on smoke. This is true. But just as we were told that, weren’t we told there are no “nevers” in the fire service? This also is true. The reason you can open the line on smoke in this situation is that the black-black smoke, a warning sign of flashover, is called “black fire.” If you see that black smoke and feel the rapid heat rise through your bunker gear, even if you can’t see the rollover, you must open the line on the “black fire.” This will reduce the possibility of flashover. This is an emergency situation, and the hoseline can be operated in this manner.

Escape FROM Flashover

Escape from flashover is nearly impossible. Recognizing the warning signs and knowing how to prevent them will do more to ensure your safety. We know the warning signs, but you must be able to react to them. A flashover can occur in 10 seconds. This will allow the average firefighter the time to travel approximately five feet to an exit. If operating a hoseline is not an option, then go out a window or move to another room and close the door if possible. This will give you some protection.

A fully protected firefighter has the best chance of survival in a flashover. Even a totally encapsulated firefighter is at risk. Human skin burns at 124 F. Without an SCBA, it would take only several breaths of 300 F air to sear your throat. This would cause you to die from asphyxiation. During a flashover, temperatures are measured in the thousands of degrees-well beyond the limits of bunker gear.

At a taxpayer fire in Monsey, New York, on November 23, 2004, three firefighters were removed from the fire occupancy seconds before the store “lit up.” The report of the fire was delayed when the store employee attempted to extinguish the fire before dialing 911. Firefighters arrived and found heavy smoke venting from the store and advanced a hoseline into the occupancy. Chief Andy Schlissel saw the warning signs of flashover and ordered the members out. “Twenty seconds later, you would have had three fried guys in there,” Schlissel said. A coordinated attack was mounted, and the fire was brought under control with only minor extension to adjoining stores.

Schlissel credits instruction received at the Rockland County Fire Training Center in Pomona, New York, for preventing injuries at this fire. Rockland County is one of the few counties in New York that has a flashover simulator. This modified shipping container provides students with a live-fire experience and can produce “controlled” flashovers.

BACKDRAFT

A backdraft is a smoke explosion that can occur when additional air is introduced into a smoldering fire and heated gases enter their flammable range and ignite with explosive force.3 A backdraft is an “air-driven event,” unlike a flashover, which is temperature driven. The fact that most fires are air regulated and not fuel regulated makes the understanding of backdrafts so important.

A fire has begun and grown, it may or may not have the thermal radiation feedback to flash over, but it has consumed the contents of the room and is now “burning itself out” by using all the available oxygen in the room. The normal oxygen level in air is approximately 21 percent. Below 14 percent, visible flame is reduced. The room’s contents are at their ignition temperature and will erupt in flames when oxygen is introduced, and the hot gases will ignite with explosive force. This is a backdraft. The explosive force at which the backdraft occurs is a result of the amount of superheated gas in the space and the amount of oxygen introduced. The explosive force can break windows, knock down walls, and injure firefighters.

Backdrafts can occur anytime during the decay stage of fire development, anytime before the gases have cooled below their ignition temperature. If the room was left and the gases cooled before any oxygen was introduced, no backdraft would occur, but the room would have been consumed by fire.

All members on the fireground must know the warning signs of backdraft. Often, the first units do not recognize the heavy dense smoke or the smoke that appears to be “puffing” or being drawn back into the building; someone at a distance from the scene generally recognizes these telltale signs. Chiefs or their aides setting up a command post across the street are most likely to see that kind of smoke. Engine chauffeurs hooking up to hydrants at the end of the block may also be positioned to see it. The truck chauffeur can see it when giving the building “a good look” while at his position on the pedestal. Hopefully, it is not too late. A small smoldering fire or some stained or cracked glass may not be recognized for what it could be. Often, we are drawn into “tunnel vision”: We see a small fire and force our way in and create a dangerous situation for ourselves.

If backdraft conditions are suspected, proper ventilation will prevent the explosion. Openings must be made above the fire to allow the superheated gases to escape before oxygen is introduced at lower levels-the lower levels where we would be entering.

Backdrafts and flashovers are equally dynamic and deadly. Recognizing the signs and being able to remain calm will give you the best chance for survival. It goes without saying that you should be in full protective equipment.

Backdrafts don’t always happen in the closed taxpayer behind the roll-down gate at 3:00 a.m. Remember that the backdraft was also known as the “smoke explosion.” This term came from the small-sized events that occurred when firefighters were opening up to check for extension.

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A fire on a lower floor of a multiple dwelling could extend heat and smoke throughout the fire building. This smoke would get trapped in the cockloft, the space between the top-floor ceiling and the roof. Members sent to check for extension would pull the ceilings and allow oxygen to enter the cockloft, and the hot gas would mix with air and the cockloft would backdraft. How did this occur? Knowing what we know about carbon monoxide (CO) today, the space was probably filled with CO. Remember, in addition to its other dangerous properties CO is flammable. Figure 2 compares CO with some common flammable gases. What does this mean to us as firefighters? CO will burn at a relatively low temperature (for fires anyway) and at almost any mixture.

When sent to check for extension, make a small examination hole first. Make this hole from the doorway, and make it small at the start. Poking a small hole will limit the amount of oxygen that enters the cockloft or other enclosed space. If you let enough air into the space to ignite the mixture of gas, you will be standing in a position of safety in the doorway. If you pull a large section in the middle of the room and the cockloft lights up, the whole ceiling will drop on your head.

If your examination hole indicates some fire may be present, call for a line before any more of the ceiling is pulled. Remember, you were sent to find fire. If you find it, what will you accomplish by exposing all that fire with no way to put it out?

Flashovers and backdrafts need to be understood because several factors are at work to increase their frequency. Fire load is probably the single biggest factor working against today’s firefighter. Yes, fires are hotter today. This is a combination of two elements of fire load-more plastic and more stuff.

The British thermal units (Btus) given off by wood, cotton, and paper in the house fire of the 1950s averaged about 8,000 per pound. Polyurethane, the soft plastics, gives off 12,000 Btus per pound. These soft plastics are the foam cushions, carpet, and the like. The hard plastic polystyrene from which TVs, VCRs, toys, and all other plastic items in the home are made gives off 18,000 Btus per pound.

Look at what you have in your home. Do you remember having that much stuff as a kid? Maybe you need to ask your parents or grandparents to get the true perspective. Each generation wants to see its generation do better than the last. How is that success measured? With more stuff.

Since the energy crisis of the ’70s, buildings have become sealed as tight as construction materials would let them. National energy codes are now in place to enforce energy-efficiency standards. In addition to making walls and ceilings more efficient, energy-efficient windows are designed to hold the heat through what was the best indicator of fire conditions. Insulation in walls and ceilings trap heat inside the structure. This trapped heat increases the thermal radiation feedback required to produce a flashover. The thermal-pane windows hold the heat in the “dead air” space between the panes of glass and delay breaking, preventing the fire’s self-venting. The double-pane windows are difficult to break when venting is required.

Electronic detection equipment has developed into a common market item. Burglar and fire detection equipment can now be found in every type of occupancy. This early detection equipment gives the fire service the earliest possible notification of fire. This benefit also has a drawback, however. Early detection may lure firefighters to enter a structure with a fire in the incipient stage (the I’m-not-going-to-kill-you stage). There is the possibility of missing some of the warning signs of the fire’s true growth (the I’m-going-to-kill-you stage). The danger is when we are in the structure when the fire enters the latter stage.

The only way to protect firefighters is to train in understanding the fire environment. We must be able to recognize the signals the fire gives. We all must look at the whole picture and be conscious of the building we’re in and our surroundings so we don’t get lured into a trap. There is little first-hand information available on the subject. Most of the people who have the experience with these events are not here to share them with us. We need to learn so we can survive.

Endnotes

1. FDNY Firefighting Procedures, Engine Company Operations, Chapter 4, Sec. 4.6.2.

2. FDNY Firefighting Procedures ... Chapter 4, Sec. 4.6.3.

3. FDNY Firefighting Procedures … Chapter 4, Sec. 4.6.4.

CHRISTOPHER FLATLEY, a 16-year veteran of the Fire Department of New York, is a lieutenant assigned to Ladder 21 in Manhattan. Before promotion, he was assigned to Ladder Company 2. He is a nationally certified fire instructor I and an instructor at the Rockland County Fire Training Center in Pomona, New York. He has been a presenter at FDIC and FDIC East.

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