Knowing the signs of flashover and backdraft will protect you and your crews against unnecessary injury and death. A user on Quora recently asked, “What are some signs of a backdraft and how can we identify them?” A few answers stood out to us, especially one by a former fire lieutenant. You can read his reply below:
A backdraft is caused by the sudden introduction of air into a fire that has depleted most of the available oxygen in a room or building.
Since a fire requires air, fuel and heat, the latter two must be present as well. Fuels, such as wood, have an ignition temperature that when reached will begin burning on its own without any direct flame contact. When a fire has depleted most of its oxygen, the flames will die down, but the fuel will still burn in a smoldering state and still generate heat, like with charcoal. That heat, along with the heat generated beforehand by the flames, will heat the room’s fuels well beyond their ignition temperatures.
When a room or building is closed up, the heat cannot escape and the fuel still burns, but does so more slowly in the smoldering phase after most of the oxygen has been used up. So, as the heat continues to build, the heat still increases even though the combustion is not complete and no longer generates flames. Some fires even burn themselves out at this stage if the oxygen is totally consumed and the heat decreases.
At any rate though, a fire without sufficient oxygen tries to suck in oxygen in order to sustain itself. It will also burn less effectively and will generate carbon and soot in the form of smoke which will be very dark brown or black, depending on the amount of oxygen it can draw in.
All smoke is the result of incomplete combustion to some extent; the darker it gets, the more incomplete the combustion is. So, as firefighters, we put all of this knowledge together and look for signs of high heat and incomplete combustion.
This heat can be determined by evaluating the color of the smoke which might be be very dense and actively seeking release from the upper portions of a structure. There is also no more room for the fire area to contain the smoke which is still being generated. This smoke can often be so hot that it produces flames on fuel where it is exits, such as eaves.
At the same time, the fire tries to draw in as much oxygen as it can to sustain itself. This is often evident because we will see smoke being drawn in along with the air under doors, windows, etc. The windows will also show signs of excessive heat, such as brown stains and cracking.
These signs are what we look for so that we do not get caught in a backdraft as we open the door and introduce oxygen. This will cause the heated fuel to burst into flames in an explosive manner because of the force it generates within a tightly sealed environment. This occurs because all of the superheated fuel ignites instantaneously.
So, we try to avoid ventilating through horizontal openings, such as windows and doors because either may cause a backdraft. The only effective way to eliminate the heat safely is by ventilating the roof or other vertical openings which will relieve the heat buildup and allow us to enter.
While the fire will still rage until we go inside with water, it will not erupt in an explosive manner, making it much safer to enter wearing the appropriate SCBA and other gear as we begin to cool it with our hose lines.
So, that is how we recognize a potential backdraft and how we prevent one from injuring our crews. Please keep in mind that we are usually going into a very, very hot environment after we ventilate and that it can still be a tough and dangerous fight. We also try to stay on the safe side by using vertical ventilation even when the obvious warning signs may not be present. Strange things can always happen in this business, so it is better to play it safe whenever possible.
This article, originally published in 2017, has been updated.