Everyone knows that fire is deadly, but when it comes to backdrafts the risk is heavily increased. Firefighters have the skills and knowledge to help try and avoid these dangerous events – or at least deal with it when it’s unavoidable – but the average person may not even be aware of what a backdraft is.
To help you understand this terminology, as well as how they happen, how to prevent them and what makes them so dangerous, we’ve compiled a short guide on the topic. Of course, you would never be expected to deal with a backdraft – that’s a job for the firefighters – but it’s always useful to arm yourself with as much knowledge as possible when it comes to fire.
What is a backdraft?
Once a fire begins to burn up all the oxygen in a sealed area, it will inevitably run out, replacing it with combustible gases. The heat, smoke and unburnt gases remain behind at an intense temperature. A real-world example of this would be a fire which starts in a room, but which becomes limited to that compartment. Once you then open any door or smash a window, the influx of fresh oxygen cause the fire to rapidly expand inside the room and burst outwards with great ferocity and force.
While the fire in a room may appear to be dying back, it’s important that you don’t risk re-entering until the fire services have arrived as this could be a backdraft situation.
How do backdrafts happen?
As mentioned above, backdrafts are always the result of a fire being confined to space and starved of oxygen. Where there is a constant source of ventilation for the fire to feed on, it’s unlikely that a backdraft could occur, but fires in offices or buildings where compartmentalisation is in effect are more likely to see this.
Of course, you yourself would never be expected to deal with a fire once it reaches this size, since the sealing of rooms with fire doors and other fire stopping techniques is designed to hold a fire back to allow time for people to evacuate the building.
Why are backdrafts so dangerous?
Any type of fire is dangerous, but one with such unpredictable timings, expansions and explosions is always more tricky and, potentially, could have devastating consequences.
One of the things which makes them so dangerous is that you can never quite tell if there will be a backdraft explosion of fire. Often, after reintroducing oxygen to a room by opening a door or smashing a window, it can take time for the backdraft to occur since a lot depends on the mixture of combustible gases and the size of the room.
A backdraft is unpredictable and usually well over 500℃ in temperature, this is the reason you’re advised to test a door by touching it with the back of your hand before opening it when a fire has taken hold of a section in a building.
How do you prevent a backdraft?
Should there be a risk of backdraft (and again, we can’t stress enough that you shouldn’t be personally responsible for dealing with one) a vertical ventilation opening is usually created to try and alleviate the pressure inside. This is something the fire and rescue services will carry out where possible – especially since they’ll be able to better spot the signs of a backdraft.
Before water and other tactics can be used to tackle a fire, this work of preventing backdrafts will be implemented. This might seem strange – even more so if your property is the one on fire – but you can trust that the firefighters will know what they’re doing.
While not directly linked with preventing a backdraft, a fire risk assessment could help the responsible person for your premises to establish if this is likely to occur in your building and where. If you’re aware of where the fuel and possible ignition sources are, you can take steps to try and avoid the event of a fire completely. Similarly, monitored fire alarm systems can alert the fire services much earlier, allowing more time for them to stop a fire taking over a building.
To learn more about monitored fire alarm systems, fire risk assessments and how to protect your business from the threat of fire, contact our expert team today.
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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.
Actualizado a fecha: 18 July, 2019
Last week we talked about flahovers. Today, we wanna explain show another dangerous phenomenon that happens when a house is burning. Did you know what is a Backdraft? A backdraft is a dramatic event caused by a fire, resulting from rapid re-introduction of oxygen to combustion in an oxygen-depleted environment; for example, the breaking of a window or opening of a door to an enclosed space. Backdrafts present a serious threat to firefighters.[1] There is some controversy concerning whether backdrafts should be considered a type of flashover.
Source: Wikipedia