Hot Gossip – 2nd Flashovers

by George Farenden, Emergency Response Consultant, DuPont Protection Technologies.

With 30 years fire-fighting experience and 7 years working as an Emergency Response Consultant I was confused and a bit amazed when I heard about second flashovers and how some materials can protect you from this deadly occurrence, WOW how fabulous, what second flashover?. Talking with Fire-fighters at the sharp end of the business they are as mystified as me as to how this can happen.

Back to Basics

There has been a lot written about the development of fire and more specifically about the development of fire in buildings, but all here relates to compartment fires and for our purpose let’s call them Rooms.

People generally think that a room fire starts and progresses like a bonfire in a garden. That is a piece of fuel gets lit, the fire burns, the heat from that piece of fuel heats a piece adjacent to it, to its ignition point, which ignites and the fire spreads to the next piece of fuel and so on until the whole mass of the bonfire is burning nicely and the smoke rises up and away, easy, everyone knows how fires work….. But It doesn’t happen like that in a room. Here I will give the scenario of a simple but decent bedroom or lounge fire, fire-fighters bread and butter. They call them house fires, but house fires are usually only one room, they may involve a hallway or a landing, but basically are confined to one room.

Smoke is not, in this instance the waste product of fire which is a nuisance albeit a hot nuisance, and although you can’t breathe it and it stops you seeing what you’re doing, it’s relatively harmless. This is a popular misconception.

So let’s watch our bedroom fire developing nicely in a T.V or chair in the corner of the room, door and window closed. The fire builds up using up the available air in the room yet the smoke cannot rise up and away as in a bonfire so it hits the ceiling and a layering of the atmosphere in the room takes place. Post fire, fire-fighters have often seen the black lines around the walls marking the layers.

The three layers:


While we’re talking about it we call it smoke, but it’s a combustible gas and is probably a fire-fighters greatest enemy. Energy rich gases like those from polyurethane foam, plastics, and paint can have an energy content similar to that of methane gas, so would you be so self assured wandering around the house and fighting a fire knowing that the house was full of methane gas?

Anyway, back to our bedroom fire. The three layers have now developed and the fires burning brightly using up the available oxygen and already something has started to happen.

A Lean, Mean Flashover

Flames start chasing up the smoke column curling and chasing across the ceiling, some Fire-fighters call these “dancing angels” this may cause wallpaper to burn and flake off, or curtains away from the original fire, to catch and drop to the floor, (maybe even starting a second seat). This does happen, yet how can the flames from our TV/armchair reach that far? They don’t it’s the smoke (combustible gas) that’s burning. Now this would be a good time to intervene but especially with today’s high fire loaded double glazed well insulated houses fire-fighters are probably only just on their way.

So now we have two fires, the original fire, and a gas pillow fire but it’s using up all the available oxygen and slowly dies back, but loads more flammable gases were released from the ceiling and walls when they were heated by a lean flashover and by the radiated heat from that flashover down onto the room furniture and even the floor coverings. Our TV/chair, the original fire, dies back to a smoulder, the air temperature falls and thus contracts and fresh air is sucked into the room. The fire momentarily flares up; using up this new air producing more combustible gas for the gas pillow, again and again this happens fire-fighters know this as the pulsation cycle. The fire is now beginning to breathe and move off to its next stage.

The Sleeping Dragon

Now is not a good time to throw open the door with the hose reel in jet mode and make like John Wayne entering a saloon in a rough part of town. Yet fire-fighters have to get in, if the door is opened or a window fails the consequence is much the same the rush of air feeds the fire on a much larger scale than described above. The smouldering fire bursts back into life and ignites the combustible gas layer in the room. The heat generated by the flames working into the gas layer is colossal and produces even more combustible gases due to the decomposition of the room contents.

Now the opening has made a basic change to the mechanism of the fire, most of the combustible gases are leaving the room by the over pressure layer this may be exiting the window or onto the landing and fresh air is entering by the under pressure layer, the movement of air is called the “air track” by fire-fighters and is the constant supply for the combustion process raising the neutral layer and allowing the flame front to work it’s way from the original fire back into the combustible layer. Now this layer is remember, full of combustible gases close to or higher than their ignition temperature and rapidly being diluted to it’s ideal mixture. The flame front reaches the upper part of the window or hallway and the flames (those combustible gases lighting progressively) connect to the outside of the room terminating in this type of flashover which is known as a “BACKDRAFT” leaving behind in its wake a full room fire.

This happens quicker than it took me to write it and faster than you can read it.

Fire-fighters have seen the evidence when they clear-up, the whole room contents has been destroyed by fire but only one small area of floorboards burned in the corner where the TV/chair once stood, there never was a bonfire in the room with a fuel – flame – fuel – flame contact, the bed and other furniture has all but gone yet the fire never crossed the floor. This is how compartment fires work. So now we have an appreciation of what happens in a room when its on fire so by the very nature of Backdrafts they must use up the available combustible gas so, how do you get caught a second time, You can’t, it’s as simple as that.