Preventing backdraft through vertical ventilation

Take care when implementing positive-pressure ventilation when firefighters are inside a structure fire to prevent a flashover


The backdraft phenomenon is one that firefighters do not want to face. We have been taught from basic training that a backdraft occurs during a lack of oxygen levels sufficient enough to sustain combustion. When oxygen is introduced into the smoldering environment, rapid combustion or an explosive combustive situation occurs.

We have also witnessed smoke explosions at certain fires where the same similar events are present, but when air is introduced into the environment, an explosive result occurs but without the flaming combustion.

Positive-pressure ventilation

Regardless of the type of rapid fire event, firefighters want to avoid it all together. The video below illustrates the end results of the rapid-fire backdraft. Though older, the video is still valid.

In the video, we can see a house that has some moderate smoke conditions, no visible flames or fire, two ground ladders raised into position and a positive-pressure ventilation fan at the front door.

Now we cannot tell if the PPV fan is on and running in the video, but we are going to assume that it is, based on the fact that it is positioned in front of the door. We are witnessing the transition from a stable event, through a rapid change in events with the introduction of air.

Most departments will grab the PPV fan for horizontal/mechanical ventilation, but will use it in the wrong context or application. Positive-pressure ventilation, or PPV, should actually be renamed post-fire ventilation or PFV. This is because the PPV fan or tactic is designed for ventilation after the fire has been suppressed and completely extinguished.

When a fire is being extinguished or is still smoldering, it has the potential for a recycle back to the flashover phase (or the backdraft or smoke explosion phase) of our fire development. The smoke that is being produced is unburned fuel and, when mixed with the right concentration of air, it will ignite.

The concussive force that is being produced is enough to blow out the one wall of the house – imagine how that feels on the body of a firefighter! They may not receive external injuries, but will certainly sustain internal injuries.

The best way for us to prevent this phase from occurring is to ventilate vertically as much as possible when suppressing the fire. Only when the fire has been fully extinguished can we use a PPV fan to help with clearing out the rest of the structure, in a systematic fashion.

Make sure the conditions support the use of PPV when dealing with a structure fire with crews inside.

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