Clearing the Air: Common Ventilation Misconceptions


Positive pressure ventilation (PPV) definitely works and improves conditions during ideal fire and smoke conditions-for example, when the fire we are looking to attack has a vent exit directly behind it, in the same room, or in the vicinity of the fire. But the fire service seems to struggle with ventilation at fires in which we can’t immediately determine the seat of the fire during a walkaround, victims are trapped, fire is at or near the hose team’s entry point, and unvented fire is in the attic space. How, where, and when to ventilate are the questions during these “nonideal” fires. In my experience, we run the nonideal fire more often than not. Let’s look at the fires that do not fit our “PPV profile.”


A PPV profile consists of the existing fire and smoke conditions (some are listed above) that support the successful use of PPV. How do we ventilate the fires that do not fit our PPV profile? If we can’t or have decided not to use PPV and we are not going to vertically ventilate, how will we ventilate the structure we are entering?

One such method is selectively breaking glass-i.e., natural horizontal ventilation. Firefighters have used this method for more than a century for good reason. With coordination, breaking glass on your 360° walkaround at or nearest the seat of the fire will assist you and your crew immensely. Breaking glass at a window or forcing a door nearest the heaviest, darkest, fastest-moving smoke, without any visible flame seen yet, will likely help you find the seat of the fire. If you do not do this, you and your crew will be walking directly into the vent exit on entering the structure (usually through the front door). This will subject you to increased exposure to hostile conditions as you try to locate and extinguish the seat of the fire-all this while inside with little or no visibility and a rapidly building heat condition and the only vent exit behind you. Beware! Flashover coming! By doing this simple task, your visibility will improve and crews can operate with increased confidence.

For crews preparing to enter a hostile environment where the extent of fire spread and smoke volatility are unknown, breaking the glass will allow the smoke to do what it needs to without endangering crews as they prepare to enter or those who have entered the fire building. The smoke may in fact ignite when the glass is broken and will vent out the opening. This has been called letting the fire “orange up” and, in fact, is good. It is better to let the fire light off when we want it to rather than when it wants to. This is called “venting for fire.” If the smoke doesn’t ignite, that’s okay, too; you have just lifted the smoke off the floor where you should be operating. This is also where you will find victims, since our goal is rescue, not recovery. Remember, the ultimate goal of ventilation is to remove the toxic and superheated products of combustion (i.e., fuel) from the structure.

For search crews operating in a heavy smoke condition, taking the glass will help to increase visibility at the floor level and will build confidence as you are now aware of a secondary means of egress in case a problem develops. For heads-up drivers or the rapid intervention team, it lets them know where to place the ground ladder for your egress even before you might need it. Keep in mind that when you leave the room, and if warranted, you can close the door to control the flow path. This will limit smoke spread back into that particular room.

Let’s say you are assigned to primary search and, while searching in a limited visibility condition, you come across a bedroom window with a plywood covering. Do you immediately remove the plywood or leave it in place so that PPV might be more effective later on? Why save the windows/plywood now for a tool (the fan) that may or may not be needed later? The coordination needed to use PPV effectively and safely can sometimes be more time-consuming and personnel intensive than one trained and capable firefighter with the appropriate tool. PPV should not be the default ventilation tactic, nor should smoke removal. Understand that there is a difference. Natural horizontal ventilation should be the default with PPV as a supplement when needed.

How about plywood over a window on a walkaround with no visible fire? My answer-a no-brainer and in both above circumstances-is to immediately remove the plywood. This may save your life and is smart, safe firefighting.

What is the difference between a window with plywood over it and just a plain glass window? Not much at all in my book (photos 1, 2).

Photos by author.
Photos by author.
Photos by author.


  • “It’s bad for public relations.” Public relations should not be a tactical consideration; victim and firefighter safety should be the primary tactical consideration. When we vent, we do so primarily for life-the victims’ and ours. Remember, we are the ones wearing the gear and self-contained breathing apparatus, not the victims.
  • “It causes too much damage to property.” According to several insurance companies, the deductible on a working fire is met several minutes before the first fire unit even arrives on scene. Besides, I thought “property conservation” came after life safety and incident stabilization. Did something change?
  • “We can’t effectively do PPV because of too many broken windows.” You can still close bedroom doors to control air flow to accomplish this when needed.

Point to ponder: If our PPV profile was not met on arrival or shortly thereafter, why are we holding off selectively breaking glass to preserve the ability to use PPV when the fire is under control? This is not a safe and sound practice and will injure people.

  • “You will introduce oxygen into the structure, thus increasing the intensity of the fire.” This is true, but consider that we introduce quite a bit of oxygen when we enter a structure through the front door. It seems that only recently we have become concerned about this. We still continue to do it, though.
  • “You will pull the fire toward you.” Yes, you may do that if in fact you have not vented the fire already at or nearest to the seat of the fire. Fire is very lazy and will always take the path of least resistance to find oxygen, lower pressure, and cooler air.


Absolutely make sure to take into account wind speed and direction when deciding to ventilate and from where you will attack the fire. This is critical and, if neglected, can injure or kill you!

When considering large commercial box buildings with ceilings of 20 to 30 or more feet, understand that no matter how many fans you put into operation, ventilation is not going to be effectively accomplished. This has been proven time and time again and makes conditions very dangerous. When deemed safe, a trip to the roof may be in order (photo 3).

photo 3

There is no doubt that PPV is a good tactic when used in a planned and coordinated manner. When our PPV profile is met, remember that PPV is best used at the backs of the nozzle/entry team with strict coordination and an adequate-sized vent opening at or nearest the seat of the fire. Proper positioning of the fan will ensure that visibility and temperature will both improve as the nozzle team makes its push. This also helps to reduce the chance that the fire and smoke will beat nozzle team members back to the entry point and cut them off.

PPV can also have very detrimental effects if used improperly and without coordination. If PPV is a “tool in your toolbox,” take the time to understand when it’s working effectively and, more importantly, when it’s not. If your PPV profile was not met and cutting the roof is not a common practice for your department, you must ventilate to protect civilians and your firefighters.

Natural horizontal ventilation (i.e., selectively breaking glass) is a time-tested method for removing dangerous and volatile smoke from the structure quickly, safely, and effectively. In doing this, you have enabled your crew to operate in a much more safe environment. Most times you will find that you will even be able to see under the smoke layer. This helps everyone inside, and that’s the idea!

KEVIN R. LEWIS is a lieutenant and an 18-year veteran of Cobb County (GA) Fire & Emergency Services. He also serves with the Roswell (GA) Fire Department. Lewis has a bachelor’s degree from the University of Delaware and is a founding member of the Metro Atlanta Fire Fighter Conference and the Georgia Chapter of the Terry Farrell Firefighters Fund.

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