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## The Stack Effect The stack effect is also affected by horizontal ventilation. Positive stack effect will normally allow smoke on lower floors that is entering the stairs to continue upward, spreading out and banking down on upper floors unless vented. If the fire is on a lower floor, and windows...

## The Stack Effect The stack effect is also affected by horizontal ventilation. Positive stack effect will normally allow smoke on lower floors that is entering the stairs to continue upward, spreading out and banking down on upper floors unless vented. If the fire is on a lower floor, and windows are open or fail, the stack effect may violently draw the fire toward the stairway without the smoke going out the windows. This is especially true of lower-floor fires on cold days, when the outside air is much colder than the inside. Nothing is gained in this case by venting the windows, and, in fact, conditions may be worsened if the additional smoke also mushrooms on the upper floors. The same is true of fires on hot days on upper floors. The smoke may be violently drawn into the building toward the stairwells, including the attack stairs as well as the ventilation stairs. This type of action has resulted in severe punishment for firefighters and has produced injuries. The IC should carefully consider the consequences before issuing an order to vent the windows. A slogan to keep in mind is “*high-high* or *low-low* is a *no-no*.” In other words, if the fire occurs on the upper floor on a day when the temperature outside is high, or on a lower floor on a day when the temperature outside is high, you'll want to make sure the windows are properly vented.