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Ventilation Basics
Ventilation exchanges hazardous air in a confined space with fresh air. This means there will be an increase in oxygen and, therefore, a potential for an increase in the size and intensity of the fire. Incident command must determine how this will impact both victims and fire crews in the structure. You should always follow your local standard operating procedures. Any ventilation activities must always be coordinated with interior crews and must be planned in such a way as to confine the fire, rather than spread it.
Natural Ventilation
Natural ventilation uses purely natural means to direct air flow in or out of a structure. It is most often accomplished by opening windows and doors. Always attempt to open windows closest to the fire to minimize damage to the rest of the structure. These locations are usually evident by smoke already venting through cracks. In windy conditions, you should attempt to vent leeward-side windows and doors first. In multi-story structures, always work from top to bottom when ventilating. If you plan to use positive pressure ventilation techniques, plan accordingly so as not to hamper your ability to control pressure.
Mechanical Ventilation
Mechanical ventilation is accomplished by the use of smoke ejectors or ventilation fans. This technique works best when used in conjunction with natural air flow through the structure. Try to minimize the gaps around your fan so that combustion products are not re-introduced into the structure. You should insure that the exhaust does not flow toward air conditioning units or cooling intakes. Try to position fans as high as possible so that they eject the hottest gases and most concentrated smoke. If units are used in tandem, place the exhaust fan above the inflow fan.
Positive Pressure Ventilation
Positive pressure ventilation is the primary method of ventilation used today. Air is forced into a structure, where it exits through a smaller ventilation point. This creates a vacuum that sucks out the by-products of the fire. You must insure that you only have one entrance point and one exit point, which must be smaller than the entrance to create the vacuum. This method can rapidly move the fire. Only use positive pressure ventilation when you know where the fire is. Fire crews and victims must always be kept between the entrance point and the fire, while the fire must always be kept between the crews and victims and the exit point. This insures that fire and smoke will not move toward them.