Firefighting 101                                                         by:  Dan Howell

Positive Pressure Ventilation - Fire Attack & Overhaul

The effectiveness of positive pressure ventilation is dependent on the size of the fan, the size of the structure, and the size and location of any vents. If these three main factors are taken into consideration, positive pressure ventilation can be used with a great deal of proficiency and control to support a variety of fire ground operations. The most common of these operations include overhaul and fire attack.

     Positive pressure ventilation is performed by using an exterior vent fan to force air into the interior of a structure. The vent fan is typically placed at the point of attack and vent openings made where necessary. The fan is then started and the doorway is covered with the air cone so that there is a minimal amount of air pushing back out of the entrance. These actions create a path of moving air between the fan and the vents established by firefighters, which in turn, force heat and smoke out of the structure. Understanding this path of moving air is the essential component to understanding positive pressure ventilation and using it properly during all fire ground operations.

     Supporting horizontal ventilation during overhaul is the most common use of positive pressure ventilation. During overhaul, there are less natural currents to propel horizontal ventilation and any remaining heat, smoke, and gases may linger in the structure. Firefighters can use positive pressure ventilation to force these combustion products out of the structure. The basic set-up above is used and the biggest decisions to make are where to create ventilation openings. Vent openings should be made so that the path of air created during the operation goes through the areas that need to be ventilated. By controlling the vent openings, the path of air can be directed to the areas that need to be vented and smoke systematically removed from the structure. It is important to remember that if there are no vents in a room or area, the path of air will not travel through this dead space and any smoke present will not be removed. When this situation is encountered it is helpful to use a second vent fan or smoke ejector to circulate smoke out of the dead space into the path of air where it can then be forced to the exterior.

     Using positive pressure during overhaul is a frequent operation and most firefighters are comfortable with this application. Positive pressure during fire attack, however, is the most aggressive form of ventilation available and is often feared and misunderstood. Firefighters must understand the effects that positive pressure has on fire behavior. As with overhaul, the key to this understanding is the path of air moving from the fan to the ventilation openings. Any fire that is within this path of air will follow it towards the vents. Fire that is not within the path of air is mostly unaffected by positive pressure ventilation. However, if the fire is close enough to the path of air, a venture type of affect can exist where fire is pulled towards the ventilation openings. Fire in buildings that have no ventilation openings during positive pressure has the potential spread uncontrolled into void spaces and other areas of the structure. In these situations, the fire can actually be pulled back towards the fan and any advancing firefighters.

     With this information in mind, there are a number of factors that must be determined during the initial size-up before positive pressure is used during fire attack. The first is that the location of the fire must be determined. If the fire cannot be located, the proper place for vents cannot be determined. If vents are incorrectly placed, the fire can spread into uninvolved areas of the structure. Second, it must be possible for the structure to be vented. If the vent openings are not present when positive pressure is applied, erratic fire behavior can result. Third, the location of any victims must be known. Any victims in the path of air between the fire and the vent may have the fire pushed into their location. Finally, firefighters must be ready and able to attack the fire immediately when positive pressure is begun. All ventilation tactics require a close coordination with fire attack. If positive pressure is started before fire attack is ready, the fire will be allowed to increase in severity and spread unchecked through the structure.

     There are also important fire conditions that must be ruled out before positive pressure is used during fire attack. The first is backdraft. If size-up reveals the potential for backdraft, then positive pressure should not be used until vertical ventilation removes the backdraft threat. Another important fire condition where positive pressure may have an adverse effect is an attic fire. Attic fires are characterized by large open areas where fire can quickly spread. In addition they are often very difficult to access. If the fire has involved an attic, positive pressure can increase fire spread while firefighters pull ceilings and access the fire. If an attic fire is present, positive pressure should be delayed until water can be applied on the fire.

     If all of the above factors and conditions can be accounted for during size-up then positive pressure may be considered to assist fire attack. The fan is set-up normally at the point of attack and ventilation openings are made on the burning side of the structure as close as possible to the fire. Vents placed in this location prevent the fire from spreading into uninvolved areas and towards any surviving occupants. The number of vents to make is based on the fan and building size but for most residential structures, two to four windows provide for good positive pressure ventilation. Windows should be completely cleared of all glass, panes, screen, and curtains with doors blocked open so that they will not shut when positive pressure begins. When positive pressure is applied firefighters must be ready to advance and attack the fire. If for any reason, fire attack is delayed or stalled, positive pressure should be discontinued until the problems can be resolved. With positive pressure properly applied, firefighters will operate in reduced temperatures and improved visibility as fire, heat, smoke, and steam are forced out to the exterior of the structure.