Firefighting 101                                                         by:  Dan Howell

Positive Pressure Ventilation

Ventilation is one of the most important operations to be performed by firefighters and they have three basic types of ventilation at their disposal. These include vertical, horizontal, and forced ventilation. Vertical and horizontal ventilation use natural and fire generated currents to move heated gases from the structure. Forced ventilation uses fans and hose streams to mechanically move air, which allows firefighters to vastly improve both horizontal and vertical ventilation. The three basic methods of forced ventilation include hydraulic, negative pressure, and positive pressure ventilation. Of these methods of forced ventilation, positive pressure ventilation is one of the most effective at removing smoke and heated gases from a burning structure. Unfortunately, positive pressure ventilation is commonly misunderstood and improperly used. This can be avoided if firefighters develop an understanding of the basic principles, set-up, and influencing factors of positive pressure ventilation. Once firefighters have accomplished this, they will be prepared to make proper use of one the most effective methods of ventilation.

The basic principle behind positive pressure ventilation is using a fan to move air from the exterior of the structure and forcing it into the interior. This action increases the air pressure on the interior to a level greater than the exterior. As a result, smoke and heated gases are forced out of the structure through any vents available. This process creates a path of moving air that follows the path of least resistance and travels from the fan to the vent. Any smoke, fire, and gases in this path of air will be pushed towards the vent. Firefighters control this path of air by making ventilation openings in the areas of the structure they wish to ventilate. This allows positive pressure ventilation to systematically ventilate one area at a time in the structure.

The basic set-up for positive pressure ventilation is a simple two-step process. First firefighters determine where they wish to ventilate. They will then most commonly use windows and doors to provide the needed vent openings. Windows can be broken or raised based on the needs at the time. Doors can be opened and blocked so that they will not slam closed when ventilation begins. Once ventilation openings have been made, the second step is to set-up the fan to force air into the structure. Normally, the inlet for the air will be the door that fire attack was launched through. The fan should be positioned so that the airflow completely cones the door. The fan should be just far enough back to cone the door with air. When the fan is too far back, the effectiveness of the operation will be substantially reduced. If space is limited, there may not be enough room to position the fan far enough away to cone the entire door. When this situation arises, the fan should be placed as far back as possible to cone as much of the door as possible.

There are a number of important factors that affect positive pressure ventilation. The first of these is the airflow capacity of the fan. The standard range for gasoline-powered fans is 15,000 to 27,000 cfm. Electric positive pressure fans are also available. They commonly range from 8,000 to 16,000 cfm. Higher output fans are capable of forcing more air into a structure than lower rated fans. The amount of air available determines what volume of smoke can be forced from the structure. The capacity of the fan also determines how much area can be vented and how many vent openings can exist before positive pressure ventilation becomes ineffective. Again, the greater the rating of the fan, the larger the area that can successfully ventilated.

The size of the building and the number of vent openings present are two additional important factors that must be considered. These two factors are inversely related in their relationship to positive pressure ventilation. As the size of the structure increases, the number and size of vent openings must decrease in order to maintain control of airflow and ensure effectiveness of the operation. The reverse is also true, as the size of the structure is reduced; more vent openings are necessary to prevent air from being “burped” back out the entrance. If the fan puts 20,000 cfm into the structure, 20,000 cfm must be able to escape in order to prevent smoke and heat from being forced back out the entrance.

As can be seen, the size of the fan, building area, and the number of vent openings are closely related. The “Positive Pressure Triangle” can be used to illustrate their relationship. When each side of the triangle is properly balanced, a large amount of air can be forced a great distance with a good degree of firefighter control.

                Two other factors that can influence positive pressure ventilation are the size of the opening and the amount of fresh air available. The size of the opening will determine how difficult it is to obtain a complete seal with the cone of air. Normal entrances do not present a problem but larger openings such as garage doors are impossible to seal with a single fan. A single fan placed in such an opening will have limited success since a large amount of air will escape around the cone. It may take two or three fans to seal such an opening. Fresh air becomes a factor when the fan is moved into the interior of a structure. Sometimes, due to the size and layout of structure, it will be more practical to only pressurize the effected area. If adequate fresh air openings are not provided to the exterior, the fan will only circulate smoke throughout the structure. Fresh air is also necessary for gasoline-powered fans to operate. In addition, positive pressure fans should never be used as smoke ejectors; they are not designed for negative pressure ventilation!

Positive pressure ventilation is one of the most effective methods of ventilation. Firefighters must fully understand the principles, set-up, and influencing factors to benefit from the many advantages of positive pressure ventilation. Future articles will continue with positive pressure ventilation by examining its use during overhaul and fire attack.