A Technical Article from the NZFBI - originally written by Kevin Peacock, Past President NZFBI, and published in the NZFS Star Magazine 2002.
Fire ventilation is an essential factor in efficient fire control. Yet there are other considerations and recognition of the conditions that determine the success or failure of the operation and the type of ventilation to employ.
The three types of ventilation—natural ventilation, mechanical or negative pressure ventilation and positive pressure ventilation—all involve different techniques. Failure to identify the impact on the fire conditions as they arise may jeopardise an entire operation.
However, when the correct method of ventilation is employed the advantages quickly become apparent.
Proper ventilation simplifies and expedites the rescue of victims by removing smoke and gases which endanger occupants who are trapped or unconscious and by making conditions safer for firefighters.
The removal of smoke, heat and gases from a building allows firefighters to more rapidly locate the fire and proceed with extinguishment. Correct ventilation of a building further enables firefighters to determine the path of travel of fire and to take proper steps for its control.
Backdraft and flashover
Backdraft and flashover are terms well known to firefighters. Although the causes are not the same, the conditions for both to occur are not dissimilar in that unburnt gases ignite and burn rapidly.
Backdraft conditions involve the confinement and intense build-up of heated gases in an oxygen-depleted environment. Whereas flashover is total involvement of all the flammable vapours from materials, which have reached their ignition temperature in an oxygen sufficient environment.
Adopting correct ventilation methods that control the release of heated gases or minimise the supply of air to the fire can control both situations.
The mushroom effect related to fire development and lateral spread at the top of a structure can be used to our advantage if the ability to ventilate at this level is available to firefighters.
We have all experienced the advantages in firefighting gained when sections of common plastic roof material usually installed to enhance natural light in commercial buildings has melted and allowed smoke and heat to disperse rather than spread. The advantages gained in terms of improved visibility, the ability to observe obstacles, quick identification of the fire seat, and reduced heat conditions all enhance knock down and control of the fire.
Natural ventilation both lateral and vertical by utilising building openings such as skylights, ventilation shafts, stairway doors, windows and removal of a section of roof are the most common and fundamental methods.
The decision to open a roof to assist ventilation is an operation that requires careful consideration and experience. Coordination is essential with the crews working inside the structure, observation of the wind direction is important to establish a downwind location especially on sloping roofs, utilise existing roof openings and note any roof loading such as header tanks and other building services equipment.
Avoid structural supports, work with the wind to your back or side, and cut one large hole rather than several smaller ones. Utilise rooflines and safety harnesses on all occasions.
Mechanical and negative pressure ventilation methods
A number of mechanical and negative pressure ventilation methods can be deployed such as utilising the natural venturi principle created by locating hose streams in door or window openings discharging to the exterior. This method has been used by firefighters for generations and is highly effective given the right conditions.
Other mechanical methods using Hi-X foam generators or extractor fans including some air conditioning systems can also be utilised to create negative pressures inside a structure thereby increasing air flows which assists in the removal of smoke and hot gases.
The correct utilisation of positive pressure ventilation introduces cool air and can increase localised flaming. But the advantages in removing the hot gases and replacing them with cool air reduces the temperature and ability of the fire to develop. The clear entry path created by the fan improves visibility, reduces temperature dramatically and allows firefighters to locate and extinguish the fire more quickly.
Whichever method is employed, the importance of implementing some form of tactical ventilation at an early stage in fire operations positively assists in rapid control, quicker knock down, safer operations and a more sophisticated and professional approach to limitation of damage and fire extinguishment.