Tactical ventilation

Ventilation can be defined as:

‘The removal of heated air, smoke or other airborne contaminants from a structure or other location and their replacement with a supply of cooler, cleaner air’

Ventilation is something that will occur naturally as part of the fire development and decay process. It will have an impact on the development of a fire before and after the-arrival of the fire and rescue service. However, ventilation is also a tool that should be considered as part of any overall firefighting strategy.

Ventilation can be performed after the fire has been extinguished or controlled, to clear residual smoke and heat from buildings or structures.

The benefits of controlled and co-ordinated tactical ventilation should be balanced against the hazards associated with accelerated fire growth and the introduction of oxygen into under-ventilated fires in buildings.

If applied and managed correctly, tactical ventilation can provide beneficial effects to any firefighting strategy by:

• Replenishing oxygen and reducing carbon monoxide levels
• Controlling temperature and humidity
• Removing moisture, dust, and other airborne contaminants
• Improving visibility and aiding navigation

Tactical ventilation is a planned intervention that requires co-ordination and control, to open up buildings and structures to release the products of combustion and can be defined as:

‘The planned and systematic removal of heat and smoke from the structure on fire and their replacement with a supply of fresher air to allow other firefighting priorities.’

As part of an overall firefighting strategy, incident commanders should always have a clear and informed objective before commencing any form of ventilation activity. This will ensure that the full range of benefits of ventilating can be realised including:

• Improving conditions for the survivability of people in the building
• Improving conditions for personnel to enter and search for or rescue people
• Reducing the potential for rapid fire development, including flashover, backdraught, or fire gas ignition
• Restricting fire and smoke damage to property

In broad terms ventilation can be separated into two types:

Natural ventilation – This is the process of supplying and removing air through a structure or space without using mechanical systems. In firefighting terms, this refers to managing the flow of air (flow path) into and out of a structure or location, using the prevailing atmospheric conditions, such as wind strength, speed, and direction, via structural openings, such as windows, doors, and vents, to clear any smoke or hot fire gases.

Forced ventilation – This is the process of using fans, blowers or other mechanical means or devices to assist in creating, redirecting, and managing the air flow (flow path) into and out of a structure or location, so that heat, smoke and fire gases are forced out.

In both instances, additional factors related to climatic and atmospheric conditions, such as temperature and pressure, will have an impact on the relative success of any ventilation activity.

Figure: Diagram showing positive pressure ventilation, courtesy of the Building Research Establishment

Figure: Diagram showing a heating, ventilation and air conditioning system in an atrium, courtesy of the Building Research Establishment

Figure: Diagram showing a heating, ventilation and air conditioning system, courtesy of the Building Research Establishment

The success of any ventilation plan or strategy will to a greater degree depend on the techniques employed to effectively plan and manage:

• Where air will enter a building, structure, or location (inlet vent)
• Where hot gases and smoke will leave a building, structure, or location (outlet vent)
• The route that they will take (flow path)

Personnel should be aware that creating a vent in a previously under-ventilated area can increase the risk of creating a backdraught.

In broad terms, two techniques may be considered, which present both barriers and enablers to the ventilation process:

• Vertical (or top) ventilation: Making an opening at high level to take advantage of the natural characteristics of hot gases and smoke, for example, buoyancy, allowing them to escape
• Horizontal (or cross) ventilation: Making openings in external walls, using doors and windows to aid the removal of hot fire gases and smoke

Both techniques can be employed using natural or forced methods of ventilation.

Locating the fire

Locating a fire is critical in formulating a robust, safe, and effective ventilation strategy. The following factors should be considered:

• The location of the fire may be evident on arrival, but it is possible that the fire has developed in hidden areas or not be visible at all. It is vital to identify any routes of potential fire development and any flow paths that may be created, considering the impact on firefighting operations and their potential to create or intensify undetected fire development.
• In the majority of incidents, ventilation should only be used when a fire has been located and the likely impact of ventilation has been assessed. If the seat of fire is difficult to locate, tactical ventilation can be used to clear adjacent rooms, corridors, or staircases to:
  • Help identify the seat of fire
  • Maintain safe access and egress routes to and from a hazard area
  • Reduce the potential of phenomena, such as fire gas ignition
• Monitoring systems, such as automatic fire detection systems or closed-circuit television (CCTV), can be used to identify the seat of fire
• Thermal scanning and thermal imaging equipment may help to identify the seat of the fire

Ventilation strategy

The ventilation strategy implemented at any fire will be affected by a range of factors but in broad terms, the strategy should initially be based around either one or a combination of the following:

• Offensive ventilation: Close to the fire to have a direct effect on the fire itself, to limit firespread and to make conditions safer for personnel
• Defensive ventilation: Away from the fire, or after the fire, to remove heat and smoke, particularly to improve access and escape routes and to control flow paths to areas of the building unaffected by the fire
• Control flow paths and anti-ventilation: Planned and co-ordinated confinement of fire gases and reduction of air flows into the fire, to prevent the fire and smoke from spreading, protecting access and egress, and limiting oxygen to reduce fire development

When planning and developing any ventilation strategy, it is vital that due consideration be given to the impact that any unplanned or poorly considered ventilation can have. The safety of personnel and people in the building is vital when forming a ventilation strategy. The impact and effects of ventilation and fire conditions should be constantly monitored and reassessed and, if appropriate, tactics should be adjusted.

The benefits and effects of any planned ventilation should be considered together with the:

• Location of the fire
• Location of people and protection of escape routes
• Access and egress of personnel
• Internal and external layout and design; including any fire-engineered solutions
• Likely fire dynamics and development
• Presence of natural ventilation, including local topography that may affect wind effects and pressure differentials
• Effect of heating, ventilation, and air conditioning (HVAC) systems incorporating smoke control, sprinklers, and design features, such as atriums and smoke curtains
• Impact of natural fire phenomena on fire development, for example Coandă, stack, trench or piston effects or wind-driven fire
• Potential for a dust explosion

Ventilation strategies should be reassessed continuously to ensure that safety is maintained and that any planned ventilation activities are supporting the overall incident plan, considering relevant factors including:

• Wind direction and strength
• Whether ventilation is appropriate and the correct ventilation tactics
• Whether personnel should be withdrawn while ventilation takes place
• Location of outlet vents, which should ideally be downwind and at a high level
• Whether external covering jets are in place
• Whether an inlet vent is created and kept clear, ideally as soon as possible following creation of the outlet vent
• The requirement to constantly monitor the effects of ventilation

Post-fire considerations

Consider:

• Using ventilation post-fire to assist in clearing any smoke and other airborne particles as part of the salvage activities
• Ensuring that bullseyes (hot spots) are identified and fully extinguished before the fire scene is handed over; turning over and damping down will assist in identifying such areas
• Advising the fire investigation officer or other agencies of any ventilation activities undertaken during firefighting operations, as this may have some relevance to the subsequent fire investigation in respect of fire development and post-fire indications; for more information refer to Operations – Preserve evidence for investigations