Under-ventilated Fires


A more dangerous firefighting environment

In Francais HERE

In Italiano HERE

For compartment fires, the transition period between a fuel and ventilation controlled fire is usually defined as ‘flashover’. An under-ventilated fire may be defined as one where fire development has been curtailed by a lack of air supply and an accumulation of highly flammable gases have formed within an enclosed fire compartment. The hazards associated with under-ventilated building fires are increasing, as fire loads incorporating an increased hydrocarbon (plastic) content, combine with modern energy efficient construction to create exceptionally dangerous working environments for firefighters. It is a fact that firefighter life losses in the UK have near doubled since 2005 as a ratio of the number of building fires and the relationship between under-ventilated fires and tactical ventilation by firefighter’s demands far greater attention, if such life losses are to be curtailed.  

  • Fuel-controlled fire - plenty of air available
  • Ventilation-controlled fire - limited air available
  • Under-ventilated (unventilated) fire - almost zero air available

Why would you ventilate?

At every fire the default position should be to begin by immediately controlling the air flow-paths, where practical. This means that any open access doors might need to be partially closed to prevent air flowing in to feed the fire, whilst we prepare our deployment with charged hose-lines. In 1987 Paul Grimwood's Tactical Ventilation strategy was proposed as a compromise between US and UK ventilation approaches and was defined as follows:

‘Tactical ventilation is the containment or venting actions by firefighters, used to take control from the outset of a fire’s burning regime, in an effort to gain tactical advantage during interior structural firefighting operations’.

Such an approach recognised that there were clear benefits in some cases of tactically creating openings in the fire building but in many more situations there was a danger that creating such openings without an objective, without precision and timing, that the situation for building occupants may become worse. This was particularly evident where firefighters were occupying the building.

To be able to understand when, where, why and how to ventilate fire buildings demands awareness and knowledge of practical fire dynamics. It also requires on-scene intelligence of where the fire is located and where occupants/firefighters may be sited. It needs an understanding of flow-paths (previously termed 'air-tracks' in the UK).

In EuroFirefighter-1 (2008) on p78 I repeated a critical piece of guidance that has preceded flow-path research by some decades. It is a basic fire-ground rule that many seem to forget - 'Don't vent in situations that place interior crews between the fire and vent opening you are creating'. For example, if crews are fighting a basement fire, any openings you might make behind and above them to relieve smoke can literally cause the newly created flow-path to 'pull' intense flaming or hot smoke directly towards them. 

Firefighter Fatalities in Flow-path events

A review of firefighter fatality reports in the UK and USA over the past 15 years demonstrates several incidents where firefighters lost their lives, as a result of being located between the fire and the outlet vent that was created, or on occasions - where the outlet created was at a higher point than the firefighters location (drawing the fire to them). On occasions there may have been flow-path reversals as interior pressure differentials balanced each other where outlet vents were under-sized. In other situations, the venting of fire buildings by fire commanders caused the fire to intensify to a point where conditions became untenable for firefighters, leading to heat effects on the body causing firefighters to make bad decisions when trying to exit the building, becoming lost and entrapped in the process. 

In the UK there were three fatal fires where tactical venting actions occurred, placing firefighters between the fire and the outlet vent. There were other fatal incidents where tactical venting actions caused fire intensification to a point where firefighters became disoriented and entrapped.

In the USA there have been several incidents where firefighters deployed from the leeward side of the fire building, resulting in a wind driven fire trapping and killing them as windows on the other side of the building failed through heat. In other situations firefighters deployed from the front into the upper level of a split level building, causing unfavourable flow-paths to lead the fire right at their location.

In each of these cases, firefighters were caught by flow-path reversals, causing fire to head back towards their entry point and causing untenable levels through the venting actions.

When is tactically venting a fire beneficial?

In some well equipped big city fire departments, with adequate or high staffing levels, a pre-planned procedure for vertical venting of fire buildings under strict protocols may be beneficial in certain types of buildings. Roof venting of scuttles, hatches or doors may be very useful after the fire has been controlled. The strategy of making roof cuts with power saws is a high-risk operation that may increase firefighter life loss rather than prevent it and demands highly experienced firefighters and a sound knowledge of roof construction. The likelihood that older roofs have been reconfigured and redeveloped over decades means that roof operations become even more dangerous.

Horizontal ventilation is a strategy well within the reach of even the most limited of staffed fire crews but again, an awareness of practical fire dynamics and street-wise fire commanders/firefighters are essential. Before creating horizontal openings, the following considerations should be foremost in the commanders/firefighters mind:

  • Have a clear pupose or objective in mind
  • Be acting under a clear directive to do so
  • Know where the fire is and where firefighters are located
  • Know where the wind direction is heading and how this may impact on any openings
  • A wind from behind us heading into an open entry door is like a PPV airflow - do we need to make an outlet, or do we need to close the door?
  • Only vent with ALL interior fire crews guidance and feedback in mind!
  • Consider any likely impact on fire intensification and how this may affect firefighters and occupants who may be inside at various locations
  • Consider if the water is being applied effectively to the fire
  • Align the inlet and outlet vents, making sure that in general, the outlet is larger or equal to the inlet vent; and that outlets are higher than inlets
  • Most importantly, don't vent if firefighters may be located between the fire and the vent you are about to make
  • Consider the density of the fire load and access to it for extinguishing purpose - a very dense fire load may burn more fiercely if more air reaches it
  • Communicate to all sectors what is about to occur in terms of vent location

Through a series of articles in the late 1980s Paul Grimwood  presented a case for a review of ventilation tactics in the UK and this  resulted in a government sponsored report by Adrian Hay (1994) looking  at ventilation by firefighters. In this report the author introduced  Grimwood's term 'Tactical Ventilation' but failed to define the term  correctly and omitted to talk of fire isolation tactics and controlling  air-flow feeding into the fire (Grimwood 1987). He then concluded that a  US approach to venting fire buildings by UK firefighters may well be  beneficial but this would be subject to further research. Paul Grimwood did urge caution in Adrian Hay's 1994 interview with him and this was reported in the document as follows:

  • “Mr.  Grimwood began by highlighting the fundamental differences between the  US and UK approaches to firefighting. In his experience, US firefighters  are, in the tactics that they employ and in their mental attitude, both  practically and psychologically more committed to aggressive  firefighting than their UK counterparts. He cautioned, however, that  this approach has both advantages and disadvantages. He summed up the  fundamental difference in strategy by explaining that US fire  departments are organised on a team system to go looking for trouble in  the early stages of a fire [proactive], whereas the UK firefighters  react to trouble as it occurs [reactive]. In terms of tactical  ventilation, he felt that the US firefighters have a tendency to  over-vent, in that they sometimes vent structures unnecessarily simply  because it is standard practice (this claim was strongly refuted by US  firefighters contacted during the course of this research). However, he  also believes that the UK fire service tend to under-vent in the  opposite extreme. He cannot recall any fires where ventilation was used  as an early tactic by the UK fire service but can quote many examples  where failing to ventilate at all has resulted in unnecessary fire  damage and even the total loss of a building. He suggests that the  correct balance in tactics lies somewhere between the two extremes.  Mr.  Grimwood highlighted four essential requirements for successful  ventilation operations: communication, co-ordination, precision and anticipation.  Any attempt to ventilate a building must be co-ordinated with interior  attack and this requires good communications between the different teams  and the Officer in Charge. Openings in the structure of the building  must be made precisely to ensure that they do not cause fire spread.  Anticipation of the effects of ventilation is required so that all the  outcomes can be prepared for and the risks covered.  Manning levels and  equipment differences between US and UK fire brigades were discussed.  Mr. Grimwood observed that manning levels in the USA were generally  higher than those in the UK and that consideration would need to be  given to increasing the number of firefighters in attendance if there  were a move toward ventilation in the UK. More importantly, however, he  felt that more hydraulic platforms and more cutting equipment would also  need to be made available.  In closing, Mr. Grimwood drew attention to  the tactic of fog attack, which he strongly believes is the firefighting  tactic of the future. He questioned whether UK fire brigades should be  introduced to ventilation before they were made better aware of the  benefits of offensive fog attack. He pointed out that the latter tactic  would be easier to introduce and that it worked effectively in  ventilated rooms. In summary, Mr. Grimwood made a very useful  contribution to the practical aspects of ventilation operations and the  important differences between the UK and US fire services. His book  contains a number of case studies which assist in developing an  understanding of the advantages and disadvantages of ventilation”.

Adrian Hay 1994

Golden rules of tactical ventilation

  1. Start from a default position of closing down the structure and closing access points until ready to deploy
  2. Every door is a ventilation point! Consider if by closing the entry door, the growth and intensification of the fire will cease or reduce dramatically
  3. Never create an opening that may place firefighters between the fire and the vent opening, or cause the flow-path to reverse in their direction
  4. Never vent on the windward side of the building, where firefighters or occupants may be on the leeward side 
  5. The vent outlet should generally be equal in size to, or be larger than, the vent inlet
  6. If venting a building containing dense fire loading, understand the temperatures may become untenable for firefighters who will be unable to maintain an interior attack for any reasonable time period
  7. Consider Positive Pressure Attack (PPA) before entry and take 30-60 seconds to see the effects on fire and smoke

Vent-Enter-Isolate-Search (VEIS)

One life safety strategy, termed VEIS, is something that may be adapted to suit both well-staffed or limited-staffed fire departments. In situations of known, likely or confirmed occupant entrapment in a room easily reached from the exterior, a VEIS operation should encompass the following actions, even in situations prior to fire attack taking place:

  • Close all access doors into the building to slow fire development
  • Locate the target room and place a ladder for access
  • Feel the window for heat and observe smoke or oily deposits on glass, denoting a high heat level within the room and an under-ventilated flow-path forming - consider other options first
  • If closed, create an opening large enough for one firefighter to enter the room as quickly as possible
  • Briefly utilise a thermal image scan before entering the room and then hand this back to the second firefighter at the head of the ladder
  • On entering the room to search the priority is to close the room's door to gain protection, even before attempting to extract a person from the room
  • Consider taking a 5 metre line/rope into the room from the head of the ladder, to assist in locating the window again in heavy smoke
  • If the fire is within the room itself, consider taking a hose-line into this opening or apply water in transitional fashion
  • The second firefighter should site themselves at the head of the ladder to assist where necessary
  • Do not search beyond the target room or enter the hallway but repeat VEIS from the exterior for other rooms as necessary
  • Before closing the room door to isolate the fire, take a brief look beyond and into the hallway to see any persons who may be overcome on the floor

HERE is a useful video of VEIS in action 

If VEIS is deployed effectively, this strategy has a medium risk/high return benefit - consider fires you have attended where this strategy may have provided a better outcome for occupants.

The application of VEIS demands great care and is often seen as a 'last resort' strategy. In effect, the entry firefighter may be placing themself directly into the exit point of a dangerous flow-path. Consider all other options first. It may be safer and more effective to deploy one of the following approaches before resorting to VEIS:

1. Attack the fire first - this may save more lives, even if working from the exterior or using water (or CAFS) off the tank supply

2. Isolate the fire on entry to the ground floor level by closing doors, or by siting a support hose-line to protect egress (at the stair) before searching beyond or above the fire compartment.

3. Coordinate search and suppression together in unison, if staffing is adequate.

Paul Grimwood 2017