Flow-path, by Paul Grimwood
29 Mar 2018

Survive in the Flow-path


The flow-path is the route that air/oxygen takes when it heads in to feed the fire and the route that fire, smoke or heat heads for, to leave the building. What determines the direction and force of this pathway is what the fire likes to do, what we like to do and also what the wind likes to do.


Paul GrimwoodFrom the book EuroFirefighter 2: Fire Engineer's Manual, by UK firefighter legend Paul Grimwood. The book, which can be ordered though this link , is also for Fire Engineers, Architects, Building Control  Officers and Students looking for information on firefighting access,  smoke control systems and practical fire dynamics.  Author's Royalties donated in aid of Katie Piper Burns Foundation.

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What is a 'flow-path' in a building fire?

Aside from any immediate rescues presenting themselves on the exterior of a structure, taking and maintaining control of the vent-profile in a building fire and deploying from the safest and most effective entry point may be life critical to both occupants and firefighters.

Deploying from the front door or from the 'unburned' side of the fire may NOT always be the best option.

What is the wind strength and direction?

Are there wind gusts that might impact on the fire?

Are there any openings where air is entering or smoke/fire is issuing?

Are there multiple openings or just one?

Is this likely to be a single one-room fire or has the fire spread to other levels?

Are we entering at the high point in a split-level building, with potential openings below us?

The flow-path is the route that air/oxygen takes when it heads in to feed the fire and the route that fire, smoke or heat heads for, to leave the building. What determines the direction and force of this pathway is what the fire likes to do, what we like to do and also what the wind likes to do!

We need to take control of all three factors. We need to be aware at all times of where the fire is heading and how that might change (or how we can change it) and how the density of the fire load might impact on firefighter heat stress where an increasing vent profile intensifies the fire.


External indicators of flow-path direction

It starts with the 360 size-up, taking into account all other information that might be feeding in to the commander, including visual, verbal, feeling and sound. This may be a very stressful time in which to stabilise a situation and effectively take control of the fire-ground.

Where are the openings? What are they doing?

Are they inlets for air, or outlets for smoke or flame?

Is the velocity of smoke or flame issuing slow and lazy, maybe wind driven, or is the fire in early stages? Is the smoke light or dark?

If the smoke is fast moving and black the fire is almost certainly grossly under-ventilated and heading towards its peak intensity.

Look closely at outlet vents - where is the neutral pressure plane (NPP) situated?

Are they fully filled from top to bottom with smoke or flame or is there a gap at the lower part, where air is feeding in?

If fully filled then there may be another point of air entering from elsewhere in the building and the door to this room is almost certainly open.

Do we deploy from the front, rear or side of the building.?

Remember, heading into a wind driven fire is our worst case scenario. If the wind is at our back, is there a vent needed on the other side in case the fire doubles back on us? In many cases we may not have the benefit of any such options.

Ideally, outlet vents should be larger than inlet vents, unless they are on the windward side of the building. The smaller the vent on the windward side, the less wind can impact the flow-path. Wind heading into floor to ceiling openings such as large sliding patio windows is likely to create the most intense of flow-paths.

Is the building split-level with one side lower than the other? In such a case, will we be deploying from above the fire's location or would it be better to enter from a lower level? If we choose the higher level to enter the building, the failure of lower level windows may cause a high velocity flow-path to head directly at us!


How our own actions can influence the flow-path

Consider PPV; Tactical venting; isolation and confinement; door entry; 360 and wind direction; (deployment point)

Firefighters should always attempt to deploy as follows:


  • Ensure an effective 360 size-up is undertaken prior to deployment, where possible
  • Start from a default position of closing down the structure and closing access points until ready to deploy
  • Feel the door for heat and observe smoke indicators to get some idea if the fire is grossly under-ventilated and searching for air
  • Consider forcing the door and utising a safe door entry procedure, with a charged hose-line
  • 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
  • Deploy from windward side of building if possible
  • Firefighters to work on cool side of flow-path at all times
  • Firefighters not to place themselves on hot side of flow-path without isolating fire from their position (VEIS)
  • Firefighters to anticipate and prepare for any flow-path reversal - consider emergency escape routes and methods!
  • Use flow-path reversal techniques to reduce heat flux at firefighter locations, or to change a bi-directional flow-path into a unidirectional flow 
  • When venting, first consider what this might do to the flow-path and fire development (IMPORTANT) 
  • 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
  • Almost Never vent on the windward side of the building, where firefighters or occupants may be on the leeward side - a coordinated and communicated VEIS action may be the only time you should do this!
  • The vent outlet should generally be equal in size to, or be larger than, the vent inlet
  • 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
  • Take every opportunity to utilise  a transitional attack starting from the outside, where this may create a safer entry for firefighters when heading against a possible wind driven scenario


Flow-path reversals

A flow-path reversal can be a very dangerous occurrence, or if planned for by firefighting actions, it may be very advantageous. Imagine it like this – you are driving fast along a three-lane motorway in heavy traffic when suddenly, all the cars have turned completely around and are now heading back towards you at the same speed! That sounds pretty scary right?!

That analogy demonstrates just how flow-path reversal can suddenly appear! The phenomenon that is referred to as flow-path reversal has been responsible for multiple firefighter deaths and is something to be anticipated and controlled (or preferably avoided).

Taking a look at fire videos of simulated or training corridor fires, one might ask; what makes a fire turn right along a corridor when leaving a fire apartment instead of left (or vice versa)? Think about that!

The hazards associated with flow-path reversal under exterior wind conditions are not solely reserved for high-rise fires. A wind-driven fire can occur at any time and in any building.

Know the wind speed and direction when you book on duty and take account of these two critical features in tactical size-up and deployment at every fire as you dismount from the fire engine.

If the fire is heading out of an opening, say a window to the exterior on a lower floor, but an opening is made on a higher floor or at the roof. The initial window outlet may then become an air inlet and the flow of hot gases and smoke will reverse in direction and head for the higher outlet.

A flow-path reversal could occur in other ways, for example if a window fails, or is vented, and a wind enters the opening causing the fire to head for lower pressure areas in the building.

Also, if automated smoke control (extract) systems are incorrectly configured to pull hot smoke and gases towards the access stair instead of away from, this can reverse the flow-path towards advancing firefighters in a corridor.

There are other occasions where the flow-path can be reversed in situations where air can actually travel down a stair-shaft as openings occur through fire spread, or are created. In fact, some smoke control systems are designed to do this, directing combustion products towards a smoke shaft in the building.

The dangers of flow-path reversal are clear to be seen where firefighters may be occupying the cool-side of the flow-path, only for it to be reversed. However, some venting actions may even pull smoke and heat away from firefighters and place them in the cool zone.


Ventilation profile and Heat profile

The vent-profile is related to the size of the openings, the wind strength and the amount of air that is able to enter the building and impact on fire development, as well as the amount of heat and combustion products that can leave the building.

The flow-path is the route between the two (or more) points. The heat-profile relates to the fire intensity (measured in MW) and the speed of fire development, or fire spread (measured on a time/temperature curve, or as a fire spread rate in square metres/minute). 


Control the flow-path to tactical advantage

stack effect inside/outside temperature differences; smoke control system design and configuration; 

Three main ways of controlling the flow-path:


  • Isolating the flow-path (close doors; use smoke blockers and wind control devices)
  • Ventilating the flow-path (reversing the flow-path with intent)
  • Cooling the fire gas layer existing near the ceiling (3D water-fog)


In taking these three tactical approaches, the use of positive pressure ventilation attack (PPA) (or Hydrovent) is most likely to have the biggest impact in controlling the flow-path to the advantage of limited staffed crews.

Corridor Fires: In situations where fire and smoke spreads into the building's accomodation corridors, there are several elements that may cause a flow-path to become established:


  • Open or fire vented window in an apartment
  • Open stair door
  • Existence and size of an opening at the head of the stair
  • Opening into a smoke shaft
  • Forced air-flow caused by smoke ventilation system


The direction and velocity of the flow-path will be determined by:


  • The height of the fire floor in a tall building (stack effect)
  • External wind conditions and direction
  • The design and activation of any smoke control system
  • The configuration of any smoke control system


The fire commander must establish the likely effects and impact on flow-path direction and velocity and consider how this may be reversed if needed, to assist the firefighting operation, particularly in long corridors. For example, door control at a stair, vent control at a stair head and deactivation of the smoke control system are all tactics that can be used to reverse or redirect a flow-path, to tactical advantage.

There is nearly always a ventilation flow-path pre-existing in these long corridors. This flow-path will determine flame velocity in the overhead gas layers and with air velocities at the floor of around 6 m/s (13.4 mph) heading towards the fire, there is a major cooling effect at firefighter locations.

This cooling effect in the flow-path can be used to advantage by firefighters and will initially prevent high heat fluxes hitting the floor from burning gases in the overhead. The faster the gas layer is moving, the greater the cool air mass is drawn in below.

In a room fire this would be dangerous for firefighters but in a long corridor the flaming combustion in the gas layer will stay high for a period, until an external wind reverses the flow-path or surface linings ignite, causing the thermal layer to drop down and eventually fill the space.

Therefore, for short runs of corridor from the stair, the light of the fire can be used to position the nozzle effectively close to where fire is entering the corridor, prior to opening the nozzle and causing the smoke layer to instantly drop to the floor. With a few pulses of water into the overhead where needed, visibility can be maintained until a high-flow direct attach is aimed at the fire.


Stairshaft Fires:

Where smoke and heat, or even fire, is entering a stairshaft this may have some major impact on occupants occupying levels above. The spread of smoke throughout upper levels is likely to force occupants to windows, if they can reach them. If they open windows this may cause the heat or fire to worsen in the stair. Those that cannot reach windows may be overcome and prevented from escaping. Therefore it is vital that stairs are protected from smoke at all times to enable firefighters to ascend and occupants to escape.

If the fire is actually in the stair shaft, conditions may be severe at upper levels. The application of water at the base of the stair can be used in conjuction with a vent at the head of the stair to create an upflow of water vapour that will control any fire in the stair very quickly.

The use of PPV to force vent a stair should be avoided until it is certain the fire has not spread into the floor levels. Creating or allowing openings at the head of the stair may impact on the fire in two ways. Before the fire is under control an opening may actually 'pull' fire or smoke into the stair causing a flow-path reversal.

If the fire has been suppressed and is under control, an open roof vent in the stair is going to raise the smoke interface and quickly release the heat and smoke from upper levels. There is much scientific research supporting roof vents in stairs but timing their opening can be critical, depending on any openings at lower levels in the stair in relation to the fire location.

Paul Grimwood