Fire Behavior Indicators and Fire Development - Part 2

Previous articles have addressed building factors, smoke, air track, heat, and flame (B-SAHF) as critical fire behavior indicators. Understanding the indicators is important, but more important is the ability to integrate these factors in the process of reading the fire as part of size-up and dynamic risk assessment. This is the second of two articles focused on fire behavior indicators and the stages of fire development.

Part 1 of FBI and Fire Development examined fire development and related fire behavior indicators from ignition through the growth stage. Part 2 examines continued fire progression into the fully developed and decay stages with a strong look at ventilation controlled decay and ventilation induced extreme fire behavior.

Fully Developed Stage
At this post-flashover stage, energy release is at its greatest, but is generally limited by ventilation (more on this in a bit). Unburned gases accumulate at the ceiling level and frequently burn as they leave the compartment, resulting in flames showing from doors or windows. The average gas temperature within a compartment during a fully developed fire ranges from 700 to 1200 degrees C (1292 to 2192 degrees F).

Remember that the compartment where the fire started may reach the fully developed stage while other compartments have not yet become involved. Hot gases and flames extending from the involved compartment transfer heat to other fuel packages (e.g., contents, compartment linings, and structural materials) resulting in fire spread. Conditions can vary widely with a fully developed fire in one compartment, a growth stage fire in another, and an incipient fire in yet another. It is important to note that while a fire in an adjacent compartment may be incipient, conditions within the structure are immediately dangerous to life and health (IDLH). See Table 4.

If the fire in our residential scenario has progressed to the fully developed stage (in the compartment of origin) what fire behavior indicators might be observed? Use the B-SAHF model to help you frame your answers.

Scenario 1:
You have responded to a fire in a one-story single-family dwelling of wood-frame construction. A fire which started in a bedroom on the Alpha/Bravo corner of the structure has reached the fully developed stage and now involves the contents of the room and interior finish of this compartment.

  1. What conditions would you expect to see from the exterior of the structure?
  2. What indicators may be visible from the front door as you make entry?

Remember that fire conditions will vary throughout the building. While the fire is fully developed in the bedroom, conditions may be different in other compartments within the building.

Decay Stage
A compartment fire may enter the decay stage as the available fuel is consumed or due to limited oxygen. As discussed in relation to flashover, a fuel package that does not contain sufficient energy or does not have a sufficient heat release rate to bring a compartment to flashover, will pass through each of the stages of fire development (but may not extend to other fuel packages). On a larger scale, without intervention, an entire structure may reach full involvement and as fuel is consumed, move into the decay stage. However, there is another, more problematic way for the fire to move into the decay stage. When the ventilation profile of the compartment or building does not provide sufficient oxygen, the fire may move into the decay stage. Heat release rate decreases as oxygen concentration drops, however, temperature may continue to rise for some time. This presents a significant threat as the involved compartment(s) may contain a high concentration of hot, pyrolized fuel, and flammable gaseous products of combustion.

Ventilation Controlled Fires
Under ventilation controlled conditions, excess pyrolizate and flammable products of combustion present in smoke are a significant hazard to firefighters. Let's go back to the fire triangle (see Figure 4) to examine the nature of this threat. While fuel, heat, and oxygen are present in proportion to support combustion where the fire is burning, the heat of the fire is pyrolyzing more fuel vapor than the fire can consume. In addition, incomplete combustion results in production of flammable gases such as carbon monoxide. The speed of fire development is limited by the availability of atmospheric oxygen provided by the current ventilation profile of the compartment or building.

When the fire is burning in a ventilation controlled state, any increase in the supply of oxygen to the fire will result in an increase in heat release rate. Increase in ventilation may result from firefighters making entry into the building (the access point is a ventilation opening), tactical ventilation (performed by firefighters), or unplanned ventilation (e.g., failure of window glazing due to elevated temperature).

It is essential to recognize when the fire is, or may be ventilation controlled and the influence of planned and unplanned changes in ventilation profile. Most compartment fires that progress into the growth stage are ventilation controlled when the fire department arrives. A bi-directional air track (smoke out the top and air in the bottom) is often a significant indicator of a ventilation controlled fire. As illustrated in Figure 5, as the neutral plane drops, smoke exiting through the opening reduces the size of the inlet opening, further restricting the air available for combustion and extent to which the fire is ventilation controlled.

The series of photos in Figure 5 also illustrates the influence of decreased oxygen supply on the color and optical density of smoke. As combustion becomes more incomplete, smoke production increases, color darkens, and optical density increases. When working inside a burning building, flames moving through the hot gas layer are also a strong indicator of a ventilation controlled fire.

Ventilation Induced Extreme Fire Behavior
When the fire is ventilation controlled, increased air supply to the fire will result in increased heat release rate and depending on conditions may result in extreme fire behavior such as flashover or backdraft. While similar, vent induced flashover and backdraft are different phenomena. When the fire is ventilation controlled and fuel gas and vapor in the smoke is below its ignition temperature, increased ventilation is likely to result in a vent induced flashover (see Figure 6). If fuel gas and vapor in the smoke is above its ignition temperature and the rate of combustion is generally much faster (deflagration) producing a more violent backdraft (see Figure 7).

While these phenomena are different, both present a significant threat to firefighters. Rapid fire progress due to ventilation induced flashover or backdraft is not an instantaneous process. Depending on a number of variables such as the location of the fire, current level of involvement, temperature of the smoke (hot gas) layer, and extent of the increase in ventilation these rapid fire progress phenomenon may take some time to occur. However, when it does, fire development will be extremely rapid! Firefighters entering a compartment or building containing an under ventilated fire must be aware of and manage the hazards presented by the potential for rapid fire progress. Remember, many if not most fires that have progressed beyond the incipient stage before firefighters arrival are ventilation controlled and present the potential for rapid fire progress with increased ventilation.

Table 5 lists the fire behavior indicators related to ventilation controlled decay stage conditions and the potential for ventilation induced extreme fire behavior. It is important to note that there are not always clear distinctions in the visual indicators for vent induced flashover and backdraft.

It is often assumed (incorrectly) that ventilation induced extreme fire behavior (flashover or backdraft) will occur immediately after an increase in ventilation. Depending on fire conditions and building configuration there may be a significant time lag between ventilation and resulting changes in fire behavior. When ventilation controlled decay conditions are indicated (or suspected), firefighters should move cautiously and take action to change conditions inside the building or compartment (e.g., gas cooling, ventilation).

You have responded to a fire in a one-story single-family dwelling of wood-frame construction. A fire which started in a bedroom on the Alpha/Bravo corner of the structure has gone from fully developed to the decay stage due to a lack of oxygen as building openings (doors and windows) remain closed and intact.

  1. What conditions would you expect to see from the exterior of the structure?
  2. What indicators may be visible from the front door as you make entry?

A fire in the decay stage (particularly when this is due to limited oxygen) still presents a significant threat as conditions can change rapidly.

The fifth edition of the International Fire Service Training Association (IFSTA) Essentials of Firefighting includes an expanded chapter on fire behavior that provides simple, but solid explanation of the fundamentals of fire behavior and dynamics of compartment fire behavior. The second edition of IFSTA Fireground Support Operations (currently under development) will expand on the basic fire behavior information presented in Essentials to address the influence of ventilation on fire behavior, fire behavior indicators, and reading the fire.

Do you have a good photograph or video clip illustrating building, smoke, air track, heat, or flame indicators? If you do and would be willing to share it, please send a copy to I will be working to include photographs and video clips in future articles to provide you with an opportunity to apply your knowledge of fire behavior and skill in reading the fire.

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ED HARTIN, MS, EFO, MIFireE, CFO, a Contributing Editor, is a Battalion Chief with Gresham Fire & Emergency Services in Gresham, OR and owner of CFBT-US, LLC, a training company specializing in compartment fire behavior training. Ed has traveled throughout the world studying firefighting best practices and has delivered training programs throughout the United States, as well as in Australia, Malaysia, Chile, Germany, Croatia, and Sweden. Ed is one of the co-authors of 3D Firefighting: Techniques, Tips, and Tactics published in 2005 by Fire Protection Publications. In addition, he has authored numerous articles for, Crisis Response, and Fire-Rescue magazine. To read Ed's complete biography and view their archived articles, click here. You can reach Ed by e-mail at