6.1: Fire Growth
- Page ID
- 66917
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Phases of Fire Review
Before examining aspects of enclosure fires, it is important to have a good understanding of the four phases of fire growth.
Take a moment to review the growth phases of fire:
- Incipient Phase
- Growth Phase
- Fully Developed Phase
- Decay Phase
Heat Release Rate
The heat release rate is the rate at which heat and energy is generated by burning.1 One of the biggest influences on modern fire behavior is the heat release rates of modern fuels. Heat release rate has a direct impact on fire growth and spread within a structure as well as oxygen availability.
Heat release rates are displayed visually on graphs called heat release rate curves that plot the amount of heat being released and the fire timeline. They can be displayed for a single item, such as a chair, or for an entire room or fuel package.
Video: Heat Release Rate and Temperature by Fire Safety Research Institutes, 2025. Used with permission; see Terms of Use.
See an example of fire growth of an office chair and the corresponding heat release rate graph.
Video "Heat Release Rate" by NYUFireResearch. YouTube Standard License.
The fire growth curve can visually depict a fire’s behavior within an enclosure. Each growth curve will show each phase of the fire from ignition to decay. However, because the ignition phase timeline varies greatly even with identical fuel packages, growth curves generally begin as the ignition phase begins to transition to the growth phase.
Traditional Fire Growth
Fires involving legacy fuels in traditional structures are typically what is considered a fuel-controlled fire. This means that the fire has the oxygen needed to consume the entire fuel load and the fuel availability controls the transition of the fire through each phase. These fires have a steady upward heat release rate through the growth phase, then level off during the fully developed phase until the fuel is consumed and the fire begins to decay. This pattern often results in a somewhat square fire growth curve.
Modern Fire Growth
Fires involving modern fuels which have much faster heat release rates, in modern structures that allow less oxygen entrainment within the structure, result in a fire that is oxygen controlled. Because of the higher heat release rates, the fuels consume oxygen much faster than traditional fuels. This results in an environment that lacks enough oxygen to support combustion.
Modern growth curves often enter an oxygen-limited phase as they reach the fully developed phase, resulting in a period of decay. If nothing changes in this environment, the fire will eventually extinguish itself due to lack of oxygen. However, often what happens is that oxygen is reintroduced. This reintroduction can be caused by a failure of a window by heat, or by someone opening a door or window. Many firefighters have been hurt or even killed by reintroducing oxygen to the fire while entering the structure to fight the fire. Even though a fire is in the oxygen-limited status, the residual heat still pyrolyzes fuel, so there is an increase in gaseous fuel waiting for that reintroduction of oxygen. Because of this, often, the fire transitions to flashover (described below) shortly after oxygen is introduced.
Flameover
Also called rollover, flameover is the condition where unburned fuel from the originating fire has accumulated in the upper layer to a sufficient concentration that it ignites and burns. This can occur without ignition of or prior to the ignition of other fuels separate from the origin.2
Example of flameover.
Video: "Fire fighting rollover demo" by DangerDad828. YouTube Standard License.
Flashover
Flashover is a transition phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously, and fire spreads rapidly throughout the space, resulting in full room involvement or total involvement of the compartment or enclosed space.3
Not every compartment fire will transition to flashover. Fuel type and availability, as well as oxygen availability, are key to the room transitioning. While a single upholstered chair contains enough energy (heat of combustion) to transition a typical residential room into flashover, there needs to be enough fuel that can be pyrolyzed in order to bring the compartment into flashover.
Flashover is considered unsurvivable for firefighters or occupants so it is important that firefighters are able to recognize when flashover may occur and either cool the space, or exit the area.
There are several indicators that can be observed by firefighters that indicate a compartment is beginning to transition to flashover. These indicators are typically categorized as technical and non-technical.
The technical indicators of flashover are temperature and heat flux. While these indicators are important, they do not mean much to firefighters inside a burning structure.
The non-technical indicators of flashover are observations that have been reported as the environment nears or transitions to flashover. Below are some of these indictors:
- Lowering neutral plane – In some instances, as the upper layer becomes hotter and more gasses accumulate the neutral plane will lower as pressures increase in the space. This will be dependent on ventilation profile of the space.
- Visible pyrolysis – often the fuels in the entire room will begin to pyrolyze due to the radiant and conductive heat transfer from the superheated gasses in the upper layer. This pyrolysis can be seen just before flashover in the above video.
- Breaking windows – Often as a room transitions to flashover, the rapid increase in temperature will result in windows breaking. This action is an indicator that flashover has occurred rather than a sign that it will happen.
Backdraft
A deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient product of incomplete combustion.5 A backdraft is relatively uncommon, but a very dangerous situation.
You can see an example of backdraft in the video below.
Video: "Daniel's Block Fire-BACKDRAFT" by Elia Clemente. YouTube Standard License.
Fuel Controlled Fire
A fuel controlled fire is one in which the heat release rate and growth are controlled by the characteristics of the fuel, such as quantity and geometry, and in which adequate air for combustion is available.
Fuel Load
Fuel load refers to the total quantity of combustible contents of a building, space, or fire area, including interior finish and trim, expressed in heat units or the equivalent weight in wood. While fuel load is the total quantity of fuel available, the type and arrangement of that fuel will determine the size and heat release rate of a fire.
Plume Formation
A fire plume is made up of the byproducts of combustion such water, carbon dioxide, carbon monoxide, hydrogen cyanide, along with other gases and particles. Because these products are heated, thermal buoyancy causes these gases and particles to rise vertically within the space. The fire plume will continue to rise until it reaches a barrier such as a ceiling, which will result in a ceiling jet, or until the gases cool to the point that the atmospheric gases cause enough drag to stop the gases from rising.
The ceiling jet is a relatively thin layer of flowing hot gasses that develops under a horizontal surface as a result of plume impingement and the flowing gas being forced to move horizontally. The ceiling jet cause convective heat transfer within the room and into other rooms. Additionally, as the ceiling jet continues to contribute to the heat in the upper layers, radiative heat transfer will heat fuel below the upper layer.
The thermal plume will always follow the path of least resistance. In an enclosure, as the thermal plume continues to flow upward and then outward forming the ceiling jet, heated gases and particles will flow horizontally until they meet a wall. When this happens, the gases will begin to fill the space, forming an upper layer of combustion products. This layer will continue to fill the space, causing the upper layer to continually grow toward the lower part of the space. This will continue until the fire begins to decay, or the upper layer is able to find another path out of the space such as a doorway or window.
You can see these concepts illustrated in the video below.
Video: Open Floor Plan Fire: Impact of Temperature and Pressure on Fire Behavior by Fire Safety Research Institutes, 2025. Used according to FSRI Terms of Use.
Ventilation Controlled Fire
Also known as oxygen controlled or oxygen limited fire, a ventilation controlled fire refers to a fire in which the heat release rate and growth are controlled by the availability of oxygen within the space.
Full Room Involvement
Full room involvement refers to a fire that has spread to all or most of the fuel in the room. Full room involvement may or may not include flashover. A similar term, fully involved structure, has the same meaning but refers to the fuel in the entire structure being involved.
References
1. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024
2. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024
3. National Fire Protection Association. (2020). NFPA 1410: Standard on training for emergency scene operations. https://link.nfpa.org/free-access/publications/1410/2020
4. National Fire Protection Association. (2024). NFPA 921: Guide for fire and explosion investigations. https://link.nfpa.org/free-access/publications/921/2024