Fire BehaviorFire Behavior
In recent years, Underwriter’s Laboratory and the National Institute of
Standards and Technology have conducted extensive research on fires in residential
buildings. The goal of this research was to discover why firefighters are being killed
and injured in residential fires. What the research has shown is that fire travels
from growth to extinguishment differently than it has in the past.
Previously, we were taught that there were three stages of fire - incipient,
free burning, and smoldering. Contemporary fire behavior now consists of five
stages of fire behavior - Ignition, Growth, Flashover, Fully Developed, Decay.
Ignition: The fire is small and typically confined to the material of origin.
Heat transfer is not generally a concern, and if left unattended or properly handled,
the fire will grow.
Growth: A fire plume begins to form above the ignited material. Hot gases
rise to the ceiling, spread laterally, and then bank down. Heat is radiated into the
room, which distills more flammable gas from the contents within the room. This
process is self-sustaining – it continues to create heat, which is radiated into the
room, distilling further gas from the contents in the room. A fire in the growth stage
will move toward to flashover.
Flashover: Flashover is the transition between the growth phase and the
fully developed fire. The thermal radiation of the walls and ceiling cause the
destructive distillation of all combustible materials in the compartment. These
gases replace the oxygen in the compartment and are gradually heated to the
ignition temperature of carbon monoxide, which is about 1200 degrees. Flashover
is the ignition of the gases within the compartment, which results in a full flame
atmosphere. Flashover typically begins the destruction of the structural integrity of
the building.
Fully Developed: A fully developed fire is one that attacks the structural
components of a building. The fire moves from the area of origin to exposed areas
of the same building or nearby buildings.
Decay: As the fire consumes the available fuel in the compartment, the heat
release begins to decline as the fire runs out of fuel to burn. Fires can also decay due
to a lack of oxygen. These fires are known as ventilation-limited fires. There is
sufficient heat and fuel in the compartment, however there is insufficient oxygen to
sustain combustion. Decay was once thought to be the last stage of a fire. What
research has shown is that ventilation limited fires can decay prior to flashover. A
ventilation-limited fire will rapidly move to flashover if ventilated and given air.
Ventilation-limited fires were a point of focus in the NSIT and UL testing. The
studies have given the fire service direction in regards to tactics at ventilation
limited fires.
Coordination: Coordination between ventilation and fire attack crews is
imperative to reduce firefighter injury and death. The ventilation crew must open
the building in the correct place while the fire attack crew extinguishes the fire. Any
delay in extinguishment will cause the fire to grow with a resulting decrease in
safety. Additionally, a door or window left open by a homeowner will accelerate fire
growth upon ventilation because of already high temperatures in the house.
Smoke tunneling and rapid air movement through the front door: Once
the front door is opened, special attention must be given to the flow of air and
smoke through the door. A rapid in-rush of air or a tunneling effect could indicate a
ventilation-limited fire.
No smoke showing: Once fire becomes ventilation-limited, smoke being
forced out of openings in houses can be greatly reduced or stopped altogether. No
smoke showing during size-up should increase awareness of the potential
conditions inside the house. Remember from previous training, “Nothing showing
means nothing!”