Residential Fires are Changing
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Residential Fires are Changing
Prepared by Past Chief Thomas Bartsch, VSFD
There has been a steady change in the residential fire environment over the past several decades.
These changes include larger homes, more open floor plans and volumes, increased synthetic fuel loads and new construction materials.
This also means shorter escape times for occupants as the egress routes may be compromised earlier due to lack of compartmentation.
Home Size and Geometry
Home Size and Geometry New home design changes; • Larger home footprints. • Open floor plan concepts. • More unventilated attics. • Increasingly airtight construction. • Increased concealed space. • Variety in plans and construction types.
Home Size and Geometry Many modern-day homes are larger than homes built before 1980. Homes have increased in average area from approximately 1500 sq. ft. in 1973 to over 2500 sq. ft. in 2008. Twenty-six percent of homes constructed in 2008 were larger than 3,000 sq. ft.
In addition to increased area, more homes are being built with two stories.
Home Size and Geometry Newer homes tend to incorporate features such as taller ceilings, open floor plans, two-story foyers and great rooms. These features remove compartmentation, add volume and can contribute to rapid smoke and fire spread. It is also common for great rooms and open foyers to directly connect the living spaces to the sleeping spaces allowing for smoke generated in the living spaces to rapidly trap potential sleeping occupants.
Home Contents Building Contents have changed significantly in recent years, contributing to the decrease in time to untenable (life threatening) conditions.
These changes include: • increased use of more flammable synthetic materials such as plastics and textiles, • increased quantity of combustible materials, • the use of goods with unknown composition and uncertain flammability behavior.
Home Contents Home Contents have transitioned from being primarily of natural materials (wood, wool and cotton) to being dominated by synthetic materials.
Hydrocarbon and synthetics such as polyurethane foam have replaced cotton as the padding found in upholstered furniture, which have significantly higher heat release rates.
Home Contents The shift to synthetic materials speeds up the stages of fire development creating an increased potential for ventilationlimited fire conditions prior to fire department arrival.
A single upholstered chair can provide enough energy to take a 10 x 12 room to flashover. A sofa can provide 3X the energy of a single chair.
Construction Materials Construction materials such as; • New engineered products. • Combustible exterior finishes. • Green or environmentally sustainable building materials. • Energy efficient windows and insulation.
Construction Materials Life and fire safety are not part of the construction material selection of builders, using less material and being more affordable are. Such as dimensional lumber floor system (Legacy) compared to wood “I” joist (Modern) Engineered wood floor assemblies have the potential to collapse very quickly under well ventilated fire conditions.
Construction Materials Changes in wall linings (unrated Gypsum board) now allows for more content fires to become structure fires by penetrating the wall lining and involving the void spaces. This change allows for faster fire propagation and shorter times to collapse. (Legacy used lathe and plaster) Changes in structural components have removed the mass of the components which allows them to collapse significantly earlier. When it comes to lightweight construction there is no margin of safety.
The Changing Fire Scene
The Changing Fire Scene LEGACY Content fires: Primarily consist of natural fiber contents such as wood, wools and cottons. These fires have a relatively low heat release rate when compared to hydrocarbon based products. MODERN Content fires: Fires that involve hydrocarbon and synthetic based contents such as foam rubber, nylon, rayon and polypropylene. These fires have a relatively high heat release rate when compared to the natural fiber products found in legacy fires.
Legacy Content Fires
Legacy Content fires will remain in the Decay Stage if no additional oxygen is added to the fire area.
Legacy Content fires develop slower, but have enough oxygen to support continuous combustion and create enough pressure to force smoke from the building, including around doors and windows.
Legacy Content Fires The smoke being pushed out of the fire compartment by the pressure of the fire clearly indicates the presence of a fire.
Flames venting from the windows of a Legacy Content fire, besides identifying the location of the fire, usually facilitates extinguishment of the fire since the fire is already vented.
Modern Content Fires Modern Content fires rapidly consume the available oxygen within the fire area and enter into an earlier oxygen limited Decay Stage. Modern Content fires rapidly consume more of the available oxygen within the fire area creating conditions favorable to a possible ventilation induced flashover.
Modern Content Fires
Modern Content fires rapidly react to ventilation and the in-flow of additional oxygen.
Modern Content fires want 4 to 5 times the amount of air as a Legacy Content fire and quickly become ventilationlimited fires due to their higher fuel load.
Modern Content Fires While in the Decay Stage, the Modern Content fire may no longer have enough pressure to push smoke from the fire area or building.
With Modern Content fires, the assumption, that the absence of smoke pushing from a building is a positive sign, is no longer accurate.
Modern Content Fires Modern Content fires require the control of ventilation to prevent the potential escalation of interior fire conditions. With Modern Content fires, heavy flames out the window are usually an indicator of high heat and smoke conditions within the interior, including areas remote from the fire. Modern Content fires generate a greater volume of heat and smoke that may overwhelm the ventilation point(s).
Impact on Firefighting Operational Time Frames
Impact on Firefighting Operational Time Frames The most significant impact of the changing residential fire environment on our firefighting tactics is the dramatic shift of the safe operational timeline. Operational timeframe begins with our arrival on the scene and ends when the fire is placed under control. To compare the Modern and Legacy content fire environment, it is important to examine the time prior to fire department arrival.
Impact on Firefighting Operational Time Frames
Modern versus Legacy fire timelines
Impact on Firefighting Operational Time Frames The UL Modern Content room test flashed over in 3:40 minutes and the UL Legacy Content room test flashed over in 29:30 minutes. Other UL tests have shown: Unprotected Modern floor system (engineered wood “I” joist) collapsed in 6:00 minutes and adding a layer of gypsum board increased the collapse time to 26:43 minutes. Unprotected Legacy floor system (dimensional lumber 2 by 10) collapsed in 18:35 minutes, and adding a layer of plaster and lath increased the collapse time to 79:00 minutes.
Modern Content and Legacy Content fires demonstrate very different fire behavior.
UL tests have shown that Modern Content fires transition to flashover in less than 4 minutes while the fastest Legacy Content fire to achieve flashover did so at in over 29 minutes.
Fire Dynamics The Modern Content fire, in most cases, has either transitioned to flashover prior to our arrival or became ventilation-limited and is waiting for a ventilation opening to increase in burning rate.
This transition has a substantial impact on occupant and a firefighter safety. This change leads to faster fire propagation, shorter time to flashover, rapid changes in fire dynamics, and shorter escape times.
Fire Dynamics It also highlights that the operational timeframe begins after potential flashover. In many cases this means that if sufficient ventilation is available, the fire will spread significantly prior to fire service arrival.
If sufficient ventilation is not available the fire will become ventilation-limited and be very sensitive to initial fire department operations.
Fire Dynamics Fast fire propagation, and rapidly changing fire conditions should be expected in the Modern content fire.
In a Legacy Content fire, arriving at the 8 minute mark, it would still be in the Growth Stage and less volatile.
Firefighters may not be in the house yet or may be just entering to search for occupants.
Fire Dynamics In the Modern Content fire, a FD arriving at the 8 minute mark, collapse is possible as soon as 1:30 minutes later.
The Legacy Content fire collapse hazard begins 40 minutes after arrival of firefighters.
Door Control Door control is a temporary action, if you limit the air inlet you limit the fire’s ability to grow.
Door control does not completely cut off the oxygen supply, it slows it, which slows fire growth and delays flashover.
Air is feeding the fire, the clock is ticking, get water on the fire or it will grow to an untenable condition.
Door Control The door should be controlled until water is applied to the fire. Once water goes on the fire, the door may be fully opened to ventilate.
The simple action of door control after forcing entry until the coordinated attack will limit the air to the fire and slow the potential rapid fire progression.
Reading Smoke “Reading smoke conditions is a very important component of size-up.” Do not get complacent if there is nothing showing on arrival. In many UL experiments, the smoke color changed from black to grey as the fire became ventilation-limited and the pressure within the house decreased, ten seconds later there was no visible smoke showing at all.
Reading Smoke Modern Content fires generate greater volumes of smoke until the fire is extinguished. Modern Content fires enter an early Decay Stage due to the limited available oxygen, producing heavy smoke and varying heat conditions. Smoke is not always an indicator of what is happening within the structure. Little or no smoke showing does not mean that conditions are safer than having fire showing.
Reading Smoke Little or no smoke showing could mean a fuel-limited fire that is producing little smoke or it could mean a ventilationlimited fire that is in the initial Decay Stage and starved for air.
If smoke and fire are venting downward, horizontally or pulsing from an opening in the building, this indicates the fire conditions may be wind-impacted. Consider treating every fire like it is ventilation-limited until proven otherwise.
Flow Path Flow paths can be defined as the movement of heat and smoke from the higher air pressure within the fire area to all other lower air pressure areas both inside and outside of a fire building.
As the heated fire gases are moving towards the low pressure areas, the energy of the fire is entraining oxygen towards the fire, as the fire is rapidly consuming the available oxygen in the area.
Flow Path A Flow Path is changed with: • ventilation and not water application and • with water application Flow paths and timing are very important to understanding fire dynamics and the impact of firefighter tactics on the fire ground. The closer the air is provided to the seat of the fire, the faster it will intensify.
Limiting flow paths until water is ready to be applied is important to limiting heat release and temperatures.
Ventilation As homes become more energy efficient and fuel loads increase, fires will become ventilation-limited making the introduction of air during a house fire extremely important.
If ventilation is increased, either through tactical action of FF’s or unplanned ventilation (e.g., failure of a window or a door opened by occupant or a neighbor) heat release will increase, potentially resulting in flashover conditions.
Ventilation Modern Content fires rapidly react to ventilation and the in-flow of additional oxygen. These ventilation induced fire conditions are sometimes unexpectedly swift, and provide little time for firefighters to react and respond. Therefore, ventilation should be controlled and communicated between interior and exterior members and coordinated by the IC or appropriate officer inside the fire area to be vented.
Ventilation UL tests, on average, showed timeframes until conditions become untenable, • 100 seconds for the one-story 1200 ft2 test house and • 200 seconds for the two-story 3200 ft2 test house. • In many of the experiments from the onset of firefighter untenability until flashover was less than 10 seconds. These times are very conservative, fire is going to respond faster to additional ventilation opening because the temperatures are going to be higher.
Ventilation UL tests have shown that the time between tactical ventilation and flashover are 2 minutes for the Modern Content fire and over 8 minutes in the Legacy content fire.
In the Legacy Content fire, you have more time to recover from poorly venting times or uncoordinated attack prior to flashover, 8 minutes.
The time to recover in the Modern Content fire was approximately 2 minutes.
Ventilation Remember - Forcing the front door is ventilation.
If you notice a rapid in-rush of air or a tunneling effect, it could indicate a ventilation-limited fire.
Air is feeding the fire and there is little time before either the fire gets extinguished or it grows until a untenable condition exists.
Ventilation Fire showing does not mean that the fire is vented; it means that it is venting and still remains ventilation-limited. The fire is burning outside of the window because there is no air available inside to burn. Other areas within the fire compartment can be “underventilated”, and additional venting creates a flow path allowing the heated gases to ignite and flashover.
Ventilation “Every new ventilation opening provides a new flow path to the fire.”
Primary importance during VEIS is closing the door to the room. This eliminates the impact of the open vent and increases tenability for potential occupants and firefighters while the smoke ventilates from the now isolated room.
Ventilation Closing a door between the occupant and the fire or a FF and the fire, can increase the chance of survivability.
If you are searching ahead of a hoseline or become separated from your crew and conditions deteriorate, then a good choice of actions would be to get in a room with a closed door until the fire is knocked down or escape out of the room’s window with more time provided by the closed door.
Ventilation It is of paramount importance that horizontal and vertical ventilation must be coordinated.
Operations conducted in the flow path can place firefighters at significant risk due to the increased flow of fire, heat, and smoke toward their position.
In ventilation, “Timing is Everything”.
Summary Changes to building materials as a result of the desire to be environmentally conscience and to increase profit, the fire environment is going to continue to change and if the current trends continue it will not be in favor of firefighter safety.
“You are not fighting your grandfather’s fire anymore.”
Referenced Material: UL “Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction”, Steve Kerber PE UL “Study of the Effectiveness of Fire service Vertical Ventilation and Suppression Tactics in Single Family Homes”, Steve Kerber, PE UL “Analysis of Changing Residential Fire Dynamics”, Steve Kerber, PE USFA “Changing Severity of Home Fires Workshop Report”. FDNY “Firefighting Procedures, Ventilation” A special THANK YOU to Dave Walsh, Program Chair-Fire Science, Dutchess Community College for his comments and suggestions.