BY STEVE KERBER
How has the residential fire environment changed in the past several decades, and what impact does it have on fire service ventilation? Underwriters Laboratories set out to answer these questions. Statistics show that there is a continued tragic loss of firefighter and civilian lives. It is believed that a significant contributing factor to these deaths is a lack of understanding of how natural ventilation in residential structures and using ventilation on the fireground affect fire behavior. Under the United States Department of Homeland Security (DHS) Assistance to Firefighter Grant Program, Underwriters Laboratories examined fire service ventilation practices as well as the impact of changes in modern furnishings and house geometries.
The residential fire environment has changed steadily over the past several decades. These changes include larger homes, more open floor plans, and an increase in synthetic fuel loads. Underwriters Laboratories conducted several room fire experiments and a series of 15 full-scale residential structure fires to examine this change in fire behavior and the impact of firefighter ventilation tactics. This fire research project developed the data needed to quantify the fire behavior associated with these scenarios and immediately develop the necessary firefighting ventilation practices to reduce firefighter death and injury.
IMPACT OF MODERN FURNISHING EXPERIMENTS
The increased use of synthetic materials in the home has resulted in the availability of more fuel to burn. If there is adequate ventilation, the time to flashover will be significantly reduced. Two side-by-side comparison experiments with adequate ventilation demonstrated flashover times of less than four minutes with modern furnishings as compared with more than 29 minutes with legacy furnishings (photos 1-4). Modern furnishings were purchased new from local retailers and are commonly found in today’s homes; the legacy furnishings were purchased used and were typical of furnishings found in homes of the 1950s through 1970s.
(1) A modern room. (Photos courtesy of Underwriters Laboratories.) |
(2) A modern room at three minutes, 30 seconds after ignition. |
(3) A legacy room. |
(4) A legacy room five minutes after ignition. |
This difference in flashover times has a substantial impact on occupant and firefighter safety. A time of less than four minutes from a candle placed on the cushion of a sofa until flashover does not leave much time for occupants to escape and highlights the need for early warning from smoke alarms and early suppression from residential sprinklers.
FULL-SCALE HOUSE EXPERIMENTS
Two houses were constructed in the large fire facility of Underwriters Laboratories in Northbrook, Illinois. The first of two houses constructed was a one-story, 1,200-square-foot, three-bedroom, one-bathroom house with eight rooms total (photos 5-6). The second house was a two-story 3,200-square-foot, four-bedroom, 21⁄2-bathroom house with 12 total rooms (photo 7). The second house featured a modern open floor plan, a two-story great room, and an open foyer (photo 8). Fifteen experiments were conducted with varied ventilation locations and number of ventilation openings. Ventilation scenarios included ventilating the front door only, opening a window only, opening the front door and a window near the seat of the fire, opening the front door and a window remote from the seat of the fire, opening a door and a higher window in the two-story house, and sequentially opening multiple windows. One scenario in each house was conducted in triplicate to examine repeatability.
(5) A one-story house. |
(6) A 3-D rendering of a one-story house. |
(7) A two-family house. |
(8) A 3-D rendering of a two-family house. |
The furnishings in both houses were similar for all of the experiments (photos 9-10), and every room in each house was furnished. Instrumentation was placed throughout the houses to measure temperature, gas concentrations, and gas velocities. Video was recorded in six locations inside the houses and two locations outside the houses.
(9) One-story living room furnishings. |
(10) Two-story family room furnishings. |
EXPERIMENTAL TIMELINE
All of the experiments began with all of the exterior doors and windows closed and all of the interior doors in the same locations either open or closed for every experiment. The fire was ignited in a sofa in the living room of the one-story house and in a sofa in the family room for the two-story house, using a remote ignition device comprised of three stick matches. The flaming fire was allowed to grow until ventilation operations were simulated by making openings. The one-story house was ventilated eight minutes after ignition. This was determined based on two main factors, time to achieve ventilation-limited conditions in the house and potential response and intervention times of the fire service. The ventilation time for the two-story house was 10 minutes, for the same reasons as for the one-story house; time was added mainly to achieve ventilation-limited conditions. It took a longer time for the larger volume to consume the oxygen inside the house.
After ventilation, the fire was allowed to grow until flashover or it was perceived that the maximum burning rate had been achieved, based on the temperatures, observation of exterior conditions, and monitoring of the internal video. Once the fire maintained a peak for a specific time with respect given to wall-lining integrity, a hose stream was flowed in through an external opening and the experiment was terminated.
EXPERIMENT RESULTS
A core mission of the fire service is life safety of firefighters and occupants. These experiments allowed for the assessment of tenability prior to fire department intervention and after fire department intervention. Ventilation is viewed as a tactic that increases tenability for the fire service and occupants if it is used properly. However, when not used properly, it can decrease tenability. Two measures of tenability were used during these experiments, temperature and gas concentration.
Tenability in these two homes was limited for occupants as temperatures throughout the home at standing height (five feet) exceeded 300°F within five minutes of ignition in the one-story house and within seven minutes in the two-story house (photos 11-12). Oxygen concentrations throughout the homes decreased to below 12 percent shortly after the temperature tenability threshold was exceeded in most of the house fires. The fire service should consider the possibility of savable lives, especially behind closed doors, in its risk analysis. Tenability thresholds were never exceeded when a bedroom door was closed throughout the experiment. When educating the public, the fire service should emphasize the importance of closing doors.
(11) The one-story experiment. |
(12) The two-story experiment. |
Tenability for firefighters can also be quantified for these experiments. Firefighters had one minute, 40 seconds in the one-story house and three minutes, 20 seconds in the two-story house after ventilation before water would have to be applied to remove the hazard or the firefighter would have to exit the house. A significant portion of the one minute, 40 seconds and the three minutes, 20 seconds until firefighter untenability was fresh air being entrained into the ventilation-limited fire. In many of the experiments, the time from the beginning of temperature escalation until untenability was less than 10 seconds in the room of fire origin. This provides little warning that the fire is going to flash over and highlights the need to understand that ventilation openings are not only allowing hot gases to escape but are also allowing fresh air to enter.
Several ventilation comparisons could be made from the experimental series. First, the more ventilation openings made, the faster the fire room transitioned to flashover. This shows that even in these modestly furnished homes, fuel is not the limiting factor and that more air will create more burning and less tenability. Ventilating near the seat of the fire localized the combustion and temperatures within the house. Ventilating remote from the seat of the fire created a flow path with expanded area available to burn and further decrease tenability within the homes. Allowing air into a ventilation-limited fire low and letting the hot gases out high can create prime conditions for a flashover, even in a large volume like the two-story family room. More efficient ventilation can mean more efficient air entrainment, which can lead to faster flashover times if water is not applied in the shorter tenability window.
FIRE SERVICE TACTICAL CONSIDERATIONS
It was paramount that the results of these experiments be made known to the fire service. Therefore, several tactical considerations were developed for the fire service with the assistance of a technical panel of fire service leaders from across the world assembled for this project.
• Stages of fire development.The stages of fire development change when a fire becomes ventilation limited. It is common with today’s fire environment to have a decay period prior to flashover, which emphasizes the importance of ventilation and its timing.
• Forcing the front door is ventilation.Forcing entry has to be thought of as ventilation as well. Although forcing entry is necessary to fight the fire, it must also trigger the thought that air is being fed to the fire and the clock is ticking before either the fire gets extinguished or it grows until an untenable condition exists, jeopardizing the safety of everyone in the structure.
• No smoke showing. A common event during the experiments was that once the fire became ventilation limited, the smoke being forced out of the gaps of the houses greatly diminished or stopped altogether. This happens because as the fire runs out of air, the temperatures begin to decline significantly; therefore, the pressure reduces significantly as well. No smoke showing during size-up should increase awareness of the potential conditions inside.
• Coordination.If you add air to the fire and don’t apply water in the appropriate time frame, the fire gets larger, and safety decreases. Examining the times to untenability gives the best case scenario of how coordinated the attack needs to be. Taking the average time for every experiment from the time of ventilation to the time of the onset of firefighter untenability conditions yields one minute, 40 seconds for the one-story house and three minutes, 20 seconds for the two-story house. In many of the experiments, from the onset of firefighter untenability until flashover was less than 10 seconds. These times should be viewed as very conservative. If a vent location already exists because the homeowner left a window or door open, the fire is going to respond faster to additional ventilation openings because the temperatures in the house are going to be higher. Coordinating the fire attack crew is essential for a positive outcome in today’s fire environment.
• Smoke tunneling and rapid air movement through the front door.Once the front door is opened, pay attention to the flow through the front door. A rapid inrush of air or a tunneling effect could indicate a ventilation-limited fire.
• Vent-enter-search (VES).During a VES operation, it is of extreme importance that the door to the room be closed. This eliminates the impact of the open vent/window vented for entry and increases tenability for potential occupants and firefighters while the smoke ventilates from the room now that it is isolated from the rest of the house. The door can always be reopened as part of a coordinated ventilation operation.
• Flow paths.Every new ventilation opening provides a new flow path to the fire and vice versa. This could create very dangerous conditions when there is a ventilation-limited fire. It can be dangerous to be in the path between where the fire is and where it wants to go.
• Can you vent enough? In the experiments where multiple ventilation locations were made, it was not possible to create fuel-limited fires. The fire responded to all the additional air provided. That means that even with a ventilation location open, the fire is still ventilation limited and will respond just as fast or faster to any additional air. It is more likely that the fire will respond faster because the already open ventilation location is allowing the fire to maintain a higher temperature than if everything were closed. In these cases, rapid fire progression is highly probable, and coordinating the fire attack with ventilation is paramount. The fastest flashover times came in the experiment with the open door and four windows opened sequentially.
• Impact of shut door on occupant and firefighter tenability. Conditions in every experiment for the closed bedroom remained tenable for temperature and oxygen concentration thresholds. This means that closing a door between the occupant and the fire or a firefighter and the fire can increase the chance of survivability. During firefighter operations, if a firefighter is searching ahead of a hoseline or becomes separated from his crew and conditions deteriorate, a good choice 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, since the closed door will provide more time. In these experiments, a hollow-core wood door provided five minutes of protection until burn-through with a fully developed fire on the other side of the door.
• Potential impact of an open vent already on flashover time.All of these experiments were designed to examine the first ventilation actions by an arriving crew when there are no ventilation openings. It is possible that the fire will cause a window to fail prior to the fire department’s arrival or that a door or window was left open by the occupant while exiting. It is important to understand that an already open ventilation location is providing air to the fire, allowing it to sustain or grow, which means it will respond to the additional air faster and most likely lead to rapid fire progression.
• Pushing fire.There were no temperature spikes in any of the rooms, especially the rooms adjacent to the fire room, when water was applied from the outside. It appears that in most cases the fire was slowed down by the water application and that external water application had no negative impact on occupant survivability. Although the fog stream “pushed” steam along the flow path, no fire was “pushed.” The study did not measure humidity created by steam generation or its potential impact on survivability of occupants and firefighters.
• No damage to surrounding rooms. Just as the fire triangle depicts, fire needs oxygen to burn. A condition that existed in every experiment was that the fire (living room or family room) grew until oxygen was reduced below levels to sustain it. This means that it decreased the oxygen in the entire house by lowering the oxygen in surrounding rooms and the more remote bedrooms until combustion was not possible. In most cases, surrounding rooms such as the dining room and kitchen had no fire in them even when the fire room was fully involved in flames and was ventilating out of the structure.
ONLINE TRAINING PROGRAM
To make the results of this study more user friendly for the fire service, Underwriters Laboratories developed an online interactive training module that can be viewed for free through Underwriters Laboratories’ fire service outreach Web site (www.ul.com/fireservice) (photo 13). The program includes a professionally narrated description of all of the experiments, their results, and the tactical considerations. Experimental video is used, and graphical data are explained in a way that brings science to the street-level firefighter.
(13) Underwriters Laboratories’ online training program. |
This research study developed empirical fire test data to demonstrate fire behavior resulting from varied ventilation openings in legacy residential structures compared with modern residential structures. The data can be used to educate and guide firefighters of all levels in the proper use of ventilation as a firefighting tactic that will result in mitigating the firefighter injury and death risk associated with improper use of ventilation.
Steve Kerber will present the classroom session “Ventilating Today’s Residential Fires” on Thursday, March 24, 1:30-3:15 p.m., at FDIC 2011 in Indianapolis, Indiana.
Steve Kerber and Daniel Madrzykowski will present the workshop “Fire Dynamics for the Fire Service” at FDIC 2011 on Tuesday, March 22, 8:00 a.m.-5:00 p.m.
STEVE KERBER is a fire research engineer at Underwriters Laboratories. His areas of research include improving firefighter safety, fire service ventilation, lightweight construction, and smoke management fire modeling. Prior to joining Underwriters Laboratories, he was a fire protection engineer at the National Institute of Standards and Technology. He is a 13-year veteran of the fire service and served most of the time in the College Park (MD) Fire Department, where he held ranks up through deputy chief. He received his bachelor’s and master’s degrees in fire protection engineering from the University of Maryland and is working on his doctorate in fire safety engineering at Lund University in Sweden. He is also a registered professional engineer.
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