Through nearly two decades of research into firefighting strategies and tactics, engineers with UL’s Fire Safety Research Institute have generated numerous evidence-based practices for firefighters conducting ventilation on the fireground. Researchers have thoroughly studied the residential fire environment to include both single-family and multi-family dwellings (garden-style and high-rise apartments). Additional research has been conducted for an exploratory look into fire dynamics and firefighting in commercial structures, specifically strip-mall occupancies.
Ventilation research dates back to the initial identification that the fire environment was undergoing changes due to modern furnishings and newer construction practices. The fire service began experiencing close calls and, unfortunately, several line-of-duty deaths directly attributed to the changing fire environment and misaligned tactics at the time. The changing fire environment resulted in ventilation-limited fires that were highly reactive to the addition of fresh air. There was recognition that things needed to change and required a better understanding of how fires were progressing and what strategic and tactical considerations were needed to address these problems.
Three of UL’s early research projects conducted alongside the fire service looked at horizontal, vertical and positive-pressure ventilation (PPV) tactics in one- and two-story homes. Shortly thereafter, an in-depth look into suppression tactics was completed, followed by several years of learning how to best combine suppression and ventilation tactics on the fireground for an effective coordinated fire attack. Coordination means that all crews are working together using strategies and tactics that align for a common goal of ensuring life safety and overall mitigation of the incident. More specifically, suppression, search and ventilation should occur with consideration for one another, and no task should occur independently without weighing the risks and benefits of such actions against the overall incident progression.
When looking at the four ventilation methods performed on the fireground, research has consistently identified the following tactical considerations.
1. Horizontal ventilation
- 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 due to the rapid fire growth and large amount of products of combustion filling the structure and limiting the available oxygen to the fire. This can lead to a quick transition through flashover with any additional changes in ventilation made by operating firefighters.
- Forcing entry to the structure must be thought of as ventilation as well. While forcing entry is necessary for access, it must also trigger the thought that fresh air is now being drawn 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. Additionally, if a rapid inrush, or tunneling, of air is noted upon opening the door to the structure, ventilation-limited fire conditions are present. Following the direction of this inflow of air can also help firefighters locate the main fire area.
- Once a fire becomes ventilation-limited, the smoke being forced out of the gaps of a structure (around doors, windows and other leakage areas), greatly diminishes and can stop all together during the initial decay phase, pre-flashover. No smoke showing during size-up should increase awareness of the potential conditions inside and the potential for rapid fire development with any increase in ventilation.
- Horizontal ventilation also includes the control of both interior and exterior doors to the structure. Pre-suppression, isolation of any area in which a victim may be located is critical to preserving and ultimately improving survivability. Additionally, if the fire location is not known, control of the entry door to the structure will help limit the fresh air intake to the fire. Once the fire location is known, the entry door to the structure should be opened fully if a flow and move approach is taken during suppression.
- During a vent-enter-search (VES) operation, primary importance should be given to closing the door to the room of entry. 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.
- Coordination of fireground ventilation with effective suppression operations is paramount to ensure the most safe and efficient attack and results in the best chance for maintaining, and subsequently improving, survivability. In the absence of effective water being applied to the fire, the additional ventilation will yield higher heat release rates and the potential for rapid fire development. With every new ventilation opening, a new flow path is established which needs to be considered and controlled.
2. Vertical ventilation
The fire service’s workplace has changed and one of several significant factors is home furnishings. As compared to legacy furnishings, the modern home furnishings are made of synthetic materials that have significantly higher heat release rates. This shift speeds up the stages of fire development creating an increased potential for ventilation-limited fire conditions prior to fire department arrival.
“Taking the lid off” does not guarantee positive results. Vertical ventilation is the most efficient type of natural ventilation by allowing the largest amount of heat and products of combustion to exit the structure; however, it also allows the most air to be entrained into the structure from horizontal openings below. Coordination of vertical ventilation must occur with fire attack just like with horizontal ventilation. The way to make sure that the fire does not get larger, and that ventilation works as intended, is to take the fire from ventilation-limited to fuel-limited by quickly applying effective water. As soon as the water has the upper hand (more energy is being absorbed by the water than is being created by the fire), ventilation will begin to work as intended.
Ventilating directly over the fire is the best choice if your fire attack is coordinated. The closer the source of the air to the seat of the fire, the quicker it will increase in size. Placement of vertical ventilation can be a complex situation, especially if you do not know where the fire is in the structure. Optimally, where you vertically ventilate depends on the room geometry, door locations, air inlet location, and subsequent flow paths. If you ventilate in coordination with effective suppression efforts, then it does not matter where you ventilate, but the closer to the seat of the fire, the more efficient the vent will be in removing heat and smoke, which will improve conditions throughout.
The stage of the fire (i.e., ventilation or fuel limited), the distance from the inlet (door or window) to the fire, the distance from the fire to the outlet (door, window, roof vent), the shape of the inlet and outlet, and the type and shape of items (furniture or walls) or openings (interior doors) in the flow paths all play key roles in the availability of oxygen to the fire, and ultimately both occupant and firefighter safety. Operations conducted in the exhaust portion of the flow path can place firefighters at significant risk due to the increased flow of fire, heat, and smoke toward their position.
Vertical ventilation is efficient in venting heat and smoke but also causes rapid changes in the conditions in the structure. Additional considerations about timing include: The fire does not react to additional oxygen instantaneously; the higher the interior temperatures, the faster the fire reacts; the closer the air is to the fire, the faster it reacts; the higher the ventilation, the faster the fire reacts; the more air available, the faster the fire reacts; and the more exhaust, the more air that is able to be entrained.
Looking at smoke conditions is a very important component of size-up, but firefighters should not get complacent if there is nothing showing on arrival. In many of the experiments, the smoke color changed from black to grey as the fire became ventilation-limited and the pressure within the house decreased. No or little smoke showing could mean a fuel-limited fire that is producing little smoke, or it could mean a ventilation-limited fire that is in the initial decay stage and starved for air. In order to increase firefighter safety, consider treating every fire like it is ventilation-limited until proven otherwise.
3. Positive pressure ventilation
- The entry (or inlet) location cannot tell you the conditions at the exhaust location(s). Assessing both the inlet and exhaust locations with interior conditions provides the best assessment of positive pressure effectiveness.
- For positive pressure to be effective, the pressure created by the fan must be greater than the pressure created by the fire. Although fan size does play a role in the effectiveness of positive pressure, exhaust size plays a greater role. Providing enough exhaust to reduce the pressure in the fire compartments below what the fan is capable of producing in the remainder of the structure, is essential for safe positive-pressure operations.
- Positive pressure effectiveness is directly dependent on the ability of the fan to exhaust products of combustion to the exterior. Any exhaust opening created in conjunction with positive pressure should be located in the fire compartment to prevent spread into adjoining areas.
- Additional openings not in the fire compartment will lower the pressure in the adjacent compartments, allowing for more flow from the fire compartment to the remainder of the structure. This can be detrimental without effective suppression efforts.
4. Hydraulic ventilation
- Although the coordination of ventilation prior to and during suppression is important for an effective fire attack, the coordination of ventilation post suppression is equally as important for smoke removal, enabling a quick return to tenable conditions throughout the structure, providing better visibility for firefighters conducting interior operations.
- Hydraulic ventilation uses the flow of water from the hose stream to entrain air. As the air moves, an area of low pressure is created, which draws in surrounding fire gases that are a higher pressure in the direction of the water flow. Essentially, hydraulic ventilation can be thought of as a pull source or negative pressure ventilation.
- As soon as the fire is knocked-down, firefighters should employ hydraulic ventilation from any available opening in the fire compartment. This can be accomplished regardless of the type of nozzle chosen, whether it be combination or smooth bore. With a combination nozzle, the most effective means of venting hydraulically is with a narrow fog stream rotated in an O pattern. With a smoothbore nozzle, the most effective method is to remove the tip, open the bale to between one-third and one-half, and rotate the broken stream into an O pattern out of the opening.
- Air entrainment principles tell us that the more broken the stream and the more the stream is manipulated, the more air that will be drawn forward with the droplets. It should also be noted that the further the nozzle operator is from the opening, the more air that will be entrained. Creating a larger distance from the end of the nozzle to the threshold in which the water leaves the fire compartment allows for more entrainment. The nozzle operator must prevent the hose stream from impacting the frame surrounding the exhaust opening as this will stop the entrainment.
- Hydraulic ventilation is a very effective means of post-suppression smoke removal that can be accomplished by a single crew. This should be employed whenever possible on the fireground for rapid improvement of conditions on the interior.
In sum
Ventilation can have a drastic impact on the overall tenability in a structure. If ventilation is provided to a ventilation-limited fire and suppression is either delayed or ineffective, the oxygen available for combustion is increased, which supports a higher heat release rate and the potential for rapid fire growth. This leads to both higher toxic and thermal threats for any trapped occupants and a corresponding decrease in survivability. Conversely, when coordinated with effective suppression efforts, ventilation can facilitate the removal of smoke, heat and products of combustion, quickly improving conditions and survivability on the interior. In the simplest sense then, minimize ventilation prior to suppression, increase ventilation during suppression, and maximize ventilation post suppression.
Ventilation efforts prior to, but timed closely with, suppression efforts should be focused on the fire area and only to facilitate a more efficient attack through having a “vent ahead,” which allows for increased air entrainment and improved conditions behind the advancing handline during a flow and move approach.
During suppression, ventilation can be increased behind the advancing handline to allow for fresh air to be drawn into the structure, improving survivability. This is best accomplished through horizontal openings, which bring the fresh air most local to where occupants may be trapped.
Post suppression, all windows and doors within the affected areas of the structure should be opened or ventilated as quickly as possible. The fire attack should culminate with hydraulic ventilation from any handline operating on the interior of the structure.
Read the research
- Impact of Ventilation on Fire Patterns
- Study of Coordinated Fire Attack Utilizing Acquired Structures
- Impact of Natural and Mechanical Ventilation on Fire Patterns in a Residential Structure
- Effectiveness of Fire Service Positive Pressure Ventilation
- Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction
