Positive pressure attack (PPA) has been used by the fire service for more than 30 years, but many still have questions regarding its value on the fireground. While almost every apparatus has a fan on it, PPA is still a very polarizing tactic. Some see it as dangerous to add thousands of cubic feet per minute (cfm) of air into a potentially occupied fire building, while others see it as negligent not to attempt to control the flow path inside a fire building by keeping a fan at your back. While both of these views are a little nearsighted, they speak to the fact that we need more information on PPA.
So, as has become customary in recent years, we look to the Underwriters Laboratories (UL) Firefighter Safety Research Institute (FSRI) to shed some light on PPA. With the release of UL FSRI’s latest fire service summary report and online training module on PPA and positive pressure ventilation (PPV), the fire service now has the data needed to enhance our understanding of the fire dynamics of using a fan on the fireground. Now, before we get too far into this article, let’s clarify the distinction between PPA and PPV: PPA is defined as using a fan to pressurize a building before the fire is knocked down, while PPV uses a fan to pressurize a building after the fire is knocked down.
This study is the third, and (seemingly) final, installment in UL FSRI’s ventilation trilogy although ventilation will still be a large component of many future studies since it’s so important to understanding the fireground. Officially called the “Study of the Effectiveness of Fire Service Positive Pressure Ventilation During Fire Attack in Single-Family Homes Incorporating Modern Construction Practices,” the PPA study will undoubtedly lead to more informed, more efficient, and ultimately safer firefighters and firegrounds.
“This is a certainly groundbreaking study as it comes to ventilation research,” says Robin Zevotek, UL FSRI lead engineer. UL FSRI and the technical panel designed 25 experiments to try to better understand the benefits and limitations of PPA and PPV. The experiments were done using the same one-story and two-story houses, with similar fuel loads that were used in the two previous ventilation study experiments (horizontal ventilation and vertical ventilation). By controlling the conditions throughout the experiments and bounding numerous variables, they empirically quantified the impact of the following:
- Exhaust location.
- Exhaust size.
- Multiple exhaust openings.
- Fire location.
- Compartmentation vs. open concept floor plan.
- Open vs. closed doors.
- Systematic vs. multiple openings during PPV.
After all of the experiments were complete and the lab in Northbrook (IL) was cleaned and emptied, the real work began. It was then time to take the millions of data points and hours and hours of video and turn them into something usable for the fire service. Dissecting the data takes countless man-hours (and pots of coffee) to make sense of the numerous graphs and videos. After seemingly infinite discussions and rewrites, the 25-member technical panel and UL FSRI staff finally landed on 16 important, actionable, Tactical Considerations for the global fire service.
The UL FSRI staff then produced an 875-page technical report, a condensed fire service summary report, and an online training module. The goals of the fire service summary report and the online training module are to make the information easy to understand and apply to your drill ground and, eventually, your fireground. The online training module, as usual, is dynamic and engaging and can act as ready-made department-level training. This easily digestible material focuses on 16 Tactical Considerations:
- Understanding the basics of PPA and PPV.
- Horizontal, vertical, and PPA are different tactics.
- Exhaust size is more important than fan location.
- An ongoing assessment is imperative.
- PPA is exhaust dependent.
- Exhaust size and location are important.
- Creating additional exhausts not in the fire room.
- PPA safety decreased when fire size and location are unknown.
- Compartmentation is essential.
- Water application increases chances for success.
- Use the reach of your hose stream.
- Void space extension is exhaust dependent.
- Open vs. closed door.
- Multiple openings aid PPV.
- Assess for extension post fire control.
- With PPV, start the fan immediately.
Throughout the experiments, much of our current best practices of PPA were reinforced by the data, but there was also new information gleaned and, as you’d expect, new questions also arose. So what did we learn after deconstructing PPA and PPV? For an in-depth analysis, check out the fire service summary report and online training module at ulfirefightersafety.com.
“If you’re going to use PPA, when they train on it, these concepts will make them more effective,” Zevotek adds.
Here are some of the big takeaways from the study.
Back to School
To understand how PPA works, we need to first briefly revisit our high school physics class. Fire flows (smoke, air, heat, and flames/products of combustion/fire gases) always flow from an area of higher pressure to an area of lower pressure, and the larger the pressure differential, the greater the velocity of the fire flows. While this is pretty basic science, we also need to understand that having a fire inside a building alters the pressures inside; so, too, does turning a fan into a building.
The basic concept of PPA is to first create an exhaust as close to the fire as possible, which will reduce the pressure inside the fire room. Then the fan is turned into the building to increase the pressure in adjacent rooms while decreasing the pressure in the fire room. Therefore, after the exhaust is made and the fan turned inside, in theory the vast majority of the products of combustion will exit the building through the exhaust and not flow into the adjacent spaces. Ideally, this will reduce temperatures and increase visibility between the attack entrance and the fire while increasing temperatures between the fire and the exhaust.
The Importance of Exhaust
It’s crucial to understand that certain conditions must be met to increase the likelihood of a successful PPA, and this starts with the exhaust. It’s pretty obvious by looking at the titles of the Tactical Considerations that exhaust plays a vital role in the success of PPA.
All too often, PPA is taught as a “Set it, and forget it” tactic, but this is myopic and potentially dangerous. While it has been proven that, under certain circumstances, PPA can enhance fire attack, search, and ventilation, it’s also true that PPA can lead to rapid fire progression and a rapid decrease in the survivable space inside a fire building if conditions are not ideal. To gauge whether PPA is improving the intake side of the flow path and working as designed, an ongoing assessment of the attack entrance, exhaust, and interior is needed.
It’s a common misunderstanding that the attack entrance will become a unidirectional intake because of the cone of air completely covering the door, but the experiments demonstrated that this is simply not true. There will be flow (backflow) out the top of the attack entrance, and this shouldn’t surprise us. What’s important is that the backflow should diminish over time. If the backflow is increasing, then the firefighter monitoring the intake will need to attempt to troubleshoot the problem by either creating another exhaust or decreasing the throttle.
While a unidirectional intake is not possible, a unidirectional exhaust is the goal. Assessing the neutral plane at the exhaust can tell you whether PPA is working as anticipated. If the neutral plane is at or below the windowsill, then the exhaust is unidirectional. If the neutral plane stays above the sill, then there’s a problem, and we need to attempt to identify and fix the problem (increase the flow or remove an obstruction); stop the fan; or, better yet, flow some water on the fire.
Having an ongoing assessment during PPA is essential since we’re adding thousands of cfm to a probable vent-limited fire, which will increase the energy and power produced by the fire. If this energy isn’t following the intended flow path out the exhaust and away from any firefighters and civilians, then the fireground can get very bad-very fast. While having someone monitoring the attack entrance and the exhaust will require more personnel than PPA has traditionally required, having multiple personnel assessing the effectiveness of PPA could decrease the customary 30 to 60 seconds, depending on conditions, that the attack team typically waits between turning the fan into the building and making entry.
While many different variables (fan size, setback distance, angle, intake, exhaust, etc.) can affect the pressures created by a fan, the exhaust is the most critical to PPA success. Again, the fan needs to produce more pressure than the fire, and having an exhaust to inlet ratio of 2:1 is more effective than 1:1 or <1:1. It needs to be clarified that the inlet is not necessarily the attack entrance door but rather the door/opening to the fire room. While this 2:1 ratio was possible in the experiments, many buildings make this exhaust:inlet ratio difficult or even impossible to recreate. If the building isn’t cooperating, just add water. Applying water to the fire will reduce the pressure (and the temperature, energy, and power) inside the fire room, which will allow PPA to be more effective.
Probably common sense, and definitely a current best practice, we also now empirically know that making an exhaust opening outside of the fire room can cause the smoke, air, heat, and flames to spread to adjacent spaces. This could obviously decrease the survivability of the adjacent spaces and put civilians and firefighters at greater risk. When it comes to the exhaust, location, size, and an ongoing assessment are crucial to achieve PPA success. This reinforces what we’ve learned from the first two ventilation studies-coordinated ventilation is critical.
The last piece of the PPA puzzle related to the exhaust is the finding that void space extension is exhaust dependent. In the experiments, the void space examined was a stud bay accessed (inlet) by an electrical outlet or light switch. Even with products of combustion entering and filling a void space, with no exhaust, the oxygen level will predictably decrease, thus not support combustion. This means that even if fire flows can find their way into a void, if there isn’t also an exhaust in the void the fire won’t readily spread.
Fast Fires Need Fast Water
We inherently know that water wins. Our experience also backs this up and, to no one’s surprise, these experiments implicitly proved this fact. Although the current Fire Attack Study will go much more in depth about the differences between interior and exterior water application, from the data we examined (temperature, heat flux, gas concentrations, pressure, and gas velocity) it was pretty clear that water wins.
I know it’s blasphemy in some circles to say that transitional attack coupled with PPA can be effective. The data showed that, regardless of where the water was applied from, conditions inside improved. It should be noted that the water was applied with a tight stream at a steep, static angle. As already mentioned, applying water to the fire will reduce the pressure inside the fire room, making PPA more effective. This coupling of two distinct fire-attack tactics hopefully leads to the realization that different tactics (interior attack, exterior attack, PPA, door control, etc.) are not mutually exclusive but rather are all options that should be chosen, and potentially combined, based on the conditions found and resources available. If conditions and resources dictate tactics, then shouldn’t there be more than one way to fight fire. “There is never one answer,” says Steve Kerber, UL FSRI director.
It has been shown time and time again, both in the lab and on the streets, that with the continued evolution of the fireground, today’s fires are faster than ever before. This emphasizes the importance of quickly stretching, advancing, and flowing. Fast fires need fast water. To put this information into practice means that we should use the reach of our stream and apply water to the seat as soon as possible. This is nothing new, but it drives home the importance of training on a smooth, fast, and efficient fire attack-PPA or not.
Impact of the Building
Now that the fire has been deconstructed, we can look at the building’s impact on PPA. With all the attention that fire behavior has received in years past, it’s easy to forget about the importance of building construction on fire dynamics. If there was one finding from the experiments that will surprise the fire service, it’s that “compartmentation is essential” to effective PPA.
While PPA worked very well on bedroom fires in the one-story ranch house, it was much less successful on fires in the family room of the two-story house. Findings showed that PPA will not be effective on fires in the open areas of an open space floor plan, especially areas with high ceilings. This is because when the fire is in an open area, the inlet size is increased (remember that the inlet is not the front door but the opening to the room), and therefore the fan becomes incapable of increasing the pressure in the adjacent rooms. High ceilings also create another problem by effectively “churning” the products of combustion in the room. In the experiments, a fire in an open area combined with a high ceiling led to increased temperatures, decreased visibility, and fire spread into adjacent areas. Thus, the importance of knowing whether or not the fire room is compartmentalized cannot be overstated.
Although PPA is an obviously polarizing tactic, PPV is widely accepted and practiced as a safe and efficient means to ventilate a building post knockdown. While only three of the Tactical Considerations are related to PPV, there were still some important findings, specifically that multiple exhausts increase the effectiveness of PPV. There has been debate for decades on the most efficient method of PPV: systematically opening windows and doors to exhaust smoke one room at a time or opening multiple windows and/or doors at once. The experiments found that systematic PPV causes smoke to be entrained from adjacent spaces, leading to inefficient PPV. With the power of today’s fans, it was found that the more openings that are made, the more efficient PPV will be.
“If your goal is to change all the air out within a structure, the fastest way you’re going to do that is by flowing more air through the front door; and the more openings you have in the structure, the more air comes in through the front door,” Zevotek says.
Education and Application
While this study was never intended to end the debate on PPA, it was intended to make the debate more educated. I’ve only briefly touched on PPA and PPV in this article, but hopefully you’re encouraged to dig deeper. While I’ve discussed some of the important findings from the experiments, there are still many more Tactical Considerations hidden in the data. If your department uses PPA and/or PPV, you owe it to yourself, your crews, and your citizens to know all you can about it by taking the online training module and reading the fire service summary report. And after you do that, go out and train on it-a lot.