Our Protracted War: GPMs vs. BTUs

Since the beginning of time, mankind has been fighting a war against fire. Although we find utility in it through it keeping us warm and using it for cooking, the devastation that fire has brought upon our country in the last few centuries is widespread. I suspect we never win our protracted war against our long-time nemesis “fire,” but we have won all of the battles thus far. 

This is to say that there are not any fires that the responding companies keyed up their radio and said, “this is too difficult, we’re gonna let this one burn and return to quarters.” Some argue that not much has changed since the beginning of our war hundreds of years ago. I hope by the end of this article, you agree that this could not be further from the truth.

According to the United States Fire Administration, our war consisted of 1,291,500 battles (fires) in 2019. During these incidents, there were 3,704 deaths, 16,600 injuries, and $14.8 billion in damages. Although the dollar losses and deaths have increased sharply in the last decade, the number of fires is down roughly three percent from 2010. Despite the complexity of each fire, each occurring in slightly different containers (buildings), the battle is quite simplistic. It is simply a matter of GPMs (Gallons Per Minute) versus BTUs (British Thermal Units). At each battle, we need to apply water (measured in GPMs) to absorb the heat release rates (measured in BTUs). Let’s clearly define both.

Gallons Per Minute (GPM) is simply the amount of water that we can “throw” at the fire. Obviously, one GPM is one gallon of water applied per minute. Determining our potential GPM starts by examining our water supply. Whether we are provided water from an apparatus booster tank, an expandable “drop tank,” a pressurized fire hydrant, or pressurized lines from tenders, this is the first consideration for how many GPMs we can bring to the battle. Next, we need to consider our pump size. While most engines have fixed pumps capable of bringing between 1,000 and 2,000 GPMs to the fight, smaller apparatus (i.e. brush trucks with 30 GPM pumps) may clearly be a constraint on our GPMs.

Hose Line Selection

Next step is to examine our hose line selection. Smaller hose lines may limit our GPMs by virtue of the friction loss associated with their diameter. While large hose lines may supply much larger GPMs, we have to consider their mobility as well. Although a five-inch hose line would give us incredible flexibility in regards to GPMs, many of us would not likely attempt to move a five-inch hose line around the interior of a building fire. 

Nozzle Selection

We next have to consider our nozzle selection and can include solid stream versus fog nozzles as well as fixed versus variable gallonage (automatic) nozzles. Fixed gallonage (i.e., a nozzle that at most can supply 95 GPM) will obviously be a constraint whereas automatic nozzles have their GPM determined by the amount of pressure applied to the tip of the hose line. We also have to consider what we want our fire stream to do. Solid streams provide us greater penetration and the ability to produce a stream that is less likely to be converted to steam during application. Fog nozzles producing straight streams are a bit more broken than solid streams — and much more broken when you turn them to the left in a modified fog or full fog pattern. These streams can provide better coverage of an area, can be beneficial when converted to steam in a confined area (one that still has a roof on it), but likely offer a lower level of penetration when compared to solid streams. 

Hose Line Placement

Finally, we have to consider our hose line placement. Taking a hose line inside a burning building and applying our GPM’s directly to the burning contents is ideal. However, sometimes the building is not safe to enter or has already had a partial or complete collapse upon our arrival. In these cases, we have to consider the fact that even our solid streams will convert to broken streams at their breakover points when applied from the ground. Therefore, elevated solid streams — like those at the tip of a ladder — will provide us great tactical advantage to hold the stream together longer and have a better chance of reaching the burning contents without being converted to steam beforehand. 

If we look back at the history of our war with fire, our water supplies, pumps, hose lines, and nozzles have had tremendous technological advances. We definitely have tactical advantages above our ancestors who were often forced to use horses, hand powered pumps, and buckets.

British Thermal Units (BTUs) are the way by which we measure the heat release rates of the building and its contents. Much like the heater in your home, which is rated to provide you with a certain number of BTUs, our buildings and its contents will provide us with a certain number of BTUs depending upon the volume, composition and arrangement. A BTU is the amount of heat required to raise one pound of water, one degree Fahrenheit. So, to move one pound of water from 68 degrees Fahrenheit to 212 degrees Fahrenheit (the temperature at which our water vaporizes and turns to steam), we need 144 BTUs (212 minus 68). One pound of water also absorbs an additional 970 BTUs through Latent Heat of Vaporization for a total of 1,114 BTUs per pound of water. Since it takes 8.34 pounds of water to equal one gallon, we can expect that each one of our GPMs will absorb around 9,290 BTUs (1,114 x 8.34). This means we can expect our 1.75-inch handline with an automatic nozzle on it that is flowing 125 gallons per minute to absorb around 1.1 million BTUs per minute (9,920 x 125). 

NIST has performed a considerable amount of research on the heat release rates of various furniture, building materials and furnishings. They found that a 100-pound sofa will produce five megawatts of energy translating to around 284,000 BTUs per minute. This explains how our 1.75-inch handline (absorbing 1.1 million BTUs per minute) is able to extinguish a burning sofa successfully. Our heat release rates have changed as a result of the widespread use of synthetics. One hundred years ago, furniture was constructed of wood and cotton. A pound of cotton produces around 6,800 BTUs and a pound of wood produces around 7,000 BTUs. 

Today, our furniture is constructed of synthetics such as Polyurethane, Polyvinyl Chloride, Polystyrene, and Polyethylene (all of which contain hydrocarbon polymers). Polyurethane and polyethylene (both hydrocarbons) produce around 14,700 BTUs and 20,000 BTUs per pound respectively. Recall that gasoline is also a hydrocarbon. By comparison, a five-pound Polyethylene cushion will have a similar heat release rate as a gallon of burning gasoline (around 115,000 BTUs). A polyurethane mattress will release 340,000 BTUs. This, however, only considers one piece of furniture.

The average single-family dwelling has 8,000 board feet of lumber at 2.5 pounds per foot (20,000 pounds) and an average of 4,000 pounds of furnishings and contents (sofa, television, carpeting, drapes, and clothing). If given a 2,000 square foot house and we divide by the 24,000 pounds of combustibles above, we can estimate around 12 pounds of combustibles per square foot. If we assume all of our combustibles are Class A (ordinary combustibles), even though we know they are not, we are looking at 192,000,000 BTUs. That number divided by 60 minutes of burn time for full consumption would give us a heat release rate of 3.3 million BTUs per minute. If each one of our hand lines is absorbing 1.1 million BTUs per minute, we would need three 1.75-inch handlines. Alternatively, we could use one 1.75-inch and one 2.5-inch line. In either case, our GPMs should exceed our BTUs, which will enable us to win that battle.

NIST determined that a 100-pound sofa will produce 284,000 BTUs per minute. This would require around 30 GPM to extinguish. Since most automatic sprinkler heads can provide between 10 and 25 GPM, we really only need a sprinkler head or two to hold the fire to that item first ignited — even when it is a sofa. 

Of course, as we all know, the biggest threat to life inside a compartment fire is not the fire itself but the toxic byproducts of combustion otherwise known as the smoke. The fuel packages 100 years ago of cotton and wood produced much less toxic smoke than today’s synthetics. The resultant smoke and heat release rates have translated into a greatly reduced time to escape today’s structure fires. Even 30 years ago, we had close to 17 minutes to exit a burning structure. Today, it is more like three to four minutes. This is why our prevention efforts are so important. Smoke detectors, exit drills in the home, and sleeping with the door closed are all tactics that our citizens can use to maximize the very limited time they have to escape a fire in their home. Once we arrive, the battle of GPMs versus BTUs will begin. 

Building Construction

Don’t forget the final change that I will mention, the building construction. Remember that lightweight construction is not lightweight. It kills you when it falls on you, which is usually really early in the fire. Modern construction now sees roof trusses that are glued together and that glue will turn back to a liquid under even limited exposure to heat and flames. This will cause structural members to fail more quickly. Keep in mind also that the energy efficiency in modern construction is designed to prevent temperature exchange with the outside environment, meaning it will hold heat from a fire better than an older building, at least until the fire is sufficiently vented.

In our protracted war against fire, remember that there is no acceptable loss of life or allowable casualty rate. Given the increased heat release rates and smoke toxicity associated with our changes to building construction, we must be more cautious in our battles than we have ever been.

Be safe and do good.

David Greene has over 31 years of experience in the fire service and is currently the deputy chief with Colleton County (S.C.) Fire-Rescue. He holds a PhD in Fire and Emergency Management Administration from Oklahoma State University and an MBA degree from the University of South Carolina. He is a certified Executive Fire Officer through the National Fire Academy, holds the Chief Fire Officer Designation from the Center for Public Safety Excellence, holds Member Grade in the Institution of Fire Engineers, is an adjunct instructor for the South Carolina Fire Academy and is a Nationally Registered Paramedic. He can be reached at dagreene@lowcountry.com.

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