Construction Concerns: Fuels and Fire Behavior

By Gregory Havel

Firefighters are familiar with many of the terms used in describing fire behavior. Photo 1 shows an acquired structure used for live fire training compliant with NFPA 1403, Standard on Live Fire Training Evolutions, at the point where the remaining structure was allowed to burn down.

(1) Photos by author.


During the past few years, the laboratory and acquired structure studies of fire behavior by the National Institute for Standards and Technology (NIST) and Underwriters Laboratories (UL) have exposed us to a large volume of scientific data and reports which express measurements in the International System of Units (SI, or metric system, which is not yet in daily use in the United States). The data also includes some terms and data which may not have been included in our firefighter training.

To understand the scientific data that is available to us today, we need to understand the relationship between SI and the system of measurement that is common in the United States.

The definitions below in quotation marks are taken from NFPA 921, Guide for Fire and Explosion Investigations, 2014 edition, Chapter 3.

Temperature is “The degree of sensible heat of a body as measured by a thermometer or similar instrument”. [NFPA 921:3.3.172] It is measured by a thermometer, or by a thermocouple (Photo 2) for more extreme temperatures. Do an Internet search for “thermocouple”, “thermocouple types”, and “thermocouple wire” for an explanation of the theory and function of theromocouples.



The three common temperature scales are the Fahrenheit, Celsius, and Kelvin. The United States and a few other countries use Fahrenheit. The rest of the world uses Celsius. The scientific community uses Celsius and Kelvin.

  • The Fahrenheit thermometer dates to 1724. It has zero as the temperature at which brine freezes, and 98.6 as the human body’s core temperature. On this scale, pure water freezes at 32°F and boils at 212°F at sea level and atmospheric pressure.
  • The Celsius thermometer dates to 1742. It uses a 100° interval between pure water freezing at 0oC and boiling at 100°C. This scale has also been called “Centigrade”.
  • The Kelvin scale dates to 1848. It uses the unit of the °C with 0K as absolute zero (the point at which all thermal motion ceases at the molecular level; -273.15 oC; -459.67 oF); pure water freezes at 273.16K and boils at 373.13 K.
  • To convert temperatures between these scales, use these formulas:

oF = 9/5 oC + 32

oC = 5/9 oF – 32

K = oC + 273.2

K = oF + 459.7

Flash Point is “The lowest temperature of a liquid, as determined by specific laboratory tests, at which the liquid gives off vapors at a sufficient rate to support a momentary flame across its surface.” [NFPA 921:3.3.82]

Ignition temperature (piloted ignition temperature) is “The minimum temperature a substance should attain in order to ignite under specific test conditions.” [NFPA 921:3.3.106] This is usually higher than the flash point for a liquid.

Auto-ignition temperature is “The lowest temperature at which a combustible material ignites in air without a spark or flame.” [NFPA 921:3.3.15] This is usually higher than either the ignition temperature or flash point.

The temperature of a fire varies depending on the type of fuel; the geometry (dimensions and arrangement of the fuel); the position of the thermocouple in the flame, thermal column, or flow path; and on the amount of oxygen available for combustion.

Pressure is traditionally measured in pounds per square inch (PSI). In the SI or metric system, the following applies:

  • 1 PSI = 6.895 Kilopascals (kPa).
  • 1 kPa = 0.145 PSI.
  • 1 bar = 100 kPa, or 14.5 PSI (atmospheric pressure at sea level).

These are the units used when discussing partial vacuum; atmospheric pressure; compressed gases in containers; fuels in pipelines, tanks, and pressure vessels; and the expansion of combustion products because of heating.

Energy is usually measured as kinetic energy. From this measurement, it is possible to calculate the potential energy of a fuel as British Thermal Units (Btus) per pound or as kilowatts per kilogram in the SI or metric system.

BTUs are “The quantity of heat required to raise the temperature of one pound of water 1°F at the pressure of one atmosphere and temperature of 60°F; it is equal to 1,055 joules, 1.055 kilojoules, and 252.15 calories.” [NFPA 921:3.3.21]

In the SI or metric system:

  • A joule is “The …heat produced when one ampere is passed through a resistance of one ohm for one second, or it is the work required to move a distance of one meter against a force of one newton. There are 4.184 joules in a calorie, and 1055 joules in a British thermal unit (Btu). A watt is a joule/second.” [NFPA 921:3.3.114] (A Newton is a unit of force required to impart an acceleration of one meter per second per second to a mass of one kilogram).
  • A watt (W) is a “Unit of power, or rate of work, equal to one joule per second, or the rate of work represented by a current of one ampere under the potential of one volt.” [NFPA 921:3.3.191] (A watt = 0.00134 horsepower (HP) = 3.42 BTU per hour).
  • A kilowatt (kW) is energy released at the rate of 1000 joules per second. A kW = 1.34 HP = 3,415 BTU per hour.

In the science of fire and combustion, some additional measurements are needed. These are usually expressed in the SI or metric system.

Heat release rate (HRR) (or rate of heat release) is “The rate at which heat energy is generated by burning.” [NFPA 921:3.3.99] It is usually measured in kW. The kW is 1,000 energy units (joules) released per second. The rate of heat release can be affected by several factors including moisture content of the fuel (fuels with higher moisture content burn more slowly) and available oxygen (compare a smoldering fire in an oxygen-deficient atmosphere to a free-burning fire, a wind-driven structure fire, a fire in a blacksmith’s forge, or a fire in a blast furnace).

Heat flux is “The measure of the rate of heat transfer to a surface, expressed in kilowatts/m2; kilojoules/m2· second; or Btu/ft2· second.” [NFPA 921:3.3.97] This can be affected by a change in the rate of heat release due to the availability of oxygen.

From the scientific studies performed at UL and NIST using these units and measurements, we can make the following conclusions about fire behavior:

  • A fire that contains fuel with a high potential and kinetic energy will require more cooling before extinguishment than a fire that contains fuel with a lower potential and kinetic energy. Firefighters or an automatic sprinkler system will need to apply a larger volume of water on these fires.
  • Fuels with high kinetic energy and high rates of heat release will result in fires with rapid growth, high levels of radiant and convection heat, and rapidly-expanding thermal columns and higher-velocity flow-paths.
  • Adding a ventilation opening to a burning structure by forcible entry for fire attack will change the ventilation profile of the structure as well as the fire, adding oxygen for more rapid combustion and flashover and the potential for firefighters to be trapped in the ventilation flow path.
  • A structure fire with a high rate of heat release that is combined with the oxygen available from an added ventilation opening can produce a heat flux that will exceed the level of protection provided by firefighters’ personal protective equipment, resulting in serious injury or fatality.

View the results of NIST studies at

The results of the UL and UL Firefighter Safety Research Institute are available online at and


Download this article as a PDF HERE


Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.




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