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Sources of Heat Energy in Fire: Chemical, Electrical, Mechanical & Nuclear
Where the heat that starts and sustains fires comes from, from spontaneous heating and arcing to friction and compression.
- Heat is energy; temperature is its intensity. BTUs, calories and joules are units of heat.
- Chemical heat includes heat of combustion, spontaneous heating, decomposition and solution.
- Electrical heat comes from resistance, arcing, leakage current, static and lightning.
- Mechanical heat comes from friction, friction sparks and compression.
- Ignition sources are often hidden in voids and concealed spaces.
Heat and temperature are not the same thing
Heat is energy on the move. Temperature measures how intense that energy is. A match flame is hotter than a bathtub of warm water, but the bathtub holds far more heat. Heat is measured in joules, calories or BTUs (a BTU raises one pound of water by one degree Fahrenheit). Anything that sets a material’s molecules moving faster produces heat in it.
There are four broad categories of heat energy: chemical, electrical, mechanical and nuclear. Fire investigators call the one that starts a fire the ignition source, and finding it is part of working out where and how a fire began.
Chemical heat energy
This is the biggest category, because every fire is itself a chemical reaction that releases heat.
- Heat of combustion is the heat given off by the oxidation process itself. It is what makes a fire spread: the fire heats new fuel faster than the fuel loses heat.
- Spontaneous heating is the self-heating of an organic material with no outside heat source. It happens when a reaction generates heat faster than the surroundings can carry it away. As a rule of thumb, the speed of such a reaction roughly doubles for every 18°F (10°C) rise in temperature, so a slow warm-up can run away.
- Heat of decomposition is released when an unstable compound breaks down. Some release it slowly. Others, such as certain peroxides and explosives, can release it all at once.
- Heat of solution is released when one substance dissolves in another. Some concentrated acids give off enough heat when mixed with water to ignite nearby combustibles.
Rags soaked with drying oils, such as linseed oil in some wood stains and finishes, can heat themselves as the oil cures. Pile them up and the heat is trapped, so the pile can ignite hours later, long after the work is done. Fire departments commonly tell people to lay such rags flat outdoors to dry, or to store them in a water-filled, sealed metal container, and to follow the product label.
Modern batteries add a newer chemical heat source. In a lithium-ion cell, damage, an internal short circuit or overcharging can trigger thermal runaway, a self-accelerating, heat-producing reaction inside the cell that can vent flammable vapor and flame. Departments handle these incidents under their own procedures.
Electrical heat energy
Electricity turns into heat in several ways. Investigators distinguish them because each leaves different evidence.
| Mechanism | How it works | Typical example |
|---|---|---|
| Resistance heating | Current passing through a conductor warms it | Overloaded extension cord, loose connection at an outlet |
| Arcing | A high-temperature discharge across a gap, or molten metal from a conductor | Damaged cord, failing breaker or fault inside a wall |
| Leakage current | Current escaping through poor or aging insulation | Overheated wiring where insulation has degraded |
| Static electricity | Spark when oppositely charged surfaces discharge | Flammable liquid flowing through hose or pipe without bonding and grounding |
| Lightning | Massive discharge between cloud and ground | Strike on a roof, attic or tree |
| Induction and dielectric heating | Magnetic fields or high-frequency current heat materials | Industrial heating equipment and some appliances |
Mechanical heat energy
Motion makes heat. Friction heat comes from two surfaces rubbing together: a seized bearing, a slipping belt, a tire dragging on a rim. Friction sparks are glowing particles thrown off when hard objects strike, as with grinders and cutting tools. Heat of compression is created when a gas is squeezed into a smaller volume. A diesel engine uses it on purpose to ignite fuel without a spark plug.
Nuclear heat energy
Splitting (fission) or combining (fusion) atoms releases heat. This is a textbook category more than a daily one: firefighters rarely meet nuclear heat as an ignition source. What they do meet is radiological material, which is a hazmat concern covered in other training.
MGM Grand, Las Vegas (1980): a hidden electrical fault
On the morning of November 21, 1980, a fire started in The Deli, a restaurant on the casino level of the MGM Grand Hotel. The Clark County Fire Department’s investigators concluded that the most probable cause was heat from an electrical ground fault in a combustible concealed space, a wall soffit, in a serving station.
The fire broke out of the concealed space, spread across the large, unsprinklered casino floor, and sent smoke through the tower. Eighty-five people died. You will return to this fire in the Products of Combustion lesson, because most of those deaths came from smoke, not flame. The lesson for this page: ignition sources are often hidden. A fault inside a combustible void can burn undetected until it has a big head start.
Fireground talk
- Ignition source
- The heat source that started the fire. Investigators work backward to find it.
- Area of origin
- The part of a structure where the fire began. Heat patterns and burn damage help locate it.
- Arcing
- Electrical discharge across a gap that can melt metal and ignite nearby materials.
- Thermal runaway
- A self-accelerating heating reaction inside a battery cell.
Practice it in the simulators
Study questions
What are the four categories of heat energy?
Chemical, electrical, mechanical and nuclear.
What is spontaneous heating?
The self-heating of an organic material with no outside heat source. It happens when the reaction produces heat faster than the surroundings can carry it away, often in insulated or piled material.
How does a diesel engine ignite fuel without a spark plug?
It uses heat of compression. Squeezing air into a small volume heats it enough to ignite the fuel.
Why do investigators care about the type of electrical heating?
Each mechanism, such as resistance heating, arcing or leakage current, leaves different physical evidence, so identifying it helps determine the ignition source.
Sources and further reading
- Lessons from the Past: MGM Grand Fire — Firehouse
- MGM Grand Hotel Fire Anniversary (Clark County Fire Department findings) — FireRescue Magazine
- NFPA 921, Guide for Fire and Explosion Investigations — NFPA (print standard)
- Essentials of Fire Fighting, IFSTA — Fire Protection Publications (print textbook)
About this guide. The Fire Behavior & Chemistry series was originally informed by LBFD Training Division recruit training material. This edition has been independently rewritten and expanded using the sources listed on each page, and its diagrams were drawn for this site. Free Fire Simulator is not affiliated with or endorsed by LBFD. Spotted an error or have a better source? Use the Contact Us button.