The conditions required for ignition

Lesson 3: The conditions required for ignition to occur.

The fire must be close enough for flying firebrands or flames to contact the ignitable portions of the structure and burn long enough to cause flaming ignition. We only see 1% of what is happening in a fire, as we don’t see the heat. Enough heat must be available long enough to convert a solid fuel to ignitable gas, which is determined by its auto ignition temperature. This is why one needs a lighter or bellows to start a wood or charcoal fire.

Heat energy generated by combustion transfers to the surroundings by convection from hot to cool surfaces—eg, from an object’s surface to its cooler interior. The duration of the exposure is very important. Pine needles or leaves on a shake roof can smolder and lead to a fire this way long after the fire front has passed. Yet a thick wooden beam or tree may simply scorch without burning.

Scientific experiments show that radiation of heat decreases rapidly with distance—eg, keeping a high intensity but fast moving crown fire 30 feet away from a wood wall resulted in the wall scorching but not igniting 90% of the time.

Next: New NFPA goals for the home ignition zone.