Filed Under:Claims, Investigative & Forensics

Technical Notebook: Fire Origin Analysis

Heat-Related Damage Patterns

Heat-related damage patterns at a fire scene yield clues as to where a fire originated. This article is a third in a series that discusses burn patterns and interpretations when attempting to determine the origin of a fire. The first article1 dealt with burn and damage patterns on buildings and interpretations of the damage. The second article2 dealt with selected case studies regarding analysis of burn patterns. This third article presents additional case studies not mentioned in the previous two articles.

Photographs facilitate the discussion of burn patterns and are unique to a particular loss. Consequently, this article contains several photos of burn patterns from actual losses in a picture book format to act as a reference when analyzing a particular fire-related claim

Some thermal patterns develop before a combustible material is ignited, as shown in the pyrophoric decomposition of wood: the chemical decomposition of wood brought on by constant or periodic heat application (Figure 4). The exhaust pipe in an auxiliary heater has caused excessive heating of the wood enclosure and a brown shade to the wood surface has developed (red arrow). This heating, if unchecked, will eventually result in ignition of the wood and a fire. Examining this exemplar installation (Figure 4) was certainly helpful, since it explained why the fire started next to the heater in another identical installation.

A unique thermal damage pattern is that of metal erosion by electrical arc as shown in Figure 5. A fault current had developed near the top of the electrical panel (red arrow) causing ionization of the metal. This was the most severely damaged area on the panel with some charring of the mounting board, suggesting the top of the panel to be the fire origin. As with many other forms of electrical malfunction, the mere existence of arc erosion is not necessarily the origin of the fire. Arc erosion damage should be evaluated along with other burn patterns in the area. In this case, the surrounding burn patterns are less severe than that on the panel, leading one to theorize that the panel is the likely origin of the fire.

Some thermal patterns at the fire origin can exist inside a wall, which may not be initially visible. Figure 6A (upper photo) shows a vertical burn pattern on paper backed fiberglass insulation up through a wall (red arrow). Figure 6B shows a pipe joint that had been soldered prior to the fire (green arrow) and a low burn area on the paper backed thermal insulation. Shortly after the plumber left the home, a fire developed. The origin of the fire appears to be at the pipe joint since there is no other low burn damage in the area. The fire easily spread up the wall into the attic above, burning the paper backing of the insulation. The lack of a fire break in this vertical cavity aggravated the situation by allowing rapid spread of this fire.

The thermal pattern in Figure 7 is a result of smoke transport through or up a wall and exiting at gaps around an electrical receptacle. There is no damage to the electrical wiring or receptacle. Particulate deposition along a smoke path may be mistaken as a fire origin. The absence of damage to the receptacle should invoke caution in opining that this area is a fire origin.

Thermal damage from a furnace malfunction is of interest to the analyst. In Figure 8, the soot deposition at the side and above the furnace (red arrow) is symptomatic of flame roll out, a phenomenon usually caused by insufficient flue vent area. An inspection of the chimney revealed deteriorated masonry blocking the flue vent, causing furnace flames to roll out the front of the furnace and ignite paper garbage in a plastic container. Modern furnaces have “spill” switches that shut down the furnace if flame roll out occurs, but many older furnaces do not. Defeating the function of “spill” switches on modern furnaces has been known to occur and can result in flame roll out patterns like that shown in Figure 8 at the bottom left. 

Figure 11 shows a burn pattern on a clothes dryer with an approximate origin at the red arrow. The low burn and badly oxidized sheet metal on the dryer suggest that a malfunction inside the dryer caused a fire. There is relatively little damage around the dryer which supports the opinion that the dryer caused the fire.

A fire originated on a plumbing stack in a home after a lightning strike (Figure 12). There was evidence of burning in the form of char on roof sheathing around the plumbing vent only, with the absence of any other burn patterns throughout the home. There were no man-made electrical or mechanical ignition sources in this area. Char depth is most severe right next to the plumbing vent and decreases away from the vent.

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