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

A review of fire analysis methodology suggests that the classic “V” thermal damage pattern is used by analysts to determine the origin of the fire, the base of the V being the likely origin of the fire. The damage that forms the shape of the V can be a result of soot deposition, direct flame impingement, combustion of a fuel or melting. Fire development by convective means often results in flames or hot gases rising and diffusing upward, forming a V. The example in Figure 1 at the bottom left illustrates the base of the V-shaped pattern (yellow dashed line), which is at the ice dispensing unit (denoted by a red arrow). There is some minor damage below the V as a result of “drop down” debris (the green arrow) often discounted as an indicator of the fire origin. The red arrow points to the probable origin of the fire. The inherent assumption is that fire generated natural convection (gas movement as a result of gas density differences) has resulted in a V-shaped damage pattern. Forced convection such as wind or mechanical influences can distort the pattern such that it does not resemble a V, adding difficulty in determining the fire origin.

Figure 2 serves to illustrate a wider burn pattern with similar characteristics (yellow dashed line). This V pattern suggests a fire origin at the red arrow in the vicinity of a stove top. There is some drop down burning debris (green arrow) as in many instances, but the significant base of the V is on top of the stove. The typical smoke cloud pattern is shown on the walls, illustrating the interface between burning gases and the clear air below. These burned areas are not at the fire origin since they are high and not low, in the vicinity of the V. 

Figure 3 is a top view of a battery charger that was near the origin of a fire. The polymer housing at the right lower corner of the charger is the most severely damaged area. No batteries were being charged at the time of the fire, but the unit was plugged into a wall receptacle. The electronics driving the battery charger are at the upper left corner and undamaged (red arrow). The direction of the heat transfer appears to be from the lower right to the upper left. There is no evidence of decreasing thermal damage from the upper left to the lower right on the charger, alluding to the conclusion that the fire origin is likely not at the charger but to the lower right, outside the polymer enclosure.

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 9 is an insulated plumbing fitting (dielectric union) attached to a water heater. It is insulated to reduce the onset of galvanic corrosion between the two dissimilar metals, copper, and iron. After a lightning strike on the home, this damage was found along with other evidence of high voltage electrical current. The resistance of the dielectric union, in concert with high current, caused excessive heating of the outer nut and severe arc damage.  This is not a result of soldering, which supplies insufficient heat to cause such damage to iron. Rather, it was most likely a result of high current flow, characteristic of a lightning strike.

Figure 10 depicts a door frame in a basement area with a fire origin at the upper part of the frame. Char depth is most severe in this area (red arrow). Electrical wiring above, at the door frame, shows evidence of arcing, probably as a result of continuously chaffing against the door, causing insulation breakdown, electrical malfunction and a fire. Char damage to wood members tends to decrease as one moves away in any direction from the location of the red arrow, a good fire origin indicator.

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.

Figure 13 depicts a relatively small burn pattern inside of a heating unit. The fire origin (red arrow) was located at severely damaged electrical wire (green arrow) that was routed around a sharp metal corner where a wire had chaffed and shorted. The blower had sucked burning debris and smoke into its intake and spread it around the apartment, causing significant smoke-related damage. Note the forced air flow has affected the natural convective V pattern, which was distorted to the left as a result of the blower.

Thermal patterns on overheated boilers tend to be uniform since the whole unit tends to be overheated. In Figure 14, we see a boiler that overheated, causing a fire and severe damage to a building. A low water cut off control was found to have malfunctioned, causing the over heating. The dark grey, high temperature oxide is noted on the outer shell of the boiler, which is consistent with overheating. 

Figure 15  is a photo of a badly damaged electronic processing machine which shows uniform burn patterns throughout the machine and duct work (red arrows). This is indicative of ignition of combustible vapors in the machine and exhaust duck work. Burn patterns of this nature are often not definitive as to the origin of the fire, other than that a combustible vapor was ignited inside the machine.

Heat-related damage pattern analysis is classically used by investigators to opine as to where a fire started. Among many other tools, V-pattern analysis, evaluation of char depth gradients, evaluation of melted material, and observation of smoke deposition have been illustrated in the 15 case studies presented in this article. These case studies serve as specific examples of heat- related damage analysis but do not exhaust the myriad conditions that exist in the field. Each fire case is unique, but the burn pattern analysis methods are similar, as illustrated in this piece.

Endnotes 1 “Burn Pattern Recognition for Fire Origin Analysis,” appeared in Insurance Adjuster Magazine (later Claims Magazine), July 1987. 2“Thermal Pattern Analysis: Investigating Fire’s Fingerprints,” Claims Magazine, June 2000. http:/

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