Fire in an Air Conditioner


Charles C. Roberts, Jr., Ph. D., P.E.

Figure 1 is a view of a home where a fire appeared to originate in a window mounted air conditioning unit that had been installed by the insured. In order to assess

Figure 1

the subrogation potential of such a loss, a fire investigation as to the cause is prudent. It was winter and the unit had been turned off, but not disconnected from the home wall receptacle. The protective cover around the outside of the air conditioning unit had been removed by the insured prior to the fire. There was severe fire damage to mechanical and electrical components. Inspection of control wiring showed no evidence of malfunctions.

Figure 2

Figure 3

While investigating the fan cavity in the unit, faulted wiring was noted as indicated by the red arrow in Figure 2. Figure 3 is a view of the faulted wiring showing beading on several of the wire strands. The circuit breaker had tripped, but sufficient heating from the fault had developed to ignite what appeared to be combustible material in the air conditioning unit. But, wait a minute, what is combustible material doing in the air conditioning unit? Excavation through the material, which was a mish mash of sticks, leaves and grass, resulted in discovery of the skeletal remains of a rodent-like animal (Figure 4). Examination of the carcass suggested it

Figure 4

was that of a North American grey squirrel. The significance of this find must be factored into the assessment of whether a defect in wiring caused the fire, or some intervening cause (the squirrel). The faulted wiring is not near a sharp bend or other chafing surface (Reference 1) that could have caused insulation breakdown. Since the unit was not operating at the time of the fire, vibratory influences on chafing of wire insulation are, most likely, not a factor. The combustible material was obviously a nest that the biologic (squirrel) used to pad its new digs. Normally, when rodents smell smoke, their instinct tells them to leave the scene. This one did not make it out in time, suggesting it was incapacitated, possibly by electrocution. It is known that rodents chew on wire insulation for a variety of reasons.(Reference 2) The suggestion that the squirrel chewed through wiring, causing an electrical fault and fire could not be ruled out. The preponderance of the evidence suggests that intervention by the biologic is more likely the cause of the fire than defective design or manufacture of the air conditioner. Since the biologic does not have legal standing (at this time), subrogation potential and chance of recovery are considered poor.


1. Roberts, Charles C. Jr., “Wire Chafing, a Cause of Electrical Fires,” Claims Magazine, April 1999, p79-80.

2. Roberts, Charles C. Jr., “Rodent Damage to Automobiles,” Claims Magazine, December 1996, p 35-36.