HVAC fire

Feb 2, 2009 by

The case of the month is a residential HVAC fire.   The fire department attributed the fire to a poorly maintained furnace.   Our client was not content to stop there.   GEI was called in to answer the question, “What Really happened?”

Our expert found that the fire had started in the fuse type electrical circuit disconnect box, mounted on an exterior wall.   The HVAC unit was installed about three years before the fire, along with other general remodeling.   The general quality of the electrical installation was poor.  A large twist of ground wires was positioned above and on the right side power wires.   An abandoned power wire remnant had a cut off, loose end uncapped, in near proximity to the right side power supply wire, and to the ground or neutral clamp.

There was clear evidence of an installed direct short to ground.   A wire exited the ground wire bundle, passed in contact with and under the feed power wire to the right side fuse, that connected to the right side fuse clamp.  The wire  was secured under the neutral/ground clamp completing a direct short.

Our expert concluded that the direct short was formed at the time of remodeling and caused the enclosure and ground wires to become part of the right-hand fuse power circuit.  The fact that the disconnect box was mounted on an exterior wall acted as an insulator from ground. Hence the HVAC unit serviced by the subject disconnect had operated in a floating neutral condition for three years.  The disconnect box also was not properly weather sealed.  Rain and sprinkler water dripped behind the enclosure and eventually entered the enclosure.  The improperly wired disconnect then shorted from phase to phase.  The fire had been caused by the faulty electrical installation-it just took three years to finally start.

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