Fireplace overheats due to incorrectly sealed glass panel

A condition occurred in the operation of a residential fireplace which overheated the rear of the fireplace. This damaged the fireplace and ignited the building materials at the rear of the fireplace.

The equipment involved was a natural gas-fired direct vent fireplace installed in the living room of a single-family residence.

  • This type of equipment is designed with a balanced flue to ensure that there is sufficient combustion air entering the fireplace to replace the products of combustion exhausting from the fireplace.
  • This is sealed with a front glass panel so that it will maintain this balance and prevent combustion products from entering a living space. The air/exhaust balance requires the fireplace to only operate with the sealed glass panel in place.

An investigation showed the following:

  • The homeowner had removed the front glass panel to clean it and failed to seal it correctly which provided an open gap at the lower front of the fireplace.
  • The fireplace was operated with the glass not correctly sealed for approximately three hours. The owner reported hearing what sounded like a fan running while the furnace was operating.
  • Air from the room entered the front of the fireplace disrupting the air/exhaust balance and affecting the flow though the combustion air supply duct to the burner. This appears to have created a condition where a “hot spot” was created at the rear of the fireplace.
  • The fireplace was shut off and several hours later the house smoke detectors alerted the residents to a fire on the outside rear of the home. The home owner and the fire department were able to extinguish the fire with minimal damage to the structure.
  • During the investigation, the gas entering the inside the burner was ignited creating a sound that the owner confirmed was identical to that which was heard during the three hours that the fireplace operated before the fire.
  • The natural gas burning inside the burner would further affect the operation of the fireplace.
  • The appliance is designed so that the combustion air is ducted down through a metal duct in the rear of the fireplace and extends out to the burner location.
  • The metal cover plate over the duct was examined and exhibited signs of heat damage on the bottom side which covers the combustion air duct at the burner location.
  • The major heat and fire damage appears to be centered where the combustion air duct runs down the inside rear area of the fireplace.

The physical evidence suggests that the cool air entering the lower front space between the glass and metal frame of the fireplace became the source of combustion air for the burner.

The warm rising air in the fireplace combustion air duct created a second exhaust vent with hot gases from the burner travelling up the rear of the fireplace and out the combustion air inlet.

The incident is documented in our enhanced incident investigation report format.

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