Top front cover of the furnace blown off in residential house

Incident Investigation

Top front cover of the furnace blown off in residential house

October 6, 2019


Reference Number:

#II-917149-2019 (#15557)

The damage was contained to the furnace in a single family residential house. The top front cover of the furnace had been blown off. The plastic housing containing the inducer fan had broken away from multiple attachment points and was hanging from the front of the furnace by the tubing and wiring. There were burn marks at the upper right hand corner of the furnace on the interior insulation and some melting of the plastic venting that was inside of the furnace cabinet. There were also some wires that had burnt and melted insulation.

An explosion and small fire occurred inside a residential natural gas furnace in a single family detached home built in 2018.

Natural gas travels from the outlet of the gas utilities’ meter to appliances in the home via a piping and tubing system. Gas piping and tubing systems are made to be gas tight so the combustible gas cannot escape to atmosphere. The tubing used in this system is copper tubing and is sealed to the valves and appliances using flared fittings. The fittings are made of brass and have a male threaded portion with a beveled sealing surface and a threaded flare nut that screws on to the male portion of the fitting. The flare nut is first placed over the end of the copper tubing then the end of the copper tubing is flared out using a specialized tool so the nut can slide over the flare but not come off of the tube. The flared copper meets up with the beveled sealing surface on the fitting, then the nut is threaded over the fitting and tightened, compressing the copper to the sealing surface on the brass fitting generating a gas tight seal. (See picture #1 in pdf) The high efficiency gas furnace incorporates a draft inducer fan. The draft inducer fan is sealed to the inside of the furnace heat exchanger and is driven by a small electric motor. The purposes of the draft inducer fan are to purge any combustion gases that remain inside of the combustion chamber after the previous heating cycle, and to ensure heat is distributed evenly inside of the heat exchanger to maximize its efficiency. The fan does this by first starting 15-30 seconds before the gas burners ignite, running while the burners are operating, then continuing to run for 15 seconds after the burners shut off. This draws air from inside the cabinet area of the furnace, across the burners, through the heat exchanger and out the venting system to the outdoors.

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