Return air duct opening reduces ambient air pressure, causes fatal CO poisoning

The West Vancouver fire department responded to a 911 call and discovered four individuals with various degrees of carbon monoxide poisoning in a home. The fire department measured up to 900 parts per million of carbon monoxide (CO) in the home’s ambient air. There were no carbon monoxide detectors in the home to alert the residents of the presence of this odorless, colourless gas. Unfortunately, the incident resulted in one fatality.

The home’s mechanical system was comprised of two main components:

  • a low mass, fin tube hot water boiler which provides heat for the domestic hot water, fan coil, pool and spa; and
  • an air handling unit which moves air throughout the home to be either heated in the winter by the boiler water, or cooled in the summer by the outdoor condensing unit.

Our investigation showed the following:

  • A day before the incident, the pool system had been energized to bring it up to a comfortable temperature for use which created an extended boiler run time.
  • Almost all of the windows and doors within the home had been kept closed to allow the air conditioning system to keep the house cool.
  • The combustion air supply grill located on the exterior of the home was fully blocked with dryer lint from a moisture exhaust duct from the dryer. Additionally, when the combustion air supply ducting was dismantled, it was found to be significantly restricted due to the installation configuration.
  • Multiple operational tests were performed on the boiler during the investigation which found that when the boiler was energized with the mechanical room door closed and the air handling unit running, products of combustion immediately began to spill out through the draft hood of the boiler. Additionally, when either the mechanical room door was opened or the air handling unit was de-energized, the draft in the venting system would immediately re-correct and begin to vent correctly through the venting system to the outdoors.
  • A boiler mechanical room depressurization test conducted with the door closed and the air handling unit running indicated a negative ambient air pressure induced within the mechanical room relative to the outdoor atmospheric pressure.

Multiple smoke tests performed within the mechanical room revealed that the air within the room was mechanically being drawn into a large unrestricted opening in the return air ductwork.

It is highly probable that the main cause of the incident can be attributed to the large unrestricted opening in the return air duct work creating a negative ambient pressure condition in the mechanical room. Other contributing factors included:

  • a blocked/inadequate combustion air supply into the mechanical room
  • an extended boiler run time
  • a lack of carbon monoxide detectors

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

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