Separated boiler vent triggers carbon monoxide leak

Incident Investigation

Separated boiler vent triggers carbon monoxide leak

October 31, 2017


Reference Number:


Incident overview

The venting system had separated at the vent outlet of the boiler. The gasses produced by the operation of the boiler were released into the house causing the carbon monoxide levels to increase and the carbon monoxide alarms to go off.

Regulated industry sector

Natural gas system


Coquitlam, BC

Investigation conclusions

Site, system and components

The boiler is a two vent pipe style appliance which pulls fresh air needed for the combustion process from outside through one vent pipe, and vents the exhaust gases out through a second pipe to the outdoors. The boiler is supplied with gas from a regulator which reduces the pressure of the gas from the house gas meter to a pre-set pressure required by the boiler for proper operation of the burner. When the temperature of the heating system drops the room thermostat will send an electrical signal to an internal gas valve with-in the boiler, allowing it to open sending gas to the burner, which will then be ignited by an ignition source, the heat produced by the burner transfers to the hot water that is circulated through the boiler.

Failure scenario(s)

The boiler located in the mechanical room closet in the basement appeared to have “hard light off” during ignition resulting in a blast of pressure in the venting system. The venting system was installed without a clamp that secured the venting coming out of the boiler to the rest of the venting system. The blast of pressure in the venting system was enough pressure to separate the piece of venting allowing the exhaust gasses produced from the burner to vent inside the boiler room.

Facts and evidence

  • Venting system was found separated above boiler
  • According to the homeowners, no regular maintenance performed on boiler system
  • Missing piece of equipment vital to venting system operation
  • Gas supply to boiler not installed as per manufacturer’s instructions
  • Venting system not installed to manufacturer’s instructions

Causes and contributing factors

It was very likely the boiler was being supplied with too much gas during the initial lighting of the appliance. This caused excessive pressure within the appliance and venting system. The missing piece of the venting system and the plausible blast of pressure most likely caused the venting system to come apart inside the boiler room causing exhaust gasses from the boiler to be vented inside the house.


  • Injury
    • Qty injuries: 1
    • Injury description: One person experienced carbon monoxide poisoning symptoms.
    • Injury rating: Minor
  • Damage
    • Damage description: A section of the venting system had separated from the boiler.
    • Damage rating: Moderate

Incident rating


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