Domestic water heater causes possible carbon monoxide poisoning

Incident Investigation

Domestic water heater causes possible carbon monoxide poisoning

June 10, 2019


Reference Number:

II-864711-2019 (#13244)

Location: Vernon, BC

Regulated industry sector: Gas Natural gas system



Qty injuries: 1

Injury description: A person was taken to the hospital as a precaution for possible carbon monoxide poisoning.

Injury rating: Moderate


Damage description: None

Damage rating: None

Incident rating: Moderate

Incident overview

One domestic water heater produced high levels of carbon monoxide which entered into the occupied areas of the dwelling. Fire department arrived on site and measured a reading of 109 parts per million in the dwelling. The occupant was taken to the hospital as a precaution.

Investigation Conclusions

Site, system and components

Domestic water heaters are used for providing hot water to the dwelling. They are gas fired and are designed to remove the products of combustion through a venting system to the outdoors. The system relies on the negative pressure in the venting system to draw the products of combustion from the burner to the outdoors.

Failure scenario(s)

Negative pressure in the venting system could not be achieved causing products of combustion to enter occupancy and spill into the dwelling unit.

Facts and evidence

The flue gas was monitored with a combustion analyzer and recorded abnormal levels of carbon monoxide when the kitchen exhaust fan was operational and all doors and windows were closed in occupancy

A discussion with a licenced contractor revealed that the combustion air inlet to the water heater was blocked and a separate air supply was necessary for make-up air the kitchen exhaust fan

Causes and contributing factors

With the combustion air inlet screen blocked the required negative pressure within the venting system was not achieved causing the products of combustion to spill into the occupied space.