Carbon Monoxide
Carbon monoxide is odourless, and the only reliable protection is a working alarm and an annual combustion check. Everything else is guesswork.
Short answer
Carbon monoxide comes from incomplete combustion and from combustion products escaping into the living space — a cracked heat exchanger, a blocked or disconnected flue, inadequate combustion air, or a depressurized mechanical room. Ontario requires carbon monoxide alarms in homes with fuel-burning appliances, and the furnace needs an annual combustion check.
Where it comes from
A cracked or failed heat exchanger, which lets combustion products into the supply air and then into every room.
A blocked, disconnected or deteriorated flue, which lets them into the mechanical room.
Inadequate combustion air, where the furnace cannot get enough air to burn completely — frequently caused by a tightened house or a blocked combustion air opening.
A depressurized mechanical room, where an exhaust fan, a dryer or a range hood pulls hard enough to backdraft the flue.
A blocked or iced high-efficiency vent termination outside, which in a Toronto winter happens after a snowfall.
And other appliances: the water heater, a gas fireplace, a gas range, and anything running in an attached garage.
Check the vent after a snowfall
A high-efficiency furnace vents through a plastic pipe out of a side wall, frequently low down. Drifted snow, ice from a dripping eavestrough, or a shovelled pile against the wall blocks it. The furnace will usually shut itself down on the pressure switch, which is the safety working — but check it after every storm, because it is a two-second look.
The Ontario requirement
Carbon monoxide alarms are required in Ontario homes with fuel-burning appliances or an attached garage.
Placement is specified — adjacent to sleeping areas, and the requirement differs for a home with a fuel-burning appliance on a storey.
Alarms have a service life and expire. A ten-year-old alarm is not protecting anybody, and the expiry date is printed on it.
Test them, and replace the batteries on a schedule.
A combination smoke and carbon monoxide alarm satisfies both where it is appropriately placed.
And in a rental, this is a landlord obligation, and the tenant should raise it in writing if alarms are absent.
If an alarm sounds
Get everybody out into fresh air, immediately, and do not go back in to investigate.
Call 911 from outside.
Do not open the windows and reset the alarm, which destroys the evidence of what the level was and sends everybody back into the house.
Have the appliances checked by a licensed technician before anybody uses them again.
Treat symptoms — headache, nausea, dizziness, confusion, several people affected at once — as the emergency rather than waiting for the alarm.
And if the alarm has sounded once, find out why rather than assuming it was faulty.
Common questions
Are carbon monoxide alarms required in Ontario?
Yes, in homes with an appliance that burns fuel, or a garage attached to the house.
Do alarms expire?
Yes. The date is printed on the unit. A ten-year-old alarm is not protecting anybody.
What should I check after a snowstorm?
The high-efficiency vent termination outside, which blocks with drifted snow and ice.