Key facts about carbon monoxide at home
What it is
Carbon monoxide (CO) is a colourless, odourless, tasteless gas produced when fuel burns incompletely.
Common sources
Gas furnaces, boilers and water heaters, gas ranges, fireplaces, dryers, wood stoves, and vehicles running in an attached garage.
Symptoms
Headache, nausea, dizziness, confusion and drowsiness, which can progress to loss of consciousness and death.
Ontario law
Homes with a fuel-burning appliance, fireplace or attached garage need CO alarms next to each sleeping area and, since January 1, 2026, on every storey.
If the alarm sounds
Get everyone out immediately and call 9-1-1 from outside.
Prevention
Yearly inspection of fuel-burning appliances, clear vents, and working, certified CO alarms.
If your CO alarm goes off: what to do right now
- 1
Get everyone out of the home immediately,
Including pets. Do not stop to open windows or search for the source.
- 2
Call 9-1-1 from outside
Or from a neighbour’s home.
- 3
Do not go back inside
Until the fire department says it is safe.
- 4
Get medical help
For anyone with headache, nausea, dizziness, confusion or drowsiness, and tell responders you suspect CO.
- 5
Do not restart the furnace or other appliance
Until a licensed gas technician has found and fixed the cause.
Treat every alarm as real, even if nobody feels unwell. Symptoms can be mild at first, and people who are asleep may not notice them at all.
Chirping is not the same as an alarm
A short chirp every minute or so usually means a low battery or that the alarm has reached its end of life. A loud repeating pattern is the emergency alarm. Learn the difference from your alarm’s manual before you need it.
Where carbon monoxide comes from in Ottawa homes
| Source | How CO can build up | Prevention |
|---|---|---|
| Gas furnace | Cracked heat exchanger, poor combustion, blocked or disconnected venting | Yearly inspection with a combustion check |
| Sidewall PVC vents on high-efficiency furnaces and water heaters | Snowdrifts or ice blocking the intake or exhaust | Keep vents clear after every storm |
| Chimney-vented water heater | Back-drafting, often after the furnace was upgraded and the chimney is left oversized | Check venting and chimney liner when the furnace changes |
| Boiler | Poor combustion or venting faults | Yearly service |
| Gas range or oven | Poorly adjusted burners; using the oven to heat the home | Use the range hood; never heat the home with the oven |
| Gas fireplace or wood stove | Blocked flue or closed damper | Yearly inspection; open the flue before use |
| Attached garage | Car, snowblower or generator running inside | Never run engines in the garage, even with the door open |
| BBQs and portable generators | Used indoors or near doors and windows | Use outdoors only, well away from openings |
Ottawa winters add a local risk. After heavy snow or drifting, the white plastic intake and exhaust pipes on the side of many homes can be buried. Modern furnaces usually shut down when venting is blocked, but you should check the pipes after every storm.
Carbon monoxide poisoning symptoms
Early CO poisoning feels a lot like the flu, which is why it is easy to miss in winter. Ontario lists the symptoms as headaches, nausea, dizziness, confusion and drowsiness, progressing to loss of consciousness and death.
Clues that point to CO rather than the flu
- Several people in the home, or pets, feel unwell at the same time.
- Symptoms ease when you leave the house and return when you come back.
- Symptoms are worse in the morning or when the furnace has been running hard.
- Nobody has a fever.
If you notice these signs, leave the home and call 9-1-1, even if your alarm is silent. Alarms can fail, be in the wrong place, or be past their replacement date.
Ontario CO alarm law: what the Fire Code requires
CO alarms are required under the Ontario Fire Code (Division B, Section 2.16), and the rules were expanded on January 1, 2026.
Houses, townhouses and cottages
A CO alarm is required if the home has any of the following:
- A fuel-burning appliance, such as a furnace, water heater or stove using natural gas, propane, oil or wood.
- A fireplace.
- An attached garage.
- Heating air supplied by a fuel-burning appliance outside the home, such as one in a utility shed.
In those homes, CO alarms must be installed adjacent to each sleeping area and, since January 1, 2026, on every storey of the home, including storeys without sleeping areas. For most two-storey Ottawa homes with a gas furnace, that means alarms on the basement, main floor and bedroom level.
Apartments and condos
A unit needs CO alarms if it has a fuel-burning appliance or fireplace, sits directly above, below or beside a garage or service room, or is heated by air from a fuel-burning appliance outside the unit. Alarms go near each sleeping area and on every storey of the unit, and public corridors heated by fuel-burning appliances also need alarms.
Who is responsible
Homeowners are responsible in their own homes. In rental properties, landlords are responsible for installing and maintaining the required alarms, and tenants should report problems right away. Homes that are fully electric, with no fuel-burning appliances, fireplace or attached garage, do not require CO alarms.
Always follow the alarm manufacturer’s instructions on placement, and choose alarms that carry a recognized certification mark such as CSA, ULC or ETL.
Caring for your CO alarms
Test monthly
By pressing the test button.
Replace batteries yearly,
Or when the alarm chirps for low battery.
Replace the whole alarm
According to the manufacturer’s instructions. Most CO alarms have a limited life and an end-of-life warning; check the date on the back.
Consider hardwired or interconnected alarms
So an alarm in the basement sounds upstairs where people sleep.
Combination smoke and CO alarms
Are available, but you still need working smoke alarms on every storey as well.
Cracked heat exchangers and furnace carbon monoxide
The heat exchanger is the metal chamber inside your furnace that keeps combustion gases separate from the air blown through your home. When it cracks, combustion gases, including CO, can mix with the house air or disturb the burner flame so it produces more CO.
What causes a heat exchanger to crack
Age
Years of heating and cooling cycles stress the metal. Risk rises as furnaces pass 15 to 20 years.
Overheating
Dirty filters, closed supply vents and undersized ductwork trap heat in the exchanger.
Oversizing
A furnace too big for the home short-cycles, heating and cooling the metal more often.
Corrosion
Condensation or poor venting can corrode the metal from the inside.
Warning signs
- Soot or rust around the burners or on the furnace cabinet.
- Burner flames that flicker, lift or change shape when the blower starts.
- A furnace that repeatedly trips its flame rollout or limit safety switch.
- CO alarm activations or CO symptoms when the furnace runs.
A technician confirms a crack with a visual and camera inspection plus a combustion analysis. A cracked heat exchanger is a safety hazard, so a licensed technician will shut the furnace down rather than leave it running. On an older furnace, replacement is usually more sensible than repair. Our guides on how long a furnace lasts and furnace replacement cost can help with that decision.
How to prevent carbon monoxide problems
Book a yearly furnace inspection before winter.
Ontario recommends that fuel-burning appliances, chimneys and vents be inspected every year before the cold weather. A proper tune-up includes checking the heat exchanger, burners, venting and combustion. See our furnace maintenance service.
Change your furnace filter
Every 1 to 3 months to prevent overheating.
Keep vents clear
Of snow, ice, leaves and nests, and keep outdoor clearance around sidewall vents.
Never use an oven, stove or BBQ to heat the home,
And never run engines in the garage.
Open the flue
Before lighting a wood fireplace.
Re-check venting
After any furnace or water heater replacement.
Listen to your furnace.
New booms, rattles or roaring sounds can signal combustion problems. See furnace noises and what they mean.
Licensed help, 24/7
Oria Heating and Cooling is TSSA and HRAI approved, and every technician holds a gas licence. We have served Ottawa since 2005 and offer 24/7 emergency service for furnaces and other fuel-burning equipment.
If the fire department has cleared your home after a CO alarm, or you suspect a furnace problem, call (613) 866-1869 or see our emergency HVAC service. For an inspection before winter, book furnace repair or maintenance with our team.
Frequently Asked Questions
Can a furnace leak carbon monoxide?
Yes. A gas, propane or oil furnace can release carbon monoxide if it has a cracked heat exchanger, poor combustion from dirty or misadjusted burners, or blocked or disconnected venting. Because CO is invisible and odourless, the only reliable warning is a working CO alarm. A yearly inspection that includes a combustion check helps catch problems early.
Where are CO alarms required in Ontario homes?
Under the Ontario Fire Code, homes with a fuel-burning appliance, a fireplace, an attached garage, or heat supplied from a fuel-burning appliance outside the home must have CO alarms adjacent to each sleeping area. Since January 1, 2026, alarms are also required on every storey of the home, including storeys without sleeping areas.
What should I do if my carbon monoxide alarm goes off?
Get everyone, including pets, out of the home immediately and call 9-1-1 from outside. Do not stay to open windows or look for the source, and do not go back in until the fire department says it is safe. Afterwards, have a licensed gas technician find and fix the cause before using the appliance.
What are the symptoms of carbon monoxide poisoning?
Symptoms include headaches, nausea, dizziness, confusion and drowsiness, and high exposure can lead to loss of consciousness and death. CO poisoning is often mistaken for the flu. Warning clues include several people or pets feeling sick at once, no fever, and symptoms that ease when you leave the house. Leave and call 9-1-1.
Does my condo or apartment need a CO alarm?
In Ontario, a condo or apartment unit needs CO alarms if it has a fuel-burning appliance or fireplace, sits directly above, below or beside a garage or service room, or is heated by air from a fuel-burning appliance outside the unit. Alarms go near each sleeping area and on every storey of the unit.
How do I know if my furnace heat exchanger is cracked?
Signs include soot or rust around the burners, flames that flicker or change shape when the blower starts, repeated safety switch trips, and CO alarm activations when the furnace runs. A homeowner cannot confirm a crack safely. A licensed technician uses a visual and camera inspection plus combustion testing, and will shut down an unsafe furnace.
How often should CO alarms be replaced?
Replace CO alarms according to the manufacturer’s instructions, since the sensors wear out over time. Most alarms show a manufacture or replace-by date on the back and give an end-of-life warning chirp. Test alarms monthly with the test button and replace batteries every year, or whenever the low-battery chirp sounds.
Related Reading
Need help with your heating or cooling?
Family-owned, licensed HVAC contractor serving Ottawa and Gatineau since 2005. 5.0 stars from 396 Google reviews.