A carbon monoxide alarm is sounding in your home. This is a true life-safety emergency, not a nuisance alert. CO is odourless and colourless, and the Public Health Agency of Canada estimates it causes about 300 deaths and 200 hospitalizations in Canada every year. The good news: if you act in the next 60 seconds, the outcome is almost always fine. Here is exactly what to do, in the right order.
Immediate response in the next 60 seconds
- Get everyone OUT of the house. Adults, children, pets. Do not stop to grab belongings beyond shoes and coats. Open the front door and exit.
- Once outside, count heads. Make sure nobody is missing.
- Call 911 from outside, whether or not anyone has symptoms. The fire department will come with CO meters to find the source and make the home safe. Tell the dispatcher if anyone has symptoms (headache, dizziness, nausea, confusion, chest pain) so paramedics are sent too. CO poisoning is treatable but requires immediate medical care.
- Once 911 has been called, you can also call the Enbridge Gas 24/7 emergency line at 1-866-763-5427 about the gas appliance. Enbridge can check and shut off a faulty gas appliance.
- Do NOT re-enter the home for any reason until the fire department clears it. This includes pets, valuables, or "just to check something."
- Stay outside. If outdoor weather is unsafe, sit in a vehicle with the engine OFF, or knock on a neighbour's door.
CO poisoning symptoms (in order of severity)
| Symptom level | What to watch for | Action |
|---|---|---|
| Mild | Headache, fatigue, mild nausea | Evacuate. Call 911 from outside and tell the dispatcher about the symptoms. |
| Moderate | Strong headache, vomiting, confusion, weakness | Evacuate. Call 911. Paramedics will check blood CO levels. |
| Severe | Loss of consciousness, seizures, irregular heartbeat | Call 911 immediately. Begin CPR if needed. Continued exposure is fatal. |
| Pets first | Pets often show symptoms before humans | A vomiting or lethargic pet is an early warning. Evacuate immediately. |
Significant or repeated CO exposure can cause lasting effects. Do not "wait it out" to see if symptoms pass. Get medical assessment whenever a CO alarm has sounded, even if it self-resets and people feel fine afterward.
Real CO source vs false alarm: what responders will check
When firefighters or an Enbridge Gas technician arrive, they enter with a calibrated CO meter and check likely sources, including:
- Furnace heat exchanger (cracked exchanger is a top CO source, common on units 12+ years old)
- Water heater venting (blocked or disconnected flue can dump CO into utility room)
- Gas fireplace combustion and venting
- Gas stove / range (improperly tuned burner)
- Gas dryer venting
- Attached garage (running vehicle, generator, or BBQ can push CO through shared walls)
- Outdoor exhaust intake (any nearby vehicle exhaust, generator, fireplace can backflow into a fresh-air intake)
If a gas appliance is found to be unsafe, Enbridge can issue a Red Tag on it, shut off the gas supply to that appliance, and instructs you to have a TSSA-certified contractor repair or replace before reactivation.
False alarm scenarios (CO alarm sounds but no actual CO)
CO alarms expire. Most have a 5 to 10 year lifespan and the sensor degrades over time, becoming prone to false positives. False alarms happen most often from:
- End-of-life alarm (most common). Check the manufacture date on the back of the unit. If it is 7+ years old, replace it.
- Low battery (most modern units have a different chirp pattern for low battery vs CO detection - read the manual).
- Cleaning chemicals or aerosol sprays released near the alarm (some can trigger false readings briefly).
- Steam from showers or cooking reaching a wall-mounted alarm.
- Alarm wired to the wrong circuit and getting electrical interference.
Even if you SUSPECT a false alarm, still evacuate and call 911 so responders can verify with a calibrated meter. Do not assume. The cost of being wrong is too high.
After the source is found: HVAC next steps
Once responders identify the source appliance and it is shut off or Red Tagged, you need a TSSA-certified HVAC contractor to inspect, repair, or replace. EcoFrost handles all post-CO HVAC work across the GTA:
| Source | Typical action | Cost range |
|---|---|---|
| Cracked heat exchanger (furnace) | Heat exchanger replacement OR full furnace replacement | $1,800 to $3,200 for HX swap; $3,500 to $8,500 for replacement |
| Blocked or disconnected vent | Vent repair, may include B-vent inspection | $250 to $850 |
| Failed water heater venting | Vent replacement or full water heater swap | $350 to $2,800 |
| Mis-tuned gas valve | Recalibration and combustion test | $185 to $485 |
| Indoor air leak from attached garage | Air seal between garage and house | $185 to $650 |
EcoFrost runs a full combustion analysis on every post-CO call to confirm the repair is complete and the unit operates within safe parameters. We do not just replace the affected part and walk away.
CO alarm placement: where to put them (Ontario Fire Code and best practice)
The Ontario Fire Code (Division B, Section 2.16) requires CO alarms adjacent to each sleeping area in homes with a fuel-burning appliance, fireplace or attached garage, and since January 1, 2026, on every storey of those homes. Best practice goes further:
- Outside every bedroom or sleeping area (hallway placement is fine).
- On every level of the home, including the basement.
- Recommended, not required by the Fire Code: an alarm near fuel-burning appliances (furnace, water heater, gas fireplace), placed at the distance the alarm manufacturer specifies.
- NOT directly above or beside the appliance itself (CO disperses through the air, and direct mounting can produce false alarms from normal startup).
- NOT in kitchens, bathrooms, or garages (false alarm zones).
- At sleeping head-height, not floor or ceiling. CO mixes with air at all heights, so wall placement at chest level is fine.
Replace CO alarms every 7 to 10 years per manufacturer specification. Check the manufacture date stamped on the back of each unit. Combination smoke + CO alarms count, but verify the CO sensor portion has not expired separately.
Prevention: catching CO risks before the alarm sounds
Three concrete steps catch the most common CO failure modes before they put your family at risk:
- Annual professional furnace tune-up with combustion analysis. EcoFrost flat $149 includes a scoped heat exchanger inspection on every unit. We have caught dozens of cracked exchangers that homeowners had no clue about.
- Replace your CO alarms on schedule. Add a calendar reminder for year 7 of every alarm purchase. Replacement cost is $35 to $80 per unit.
- Service your water heater venting whenever you replace the water heater itself, and inspect annually if your water heater is 8+ years old.
For a deeper look at heat exchanger warning signs, read our 7 signs your furnace needs repair guide.
Ready to take the next step?
Book a Combustion Safety Inspection - $149?Frequently Asked Questions
What do I do if my CO alarm goes off?
Who do I call first when my CO alarm sounds: 911 or Enbridge?
How quickly does CO poisoning happen?
Why did my CO alarm go off but responders found nothing?
What appliance causes CO in homes most often?
Will my CO alarm work without batteries during a power outage?
How much does heat exchanger replacement cost in Ontario?
Can I keep using my furnace if Enbridge Red Tagged it?
EcoFrost Heating & Cooling
Toronto's Trusted HVAC Experts Since 2015
Our certified HVAC technicians have completed 15,000+ installs across the Toronto area. We write about heating, cooling, and air quality from real field experience not marketing copy. Learn about us →
