A working carbon monoxide detector going off is an emergency. Carbon monoxide is colorless, odorless, and at high concentrations causes loss of consciousness within minutes. "I feel fine" is not a useful indicator — symptoms appear as exposure accumulates, often after the danger is already serious. This page covers the emergency response (most important), then the underlying causes of CO in homes (mostly HVAC equipment), and what we do to verify a home is safe after a CO event.
Most common causes
The likely culprits, roughly in order of how often we see each one:
- Cracked heat exchanger in furnace (most serious)
A cracked heat exchanger leaks combustion gases — including CO — into your home's air supply. Most common in furnaces over 15 years old. The fix is replacement; the heat exchanger cannot be patched safely.
- Blocked or restricted furnace venting
When exhaust gases can't escape properly, they backflow into the house. Causes: bird nests in vents, snow drifts blocking sidewall vents (high-efficiency furnaces), chimney issues on older units. Sometimes immediate-fix simple, sometimes more involved.
- Improperly installed appliance
Gas appliances installed without proper venting, oversized for the home, or with miscalibrated combustion produce CO. Includes water heaters, gas stoves, gas dryers, and fireplaces.
- Vehicle running in attached garage
Common cause of CO events that get attributed to HVAC equipment. Even briefly running a car in an attached garage can produce CO levels that drift into the house through doors and air leaks.
- Generator or unvented heater being used indoors
Portable generators, propane construction heaters, charcoal grills used during power outages. Producing CO is essentially their normal operation; that operation is just not supposed to happen indoors.
- Detector is malfunctioning (rare but possible)
CO detectors have a service life (typically 7-10 years). Older detectors can produce false alarms. Treat every alarm as real until proven otherwise — but if you've had multiple alarms with no fault found, replacing the detector itself is reasonable.
What to check yourself first
Before calling, walk through these — sometimes the fix is something simple:
- EVACUATE FIRST
Get everyone out. Don't do anything else until everyone is outside. Pets too if possible without delay.
- Call 911 if anyone has symptoms
Headache, nausea, dizziness, confusion, vomiting = call 911 from outside the house. Don't drive yourself to the hospital — paramedics arrive faster than emergency rooms can treat you.
- Call us OR fire department for source identification
Once everyone is safe, we dispatch immediately for HVAC investigation. Fire departments will also respond to CO alarms with monitoring equipment.
- Do NOT go back in to "ventilate"
The air will clear on its own once you stop running whatever is producing the CO. Going back in to open windows or check anything is exactly the wrong move.
- Note when alarm started
Was the furnace running? AC? Just back from being out of the house? Any new appliance? This helps us identify the source faster.
- Don't restart any gas appliance until cleared
After response, don't restart the furnace, water heater, or any gas appliance until a technician confirms it's safe.
What NOT to do
- Don't ignore a CO alarm because you "feel fine" — symptoms develop with cumulative exposure
- Don't open windows and stay in the house — leave first, ventilate after
- Don't assume a single alarm is "probably the detector being broken" — investigate every time
- Don't restart any combustion appliance until a technician has cleared the home
When to call us
Stop troubleshooting and pick up the phone if:
- CO detector going off right now (call 911 first if symptoms, then us)
- CO detector went off briefly and stopped (still call us — trace cause matters)
- You've had multiple "false" alarms over weeks
- You feel symptomatic (headache, nausea) when home but better when away
- Your CO detector is over 7 years old (service life consideration)
- You haven't had your furnace inspected this heating season and it's mid-winter
Frequently asked questions
How do I know if my CO detector is real or going off because it's old?
You don't — and you shouldn't try to make that judgment in the moment. Treat every alarm as real until investigated. After a confirmed-safe response, if the detector is over 7-10 years old (check the manufacture date on the back), replace the detector itself rather than continuing to use it.
What CO levels are dangerous?
Detectors typically alarm at 35 ppm sustained or 70 ppm for shorter exposure. Symptoms in healthy adults start at 70+ ppm; serious effects at 200+ ppm; loss of consciousness at 800+ ppm. Children, elderly, pregnant women, and people with cardiovascular issues are affected at lower concentrations.
How often should I have my furnace checked for CO production?
Annual furnace tune-ups should include combustion analysis — measuring CO output at the burner with a calibrated meter. This catches potential CO issues before they reach detector-alarming levels in your home. It's the single most important reason to do annual professional maintenance even if everything seems fine.
Should I have CO detectors on every floor?
Yes. Ontario building code requires CO detectors near every sleeping area and on every floor that contains a fuel-burning appliance. Practically: bedroom hallway plus mechanical room minimum, additional units near gas fireplaces or attached garages.
Related services
If you need professional repair, the relevant services for this issue: