Carbon monoxide (CO) is the most dangerous by-product of gas heating systems — colorless, odorless, and immediately lethal at high concentrations. Minnesota's tight, well-sealed homes and long heating seasons make CO awareness critical for every homeowner. Here's what you need to know.
What Is Carbon Monoxide and Where Does It Come From?
Carbon monoxide is produced by incomplete combustion of carbon-based fuels — natural gas, propane, oil, wood, and charcoal. When combustion is complete, fuel + oxygen produces CO2 and water. When combustion is incomplete (insufficient oxygen, wrong fuel-air mixture, cracked heat exchanger), CO is produced instead.
In a home, CO sources include:
- Gas furnace (especially with cracked heat exchanger)
- Gas water heater
- Gas stove/oven
- Attached garage (car exhaust)
- Wood fireplace with blocked chimney
- Portable gas generators run indoors or too close to windows
- Gas-powered tools operated in attached garages
How CO Affects the Body
CO binds to hemoglobin in the blood far more readily than oxygen — preventing red blood cells from carrying oxygen to tissues. Symptoms progress with concentration:
| CO Level (ppm) | Exposure Time | Symptoms |
|---|---|---|
| 35 ppm | 8 hours | Headache, dizziness (OSHA action level) |
| 70 ppm | 1–2 hours | Headache, fatigue, nausea |
| 150 ppm | 2–3 hours | Disorientation, severe headache |
| 400 ppm | 3 hours | Life-threatening |
| 800 ppm | 45 minutes | Seizures, loss of consciousness, death |
| 1,600+ ppm | 20 minutes | Death |
Because symptoms mimic the flu (headache, nausea, dizziness), CO poisoning is often misdiagnosed. A critical clue: symptoms improve when you leave the house and return when you come back.
The Furnace Heat Exchanger: The #1 CO Risk
In a gas furnace, the heat exchanger is the metal chamber where combustion occurs. It's designed to be completely sealed from the air distribution system — combustion gases stay inside the heat exchanger and exhaust through the flue; household air flows around the outside of the exchanger and gets heated by conduction.
When the heat exchanger develops a crack or hole (common in aging furnaces), combustion gases — including CO — mix directly into the circulating air and are distributed throughout your home. This is the most dangerous failure mode in residential HVAC, and the reason annual furnace inspections matter.
Signs of potential heat exchanger cracking:
- Soot or dark streaking near the furnace or vent connections
- Visible corrosion or rust on the heat exchanger (viewed through access panel)
- CO detector alerts when furnace is running
- Household members experiencing repeated flu-like symptoms that resolve when away from home
- Furnace is 15+ years old (risk increases significantly after this age)
CO Detector Requirements and Placement
Minnesota Law
Minnesota state law (Minnesota Statute 299F.50) requires CO detectors in all new construction and after renovation in:
- Every sleeping area (or within 10 feet of sleeping areas)
- Each level of the home containing a bedroom
Best Practice Placement
- Near sleeping areas: CO poisoning during sleep is especially dangerous — you can't sense symptoms while unconscious
- Near gas appliances: Within 10 feet of the furnace room, if accessible
- Each floor: CO is roughly the same density as air and distributes throughout a home
- Not directly above stoves or in garages: Cooking fumes and car exhaust will cause nuisance alarms
Choosing a CO Detector
- Electrochemical sensors: Most accurate technology. Used in standalone detectors and combination smoke/CO units. Replace every 5–7 years.
- Plug-in vs. battery: Plug-in detectors with battery backup are preferred — battery-only detectors depend on the user replacing batteries consistently.
- Smart CO detectors (Nest Protect, First Alert OneLink): Send phone alerts and can communicate with smart home systems. Worth the premium if you travel or want remote monitoring.
- AVOID: Biomimetic (gel-based) CO detectors — these are outdated technology, less accurate, and more prone to false alarms.
What to Do If Your CO Alarm Sounds
- Get out immediately — do not stop to gather belongings. Every second of additional exposure matters.
- Leave the door open as you exit to allow ventilation
- Call 911 from outside — fire department has CO measurement equipment and will investigate the source
- Do not re-enter until emergency responders have cleared the building
- Seek medical evaluation if anyone has symptoms — CO poisoning requires medical treatment and oxygen therapy
Annual Prevention Checklist
- ✅ Annual furnace inspection including heat exchanger check
- ✅ Test CO detectors monthly; replace batteries annually
- ✅ Replace CO detectors every 5–7 years
- ✅ Never use gas ovens for home heating
- ✅ Never run generators indoors or in attached garages
- ✅ Keep garage-to-house door closed when running vehicles
- ✅ Ensure furnace and water heater flues are clear of debris and separation
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