September 13, 2026 · 6 min read
Carbon monoxide (CO) is called the "silent killer" because it's colorless, odorless, and tasteless. Every year, more than 400 Americans die from unintentional CO poisoning not linked to fires, and thousands more are hospitalized. Your gas furnace is one of the most common household sources — but with proper maintenance and awareness, the risk is almost entirely preventable.
How Your Furnace Produces Carbon Monoxide
Every gas furnace produces carbon monoxide during normal combustion. In a properly functioning system, CO is safely vented outside through the flue pipe. The problem starts when something disrupts this process:
- Cracked heat exchanger — the most dangerous failure. Combustion gases leak directly into the air your family breathes instead of going up the flue.
- Blocked or disconnected flue pipe — bird nests, ice, or deteriorated connections can trap CO inside the house.
- Incomplete combustion — dirty burners, incorrect gas pressure, or insufficient combustion air cause the furnace to produce more CO than normal.
- Backdrafting — negative house pressure (from exhaust fans, dryers, or a tightly sealed home) pulls flue gases back into the living space.
Warning Signs of a CO Problem
While CO itself is undetectable without a monitor, your furnace and your body give clues:
- Yellow or flickering burner flame instead of a steady blue flame (indicates incomplete combustion)
- Soot or discoloration around the furnace, vents, or flue pipe
- Excessive moisture on windows or walls near the furnace
- A burning or unusual smell when the furnace runs (the combustion byproducts, not CO itself)
- Physical symptoms: headaches, dizziness, nausea, confusion, or flu-like symptoms that improve when you leave the house
If multiple family members have similar symptoms that get better when they leave home and worse when they return, treat it as a CO emergency immediately.
CO Detector Placement — Do It Right
Ohio law requires CO detectors in all residential dwellings with fuel-burning appliances. Here's how to place them correctly:
- Every sleeping area — one within 10 feet of each bedroom door
- Every level of the home including the basement
- Near the furnace and water heater (but not directly on top of them — at least 15 feet away to avoid false alarms during startup)
- At breathing height — CO mixes with air evenly, so wall-mounted at 5 feet or on a nightstand is fine
Replace batteries every 6 months and replace the entire detector every 5–7 years (check the manufacture date on the back). A detector with a digital display showing PPM (parts per million) readings is worth the extra cost.
What to Do If Your CO Alarm Goes Off
Don't ignore it. Don't assume it's a false alarm. Follow this sequence:
- Get everyone — including pets — out of the house immediately
- Call 911 from outside
- Do NOT re-enter the home until emergency responders clear it
- Have your furnace and all gas appliances inspected by a licensed HVAC technician before using them again
Prevention: The Annual Furnace Inspection
The single most effective way to prevent CO problems is an annual furnace safety inspection. During our inspection, we:
- Test the heat exchanger for cracks using a combustion analyzer (measures CO levels in the flue and supply air)
- Inspect the flue pipe for blockages, corrosion, and proper draft
- Check burner flame color and pattern
- Verify gas pressure is within manufacturer specifications
- Test safety controls (high-limit switch, flame rollout switch, pressure switch)
- Ensure adequate combustion air supply
A cracked heat exchanger in a furnace over 15 years old usually means it's time for a replacement. Learn the signs it's time for a new system.
High-Efficiency Furnaces and CO Safety
Modern high-efficiency (90%+ AFUE) furnaces have a sealed combustion design — they pull combustion air from outside through a dedicated PVC pipe and exhaust through another. This virtually eliminates backdrafting risk. If you're replacing an older 80% furnace, upgrading to a high-efficiency model isn't just about lower gas bills — it's a meaningful safety improvement.