Gas furnaces are safe, reliable equipment when properly installed and maintained, but they're also the most common source of carbon monoxide risk in a home during heating season. Understanding the basics, what carbon monoxide actually is, why you can't rely on your senses to detect it, and what to do about it, is worth a few minutes even if you never end up needing it.
Does carbon monoxide actually have a smell?
No. Carbon monoxide itself is completely odorless, colorless, and tasteless. If you've heard it described as smelling faintly metallic or like exhaust, what's actually being detected is other combustion byproducts released alongside it, not the carbon monoxide itself, and those other smells aren't reliably present at dangerous concentrations. This is exactly why a working detector, not your nose, is the only real line of defense.
Where carbon monoxide detectors should actually go
A detector on every level of the home, and specifically near sleeping areas, is the standard recommendation, since carbon monoxide poisoning is especially dangerous while asleep. A detector near, but not directly on top of, fuel-burning appliances like your furnace is also worth having, though it shouldn't be your only one. Avoid placing detectors directly next to windows, doors, or vents, where drafts can affect readings.
How furnace maintenance connects to carbon monoxide risk
A cracked heat exchanger is one of the more serious ways a furnace can become a carbon monoxide risk, since it allows combustion gases to mix with the air actually being circulated through your home instead of being vented safely outside. This is one of the specific things checked during a proper furnace inspection, and it's a real reason routine maintenance matters beyond just efficiency and comfort.
What to do if a detector goes off
Leave the house immediately, get everyone to fresh air, and call emergency services before trying to investigate the cause yourself. Once it's safe, a technician should inspect the furnace and venting system before it's used again, rather than resetting the detector and hoping it doesn't happen again.
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