Carbon monoxide and your furnace: what to know
Carbon monoxide (CO) is a byproduct of the same combustion that heats your home, and unlike a gas leak, it has no smell at all. A working detector, not your senses, is what actually protects you.
Why a furnace can produce carbon monoxide
A gas furnace burns fuel to make heat, and carbon monoxide is a normal byproduct of that combustion. In a furnace that is venting correctly, almost all of it goes out through the flue and never enters the house.
The risk appears when that combustion is incomplete or when the exhaust path is not doing its job: conditions like a cracked heat exchanger, a blocked or disconnected flue. Or a vent restricted by debris, ice, or a bird's nest can let combustion byproducts reach the living space instead of leaving the building.
You cannot smell, see, or taste it
This is the detail that makes carbon monoxide different from a gas leak. Natural gas has an odor deliberately added to it so a leak can be detected by smell. Carbon monoxide has none: it is colorless and odorless, which is exactly why detectors exist as a category separate from your own senses. There is no reliable way to notice a rising CO level in a room by how it smells, looks, or feels in the air.
| What raises the risk | Why it matters |
|---|---|
| Cracked heat exchanger | Lets combustion byproducts mix into the air the blower circulates through the house |
| Blocked, disconnected, or damaged flue/vent pipe | Exhaust that should leave the building has nowhere to go |
| Vent termination blocked outside (debris, snow, a nest) | Same effect as a blocked flue. Combustion gases can't exit |
| Furnace running with the cabinet panel off or not latched | Combustion air supply and venting can be disrupted by an open cabinet |
Signs of exposure to watch for
Early carbon monoxide exposure can feel a lot like the flu or general fatigue: headache, dizziness, nausea, confusion, and shortness of breath are the common signs. One pattern worth noticing is timing. Symptoms that several people in the house share at once, that get worse the longer everyone stays home. And that ease up after time outside or away from the house, are a different situation than an ordinary illness and worth taking seriously.
If a CO alarm sounds, or you suspect a problem
There is one response here, and it does not change based on how the furnace looks or sounds: leave the house immediately, get everyone and any pets out. And call 911 or your fire department from outside. Do not stop to open windows, look for the source, or try to air the house out first. Get out, then call. Do not go back inside until emergency responders say it is safe to.
What not to do
- Do not try to identify the source of a CO alarm yourself before leaving. Get out first, investigate never.
- Do not assume you'd notice a leak because nothing smells wrong. Carbon monoxide has no smell at all; that is the entire reason detectors exist.
- Do not silence or ignore a CO alarm because it has happened before or you don't feel unwell: symptoms don't always appear right away, and a detector alarming is not a false-alarm-by-default situation.
- Do not open the furnace cabinet or inspect the heat exchanger or venting yourself. Diagnosing a suspected CO source is a job for a licensed HVAC technician with combustion-testing equipment, not a visual check.
The detector is the safeguard, not your senses
You cannot detect carbon monoxide by smell, sight or how you feel in the moment. So a working CO detector on every level of the home, especially near sleeping areas, is the actual line of defense, not noticing something is off. Detectors have a limited service life printed on the unit itself, and their batteries need regular testing. A detector that has expired or has a dead battery is functionally the same as having none.
Annual inspection is the real prevention
A furnace's heat exchanger and venting are exactly what a licensed technician checks during a seasonal tune-up, before heating season rather than after a problem shows up. That inspection, not a homeowner walk-around, is what actually catches a developing crack or venting issue early.
Frequently asked questions
Does a smoke detector also detect carbon monoxide?
No, not unless it is specifically labeled as a combination smoke and CO detector. Smoke and carbon monoxide are detected by different sensors, so a home with any gas-burning appliance, including a furnace, needs a CO detector as its own device or as part of a labeled combination unit: a smoke-only detector will not alert to a CO problem.
Can I tell if my furnace has a carbon monoxide problem without a detector?
No, and that's the point of the whole issue: carbon monoxide has no smell, color, or taste, so there is no reliable way to sense it building up on your own. Headache, dizziness, and nausea can be signs of exposure, but by the time symptoms appear, levels may already be significant. A working detector is what actually catches it early.
Is furnace carbon monoxide a different concern from a gas smell?
Yes. A rotten-egg or sulfur smell means a natural gas leak, which is a different hazard with the same response (leave and call your gas utility or 911). Carbon monoxide has no smell and comes from combustion that isn't venting correctly, which is why it needs its own detector rather than being something you'd notice by smell the way you would a gas leak.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics
- NFPA. Carbon monoxide safety