Fire alarms and CO detectors are separate devices that do different jobs

A standard fire alarm detects smoke from a fire. It does not detect carbon monoxide (CO), the odorless, colorless gas that comes from incomplete combustion in furnaces, water heaters, car exhaust, and blocked chimneys. If you want to know whether CO is in your home, you need a dedicated carbon monoxide detector — a fire alarm will not tell you.

Some combination devices exist that detect both smoke and CO in a single unit, but they are less common and more expensive than buying them separately. Most homes have a smoke alarm in the bedroom and kitchen, and a CO detector near the furnace or in a central hallway. They work side by side, not as replacements for each other.

Key Takeaways

  • Fire alarms detect smoke only; they do not sense carbon monoxide at any level.
  • Carbon monoxide detectors are a separate purchase and should be placed near fuel-burning appliances or in a central location on each level of your home.
  • Combination smoke and CO detectors exist but cost more and are harder to find than individual units.
  • CO poisoning can cause headache, dizziness, and confusion before you notice anything is wrong, so detection matters even when you cannot smell it.

Why fire alarms cannot detect carbon monoxide

Smoke alarms use one of two sensing methods: ionization (which detects fast-flaming fires) or photoelectric (which detects smoldering fires). Neither method responds to carbon monoxide gas. CO is invisible and odorless, and it does not produce particles or light the way smoke does. A smoke alarm sensor straightforward has no way to recognize it.

Carbon monoxide detectors use a different sensor — usually electrochemical or biomimetic — that reacts specifically to CO molecules. This is why the two devices cannot substitute for each other. A home with only a smoke alarm is protected against fire but not against CO leaks from a furnace, water heater, or blocked vent.

Where to place a carbon monoxide detector

The best location for a CO detector is near any fuel-burning appliance: your furnace, water heater, gas stove, or fireplace. If you have a basement furnace, place one detector in the basement near the unit and another on the main living level. If your furnace is in a closet on the first floor, a detector in that hallway works.

Many people also place a CO detector in the bedroom, since CO poisoning can happen while you sleep and cause confusion or loss of consciousness. The key is having at least one detector per level of your home, with at least one near the appliance that produces the risk. Avoid placing detectors directly above a stove or in very humid areas like bathrooms, as moisture can interfere with the sensor.

Combination smoke and CO detectors: cost and availability

Combination units that detect both smoke and carbon monoxide do exist. Brands like First Alert and Kidde make them. They cost between $40 and $80 per unit, compared to $10 to $25 for a smoke alarm alone and $20 to $50 for a CO detector alone. If you need both in the same location, a combination unit can save money and wall space.

The trade-off is that combination detectors are harder to find in stores and online than individual units, and if one sensor fails, you may need to replace the entire device. Most fire codes and insurance companies do not require a combination unit; they straightforward require that both smoke and CO detection be present in the home. Buying them separately gives you more flexibility in placement and replacement.

What carbon monoxide detectors actually sense

A CO detector triggers an alarm when carbon monoxide reaches a certain concentration in the air over a set time period. Most detectors alarm at 70 parts per million (ppm) after 60 minutes, or at 400 ppm after 4 to 15 minutes. These thresholds are set to alert you before CO reaches dangerous levels, but not so sensitive that you get false alarms from brief, harmless exposure.

If your CO detector goes off, leave the house when ready and call 911 or your local fire department from outside. Do not assume it is a false alarm. CO poisoning symptoms — headache, dizziness, nausea, confusion — can mimic the flu, and by the time you feel sick, you may already be in danger. The detector is your early warning system.

Testing and maintaining both detectors

Test your smoke alarm once a month by pressing the test button for three seconds. Test your CO detector the same way. Both should be replaced every 10 years; check the date on the back of the unit. If a detector is more than 10 years old, replace it even if it still works, because the sensor degrades over time.

CO detectors with replaceable batteries need a fresh battery twice a year — many people change them when they change their clocks for daylight saving time. Hardwired smoke alarms (common in newer homes) have a backup battery that should also be replaced every year. Keep a record of when you last tested and replaced batteries so you do not forget.

Frequently Asked Questions

Can a smoke alarm ever detect carbon monoxide?

No. Smoke alarms are designed to sense particles and light from fire, not gas. Even if CO is present in your home, a smoke alarm will not alarm. You need a separate CO detector to know if carbon monoxide is building up.

What does carbon monoxide smell like?

Carbon monoxide has no smell, taste, or color. You cannot detect it with your senses. That is why a detector is essential — by the time you feel symptoms like headache or dizziness, the gas may already be at a dangerous level.

Do I need a CO detector if I have an electric furnace?

Electric furnaces do not produce carbon monoxide because they do not burn fuel. However, if you have a gas water heater, gas stove, fireplace, or attached garage where a car might run, you still need a CO detector. Check your home for any fuel-burning appliance.

How long do CO detectors last?

Most CO detectors last 5 to 10 years depending on the brand and model. Check the back of your unit for the manufacture date. Once it reaches 10 years old, replace it. Some detectors have a low-battery indicator light that tells you when the battery is running down.

Can I use a combination smoke and CO detector instead of two separate ones?

Yes, if you want both functions in one location. However, you may still need a second detector elsewhere — for example, a smoke alarm in the bedroom and a combination unit near the furnace. Check your local fire code for requirements in your area.