Smoke detectors and carbon monoxide detectors are separate devices that sense different hazards

A smoke detector senses smoke particles in the air and alerts you to a fire. A carbon monoxide detector senses an odorless, colorless gas produced by burning fuel and alerts you to a dangerous buildup. They work on completely different principles and cannot replace each other. You need both in your home.

The confusion is understandable—they look similar, sit on ceilings or walls, and both save lives. But a smoke detector will not catch carbon monoxide, and a carbon monoxide detector will not catch smoke. Installing one type does not protect you from the other hazard.

Key Takeaways

  • Smoke detectors sense smoke particles; carbon monoxide detectors sense a toxic gas—they detect completely different things.
  • A smoke detector cannot warn you about carbon monoxide, and a carbon monoxide detector cannot warn you about fire.
  • You should have smoke detectors on every level of your home and in bedrooms; carbon monoxide detectors belong near sleeping areas and fuel-burning appliances.
  • Some combination units exist that detect both smoke and carbon monoxide, but they cost more than buying separate detectors.
  • Both types need batteries checked monthly and should be replaced every 5 to 10 years, depending on the model.

How smoke detectors work

Smoke detectors use one of two methods to sense fire. Ionization detectors contain a small radioactive source that ionizes air molecules, creating a tiny electrical current. When smoke enters the chamber, it disrupts this current and triggers the alarm. These are better at sensing fast, flaming fires.

Photoelectric detectors use a light beam and a sensor. Smoke particles scatter the light beam onto the sensor, triggering the alarm. These are better at sensing slow, smoldering fires. Many homes use dual-sensor detectors that combine both methods.

Smoke detectors do not sense carbon monoxide at all. They are blind to it. A home could be filling with carbon monoxide while a perfectly working smoke detector stays silent.

How carbon monoxide detectors work

Carbon monoxide detectors use a chemical or electrochemical sensor that changes color or resistance when exposed to carbon monoxide gas. The detector measures the concentration of the gas and the time of exposure. If levels reach a dangerous threshold, the alarm sounds.

Carbon monoxide is produced by any fuel-burning appliance: furnaces, water heaters, gas stoves, fireplaces, and car engines. It is colorless and odorless, so you cannot sense it yourself. A detector is your only warning.

A carbon monoxide detector will not sense smoke. It cannot tell you a fire has started. You need a separate smoke detector for that protection.

Where to place each type in your home

Smoke detectors should be installed on every level of your home, including the basement. Place one in each bedroom and one in the hallway outside sleeping areas. Mount them on the ceiling or high on a wall, because smoke rises. Keep them at least 10 feet away from cooking appliances to avoid false alarms from cooking steam.

Carbon monoxide detectors belong near sleeping areas so you will hear the alarm at night, when carbon monoxide poisoning is most dangerous. Place one near any fuel-burning appliance: the furnace, water heater, or fireplace. Install one on each level of your home. Do not place them directly above or beside a stove or oven, as heat can damage the sensor.

Some people install both types in the same location—for example, a combination unit in the hallway outside bedrooms. This works, but it does not replace the need for smoke detectors in other rooms or carbon monoxide detectors near appliances.

Combination smoke and carbon monoxide detectors

Combination units that detect both smoke and carbon monoxide exist and are sold under brand names like First Alert and Kidde. They contain both a smoke sensor and a carbon monoxide sensor in one device. When one alarm sounds, the unit tells you which hazard it detected.

Combination units cost more than buying a smoke detector and carbon monoxide detector separately—usually $30 to $50 per unit versus $10 to $20 for a single-purpose detector. They save space and wiring if you are installing hardwired models, but they do not change where you need to place detectors in your home. You still need multiple units on different levels and in different rooms.

The main drawback is that if the combination unit fails, you lose protection against both hazards. With separate detectors, a failure in one type does not affect the other.

Maintenance and replacement for both detector types

Test both smoke and carbon monoxide detectors monthly by pressing the test button. The alarm should sound loudly. If it does not, replace the batteries when ready. Many detectors use 9-volt batteries; some use AA or AAA. Check your manual to know which type yours needs.

Replace the batteries in battery-powered detectors at least once a year. A common practice is to change them when you change your clocks for daylight saving time. Hardwired detectors have backup batteries that should also be tested and replaced annually.

Replace the entire detector every 5 to 10 years, depending on the model. Check the date printed on the back of your detector. Sensors degrade over time and become less reliable. Many detectors will chirp or beep periodically when they are nearing the end of their lifespan—this is the manufacturer's signal to replace the unit, not just the battery.

What to do if a detector goes off

If your smoke detector sounds, assume there is a fire. Leave your home when ready through the nearest exit. Do not stop to investigate, gather belongings, or call 911 from inside the house. Get outside first, then call 911 from a safe location.

If your carbon monoxide detector sounds, leave your home when ready and go outside into fresh air. Once outside, call 911 or your local emergency number. Do not re-enter the home until emergency responders have cleared it. Carbon monoxide can cause loss of consciousness and death, so speed matters.

If a detector sounds and then stops, do not ignore it. Test it again. If it sounds again, treat it as a real alarm and follow the steps above. A detector that sounds intermittently may indicate low batteries or a failing sensor, but it could also mean a real hazard is present.

Frequently Asked Questions

Can I use a smoke detector to warn me about carbon monoxide?

No. Smoke detectors sense only smoke particles and cannot detect carbon monoxide gas. You must install a separate carbon monoxide detector to be warned about that hazard. Having only a smoke detector leaves you unprotected against carbon monoxide poisoning.

Can I use a carbon monoxide detector to warn me about fire?

No. Carbon monoxide detectors sense only that specific gas and cannot detect smoke. You must install smoke detectors to be warned about fire. A carbon monoxide detector will not alert you if a fire starts in your home.

Do I need both types if I have a combination unit?

A combination unit detects both hazards in one device, but you still need multiple detectors placed throughout your home. Install them on every level, in bedrooms, and near fuel-burning appliances. One combination unit in one location does not protect your entire home.

What should I do if my detector keeps going off for no reason?

For smoke detectors, check for cooking steam, dust, or insects in the sensor. Clean it gently with a vacuum or compressed air. For carbon monoxide detectors, low batteries are the most common cause of false alarms. Replace the batteries first. If alarms continue, the sensor may be failing and the unit should be replaced.

How often should I replace my detectors?

Replace the entire unit every 5 to 10 years. Check the manufacturing date printed on the back. Test batteries monthly and replace them at least once a year. Many detectors will chirp when they are nearing the end of their lifespan—this is your signal to buy a new one.