Most smoke alarms detect only smoke, not carbon monoxide
A standard smoke alarm is built to sense smoke particles in the air. It does not detect carbon monoxide (CO), an odorless, colorless gas produced by burning fuel. If you want protection from both hazards, you need either a combination alarm that detects both smoke and CO, or two separate devices — one smoke alarm and one CO detector.
The confusion is understandable: both are small wall-mounted devices that alert you to invisible dangers. But they work by different methods. A smoke alarm uses either ionization or photoelectric sensors to spot smoke. A CO detector uses an electrochemical sensor to measure carbon monoxide levels in the air. One device cannot reliably do both jobs.
Key Takeaways
- Standard smoke alarms detect only smoke; they will not alert you to carbon monoxide, which is produced by furnaces, water heaters, and car exhaust.
- Combination smoke and CO alarms exist and are sold at hardware stores, but cost more than a smoke-only alarm.
- You need at least one CO detector on every level of your home, especially near bedrooms and the furnace or garage.
- Smoke alarms and CO detectors have different battery replacement schedules and test buttons, so check both regularly.
- If your home has a furnace, water heater, or attached garage, a CO detector is not optional — it is a safety necessity.
How smoke alarms and CO detectors work differently
A smoke alarm contains either an ionization chamber or an optical sensor. Ionization alarms use a small radioactive source to ionize air molecules; when smoke particles enter, they disrupt the current and trigger the alarm. Photoelectric alarms shine a light beam across a chamber; when smoke scatters the light onto a sensor, the alarm sounds. Both types respond to visible smoke particles, not gases.
A carbon monoxide detector uses an electrochemical cell that changes color or resistance when exposed to CO. The sensor measures the concentration of gas over time. If CO levels reach a dangerous threshold — typically 35 parts per million over 8 hours, or 400 ppm over 15 minutes — the alarm sounds. This sensor cannot detect smoke, and a smoke alarm cannot detect CO.
Because the two hazards require different detection methods, manufacturers do not combine them into a single reliable device at a consumer price point. Combination alarms exist, but they are more expensive and require separate sensors for each function.
When you need a combination alarm versus separate devices
If you want one device to cover both smoke and CO, a combination alarm is available at most hardware stores and online retailers. These units contain both a smoke sensor and a CO sensor in the same housing. They cost roughly 50 to 100 dollars, compared to 15 to 30 dollars for a smoke-only alarm and 40 to 80 dollars for a standalone CO detector.
Combination alarms make sense if you have limited wall space or prefer a single device per room. However, many fire safety experts recommend separate devices because they allow you to place CO detectors exactly where you need them — near the furnace, water heater, and bedrooms — while placing smoke alarms in hallways and bedrooms according to fire code.
If you choose separate devices, you will need to test and maintain both. Each has its own test button, and each may use different battery types or have different replacement intervals. Some CO detectors are hardwired with battery backup; some are battery-only. Read the manual for each device to understand its specific requirements.
Where to place CO detectors in your home
The National Fire Protection Association (NFPA) recommends placing at least one CO detector on every level of your home, with special attention to areas near sleeping spaces. If your home has a furnace, water heater, or attached garage, place a detector within 10 to 20 feet of those sources.
Do not place a CO detector directly above a furnace or water heater, where heat and humidity can damage the sensor. Avoid placing it in kitchens near cooking appliances, which can produce brief, harmless CO spikes that may trigger false alarms. A bedroom wall near the door, or a hallway outside sleeping areas, is a good location.
If you have a two-story home, place one detector on the first floor near the furnace or garage, and one on the second floor near bedrooms. If your home has a basement with a furnace, place a detector in the basement as well as upstairs.
Testing and maintaining both alarms
Smoke alarms and CO detectors require different maintenance schedules. Test your smoke alarm monthly by pressing its test button for a few seconds until the alarm sounds. Replace the battery once a year, typically in the fall when you change your clocks. Replace the entire smoke alarm unit every 10 years.
CO detectors also have a test button, but battery replacement intervals vary by model — some need new batteries every 6 months, others every year. Check your detector's manual. Many CO detectors have a lifespan of 5 to 7 years; some display an end-of-life warning light or chirp when they need replacement. Write the installation date on the back of each device so you know when it expires.
If either alarm sounds, treat it seriously. For a smoke alarm, evacuate when ready and call 911 from outside. For a CO alarm, move everyone to fresh air, call 911, and do not re-enter the home until a professional has checked for a CO source.
Common sources of carbon monoxide in homes
Carbon monoxide is produced whenever fuel burns incompletely. The most common sources are furnaces, water heaters, and gas stoves. A malfunctioning furnace or a blocked chimney can allow CO to seep into living spaces. An attached garage is a major source — running a car engine in an attached garage, even with the door open, can push CO into the home through walls and doors.
Other sources include portable generators (which should never run indoors or in garages), grills, and space heaters. In winter, a blocked furnace vent or chimney caused by snow or ice can trap CO inside the home. This is why a CO detector is essential if you heat with gas or have any fuel-burning appliance.
If your home is all-electric — with electric heat, electric water heater, and no gas appliances — your CO risk is much lower. However, an attached garage or a neighbor's furnace venting near your home can still introduce CO, so many experts recommend at least one detector even in all-electric homes.
Hardwired versus battery-powered detectors
Smoke alarms and CO detectors come in two forms: battery-powered and hardwired. Battery-powered units are easier to install — you mount them on a wall or ceiling and insert batteries. Hardwired units connect to your home's electrical system and usually have a battery backup in case of power loss.
Hardwired detectors are more reliable because they do not depend on you remembering to replace batteries. However, they require an electrician to install, which costs money. Battery-powered units are cheaper upfront but require you to test them monthly and replace batteries annually.
Many building codes now require hardwired smoke alarms in new construction, but existing homes often use battery-powered units. For CO detectors, battery-powered models are common in homes without hardwired systems. If you are installing detectors yourself, battery-powered is simpler; if you are renovating or building, ask your electrician about hardwired options.
Frequently Asked Questions
Can a smoke alarm detect a gas leak?
No. A smoke alarm detects smoke particles only. A natural gas leak requires a separate gas detector. If you smell rotten eggs (the odor added to natural gas), leave your home when ready and call your gas company's emergency line from outside.
Do I need a CO detector if I have a new furnace?
Yes. Even new furnaces can malfunction, and a blocked chimney or vent can trap CO regardless of the furnace's age. A CO detector is your safety net if something goes wrong.
What does a CO detector alarm sound like?
Most CO alarms produce a loud beeping pattern, often four beeps followed by silence, repeated. Some models have a voice alert that says "carbon monoxide." The pattern is different from a smoke alarm's continuous beeping, but both are loud enough to wake you at night.
Can I use a combination alarm in place of separate devices?
Yes, combination alarms meet safety standards for both smoke and CO detection. The trade-off is that you cannot place the CO sensor exactly where you need it — it is fixed to the same location as the smoke sensor. If your home layout allows, separate devices give you more flexibility.
How long do CO detectors last?
Most CO detectors last 5 to 7 years. Check your manual for the specific lifespan. Many detectors display an end-of-life indicator or chirp when they need replacement. Mark the installation date on the back so you do not forget.