Smoke and Carbon Monoxide Alarms: How to Choose

Updated 6 November 2026 · Standard formula, shown below

Choose smoke and carbon monoxide (CO) alarms that are listed by a recognized testing laboratory, suit the hazards and layout of your home, and meet local fire-code requirements. For most homes, that means interconnected smoke alarms on every required level and near sleeping areas, plus CO alarms outside sleeping areas and wherever fuel-burning equipment or an attached garage creates a risk.

What the alarms detect

Smoke alarms warn of a fire; CO alarms warn of carbon monoxide, an odorless, colorless gas produced by incomplete combustion. One cannot substitute for the other. A combination alarm detects both, but its label and instructions should make clear which hazard is sounding. If you use separate devices, make sure both types of protection are covered.

Smoke alarms commonly use one of two sensing methods:

Photoelectric: Uses light sensing and is generally more responsive to slow, smoldering fires.
Ionization: Uses a small amount of radioactive material in a sensing chamber and is generally more responsive to fast-flaming fires.

Both types can detect fires, and real fires may develop in different ways. A dual-sensor alarm combines the methods; alternatively, use alarms that meet current applicable standards and follow fire-safety guidance for your area. Do not choose solely on a claim that one sensing technology is best for every fire.

Choose the power and connection type

Alarms are commonly powered by replaceable batteries, sealed long-life batteries, household wiring, or wiring with battery backup. Each has trade-offs:

Replaceable-battery models are useful where wiring is unavailable, but the battery must be replaced as directed and the alarm tested regularly.
Sealed-battery models reduce routine battery changes, but the whole alarm must be replaced when it reaches its service-life limit or signals end of life.
Hardwired models connect to household electrical power and should have backup power for outages. Installation or replacement may require a qualified electrician, especially where local rules require interconnected wiring.

Interconnected alarms sound together when one detects danger, giving people in distant rooms or on other floors more warning. Interconnection may be wired or wireless; check that every device is compatible with the others and follow the manufacturer’s instructions. A phone alert or smart-home connection can add notification, but it should not replace required local alarms that sound inside the home.

Plan locations before buying

Exact placement rules vary by jurisdiction, so check local fire-code guidance and the alarm instructions. Common safety guidance is to install smoke alarms inside each bedroom, outside each separate sleeping area, and on every level of the home, including a basement. Place CO alarms outside sleeping areas and on each level where required; additional placement near fuel-burning appliances may be advised by local authorities or the manufacturer.

For smoke alarms, ceiling placement is generally preferred. If an alarm is mounted on a wall, keep it within the distance from the ceiling specified in its instructions. Avoid dead-air corners, locations near vents or fans, and spots where cooking smoke or shower steam is likely to trigger nuisance alarms. Do not install one directly beside a stove or in a bathroom unless the product instructions and local guidance allow it.

CO alarm placement depends on the device. Although CO mixes through room air, some models specify wall or ceiling mounting, while others permit different locations. Follow the exact instructions rather than assuming every model belongs at a particular height. Keep alarms away from children and pets, and do not put them where furniture or curtains can block the sound or openings.

Compare features that affect daily use

Before purchasing, check the packaging and instructions for the following:

Approval or listing by a recognized testing laboratory and suitability for the intended use.
Alarm type: smoke, CO, or combination; for smoke, the sensing method and whether it is dual-sensor.
Power source, backup power, and whether batteries are replaceable or sealed.
Interconnection method and compatibility with alarms already installed.
Test and hush controls, end-of-life warnings, and any display or voice message that identifies the hazard.
The alarm’s replacement date or stated service life, installation requirements, and accessibility of the test button.

Voice alerts or a display can help identify whether smoke or CO triggered a combination unit, but they do not make an alarm more important than correct placement, maintenance, and a practiced escape plan. For people who may not hear a standard alarm, look for compatible equipment with visual signals or other accessible alerting features.

Maintenance, nuisance alarms, and replacement

Test alarms according to their instructions, commonly once a month. Keep them free of dust and follow the cleaning directions; never paint over an alarm or disable it because it sounds inconvenient. Replace batteries as directed, and replace the entire unit at the end of its stated service life. Many smoke alarms have a service life of about 10 years, but the unit’s label and instructions control; CO alarm life varies by model. A chirp can indicate a low battery, fault, or end of life, so consult the manual instead of assuming the cause.

If cooking sets off a smoke alarm, use its hush feature if provided, improve ventilation, or consider whether the alarm is too close to the cooking area. Do not remove its battery or leave it disconnected. Repeated nuisance alarms may mean the location or device type needs adjustment; consult the instructions or local fire service before relocating it.

When a smoke alarm sounds, get everyone outside and call the local emergency number from outside. Do not re-enter until authorities say it is safe. If a CO alarm sounds, move everyone, including pets, outdoors to fresh air and call emergency services; do not search for the source or go back inside to silence the alarm. If anyone feels unwell, tell responders that CO may be present.

Frequently Asked Questions

Do I need both smoke and carbon monoxide alarms?
Yes. They detect different hazards. A listed combination alarm can cover both functions in one unit, but you still need enough alarms in the required locations and must follow its instructions.

Are hardwired alarms better than battery alarms?
Not automatically. Hardwired units with backup power can reduce dependence on routine battery changes and may interconnect, but battery-powered alarms can provide protection where wiring is not practical. Choose a compliant type, install it correctly, and maintain it.

Where should I put a CO alarm?
Start with the locations required by your local code and the manufacturer’s placement instructions. Common guidance includes outside sleeping areas and on each level, with additional alarms where fuel-burning equipment or an attached garage presents a risk.

How often should I replace smoke and CO alarms?
Use the replacement date or service-life instructions printed on each device. Smoke alarms commonly have a service life around 10 years; CO alarm life differs by model. Replace the complete alarm at end of life, not just its battery.

Can I stop a nuisance alarm by taking out the battery?
No. That leaves the home without protection. Use the hush button if available, address smoke or steam, and check whether the device needs cleaning or relocation according to its instructions. Keep it operating while you resolve the cause.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.