Buying Guide

Smoke and Carbon Monoxide Detectors

Alarm Types

Although an alarm’s job is the same – to alert you to the presence of unsafe smoke or gas in your home – the way it detects the hazard can be quite different, depending on the type of alarm.

  • Ionization smoke alarms feature electrodes that continuously conduct low-level electrical current through ionized air. Smoke particles that enter the ionized air disrupt this current, triggering the alarm. Ionization sensors can detect smaller, less visible fire particles sooner than photoelectric alarms.
  • Photoelectric alarms detect smoke using a light sensor and light beam. Ordinarily, the light beam points away from the sensor. When there’s a fire, the smoke particles scatter the beam, redirecting some light to hit the sensor, which triggers the alarm. These alarms may detect larger, more visible fire particles sooner than ionized sensors.
  • Both ionization and photoelectric alarms are designed to detect any house fire, no matter the source. For optimal protection, consider a dual-sensor smoke alarm that incorporates both technologies.
  • Standard carbon monoxide alarms have sensors that trigger an alarm when CO gas reaches unsafe levels.
  • Combination smoke and CO alarms provide an all-in-one solution.

Power Sources

Smoke and CO alarms operate using one of three power sources: hard-wired, battery-powered, or plug in to a wall outlet.

  • Hard-wired models are powered directly to an AC electrical circuit, which is standard in new construction. Many also include a battery backup system to keep it functioning during power outages.
  • Battery-operated units are standalone and don’t require electrical wiring. They can use a 9 volt or AA battery, which should be changed twice per year. Models with lithium batteries can last up to 10 years.
  • Plug-in CO alarms plug directly into an electrical wall outlet, but work best when installed on the ceiling or top of a wall. The plug-in design isn’t practical for smoke alarms.


Look for alarms that connect to other alarms and safety systems for the most efficient coverage.

  • It is possible to connect some hard-wired units, also called wire-in alarms, so that all alarms will sound if any detect smoke or CO. These units require three-wire cables and a set of alarms from a single manufacturer designed to connect with each other. See our project guide on Installing Hard-Wired Smoke and CO Alarms for more info.
  • Some battery-powered alarms are designed to communicate with each other via radio frequency signals where if one senses any smoke, all of them sound the alarm.
  • Alarms connected to home security systems can be monitored in real time and the monitoring company can notify the fire department if the alarm is triggered.

Installation and Maintenance

The National Fire Protection Association (NFPA) offers several guidelines for where to install detectors and their upkeep.

  • Install smoke detectors on the ceiling or high on the wall 4 to 12 inches from the ceiling.
  • Drafts can interfere with smoke alarm operation, so install them away from doors and windows and A/C vents.
  • Include one alarm on every level of your home.
  • Install a smoke alarm in every bedroom.
  • Press the test buttons once per month to make sure they are functioning.
  • Replace batteries twice per year.
  • Replace the alarm every 10 years.