How to choose smoke and carbon monoxide alarms
Alarms are one of the few home purchases where the cheapest working model still does an important job. The choice is mostly about sensor type, power, and whether the alarms talk to each other. Always follow the manufacturer's instructions and your local building and fire codes.
Ionization, photoelectric or both
Smoke alarms use one of two sensing methods. Ionization alarms are generally quicker to react to fast, flaming fires. Photoelectric alarms are generally quicker to react to slow, smoldering fires, and tend to give fewer nuisance alarms from cooking. Dual-sensor alarms combine both. Fire safety groups recommend having protection for both kinds of fire, so many people choose photoelectric or dual-sensor alarms, or a mix of types.
Smoke, CO or combination
Carbon monoxide has no smell or color, so only a CO alarm can detect it. A home with gas appliances, a fireplace, or an attached garage should have CO alarms. Combination alarms detect both smoke and CO in one unit and use different alarm sounds or voice messages for each, which saves ceiling space. Where to mount a CO alarm differs between models, so place it where the manual says.
Sealed long-life batteries versus replaceable batteries
Battery-only alarms can go anywhere. Models with a sealed long-life battery, often sold as “10-year” alarms, never need a battery change; when the battery runs down, the whole alarm is replaced. Alarms with replaceable batteries cost less up front but need new batteries on a schedule and chirp when they're low. Hardwired alarms run on household power with a backup battery, and replacing them usually means matching the existing wiring plug or connector, so check the brand and model of what you have.
Interconnected alarms
Interconnected alarms all sound when any one of them detects a problem, so you hear an alarm in the basement from a bedroom. Hardwired alarms can be connected by wire, and many newer alarms link wirelessly. Wireless alarms usually only connect to others from the same brand and product line, so plan on buying one system for the whole house.
Smart alarms
Smart alarms can send alerts to your phone when you're away and silence false alarms from an app. They usually need Wi-Fi and an account. Check whether the alarm still sounds and works normally if your internet is down.
Check the date on the alarms you already have
Alarms don't last forever, even if they still beep when tested. Every alarm has a manufacture date or replace-by date printed on it, usually on the back. Take one down and check, then follow the manufacturer's guidance on when to replace it. When buying on clearance, it's reasonable to check that the stock isn't old.
Test them
Press the test button on every alarm regularly, and follow the cleaning and testing steps in the manual. A new alarm only protects you once it's installed in the right place and working.
Guide updated October 22, 2026.