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Fire & Life Safety

Smoke and Carbon Monoxide Alarms: Types, Placement and Replacement

Researched guide · last reviewed November 8, 2026

Researched from public sources, not a hands-on review. Local codes may be stricter than a national explainer. Check with the local fire marshal.

Smoke alarms: where, and how they are powered

The U.S. Fire Administration’s smoke-alarm page says:

USFA’s maintenance split matters, because the replacement clock is not identical for every power source:

If an alarm sounds while you are cooking or showering, USFA says not to take the battery out. Open a window or door, use the hush button, wave a towel, or move that alarm several feet from the kitchen or bathroom. Disabling an alarm can be fatal.

Sensor types

Ionization and photoelectric alarms detect different types of fires. USFA does not, on the page we read, say which one is faster for flaming fires or for smoldering fires.

Because no one can predict the fire, USFA recommends that every home and every place people sleep have both ionization and photoelectric alarms, or a dual-sensor alarm that contains both.

For people who are deaf or hard of hearing, USFA says there are alarms with strobe lights that flash, alarms that vibrate, or both.

Wireless alarms that talk to each other are a real category in USFA’s description. Whether two brands interconnect is a compatibility question for the instructions, not a rule this guide can state from that page. Hardwired interconnection is electrical work for a qualified electrician.

Packaging often shows a UL 217 mark on smoke alarms and a UL 2034 mark on carbon monoxide alarms. Those standard numbers were not restated on the USFA page we read. Confirm the mark on the box.

Carbon monoxide alarms

EPA describes carbon monoxide as an odorless, colorless, toxic gas. You cannot see, taste, or smell it.

USFA, crediting CPSC on its carbon-monoxide page, says to install CO alarms in a central location outside each separate sleeping area and on every level of the home. The NFPA carbon-monoxide page is linked below and did not return article text to our research, so this guide does not attribute a narrower “only if you have fuel-burning appliances” rule to NFPA. Local code can be more specific. Follow it.

CO sensors expire. Follow the replacement date printed on the unit. This guide does not assign a universal number of years, because the USFA page it read did not print one.

Combination smoke-and-CO units mean fewer devices and a single replacement when either sensor is due. Separate units let you place each one where its own instructions say. That is a buying tradeoff, not a test.

Phone alerts from a “smart” alarm need a network and an app, and the app may collect data. The alarm on the ceiling still has to work if the phone does not.

Sources