Best combination smoke + carbon monoxide alarms

How to choose

Put combos in hallways and smoke alarms in bedrooms

CPSC, the federal product safety agency, wants a smoke alarm inside each bedroom, outside sleeping areas and on every level. It wants CO alarms on each level and outside sleeping areas.³ So you need fewer CO sensors than smoke sensors. The layout that fits is a combo in each hallway and on each level, plus a photoelectric smoke alarm in each bedroom, which our smoke alarm guide covers.

That mix also keeps your costs down. In a Washington State survey, only half to two-thirds of equipped homes had the required number of alarms, so don't let a combo's higher price cut your count.²³ Remember too that one failed combo takes out both protections at once.

Basements are the one point of conflict. CPSC says every level, yet the same page says not to install CO alarms in basements without a sleeping area, while First Alert's store says every level.^{3 24} Either way, that level needs a smoke alarm.³ We'd use a combo there and keep the distance from the furnace that the manual asks for.

An all-electric home with no attached garage, fireplace or fuel-burning appliance, and no generator, has the fewest CO sources, so it can use plain smoke alarms in most spots. Keep at least one CO alarm anyway, because CPSC says every home should have one.^{25 26 27}

Check that the smoke sensor is photoelectric

Read the spec sheet, because the box may not tell you. First Alert's sheets show ionization sensors in the SMICO100, SMICO110 and SMICO105-AC, and photoelectric sensors in the SMCO200, SMCO210, SMCO500V and SMCO100V-AC.^{2 4 5 6 7 8 28}

In a randomized trial of 761 Washington households, 20% of ionization alarms weren't working after 9 months, against 5% of photoelectric alarms.¹ In kitchen tests by NIST, the federal standards lab, older ionization alarms sounded in 43% of cooking tests with the range hood off, and older photoelectric alarms sounded in 12%.²⁹

Stay wired, or buy sealed

If your home has wired alarms, stay wired. The National Fire Protection Association found that hardwired alarms operated in 94% of fires large enough to set them off, and battery-only alarms in 81%.³⁰ A wired CO alarm still needs a working battery backup.^{25 31}

Without wiring, buy sealed. New York stopped sales of battery-only alarms with removable batteries.¹⁷ Massachusetts requires a sealed battery and a photoelectric sensor when you replace a battery alarm.¹⁶

Link them if you can

Linked alarms all sound when one does. In a CPSC survey of household fires that weren't reported to fire departments, they sounded in 53% of fires, against 27% for alarms that weren't linked, so linking is worth more than an app or a voice.³⁰ Links stay within a brand, so choose the brand for the whole house.^{7 14}

Make sure you can tell the two alarms apart

This risk belongs to combos. One firefighter's video says people who hear an alarm and see no smoke get confused and walk around looking for it, which is the wrong move with CO.³² First Alert and Kidde sheets use 3 beeps for smoke and 4 for CO.^{7 33}

Voice or a CO display helps you tell the two alarms apart. Studies tested a parent's recorded voice, not retail prompts.³⁴ If you can't tell which alarm is sounding, get everyone outside and call 911 from there.

Don't read silence as an all-clear

CO is measured in parts per million, or ppm. First Alert's sheets say the sensor ignores levels below 30 ppm and won't alarm at a constant 30 ppm for 30 days. It alarms at 70 ppm in 60 to 240 minutes, at 150 ppm in 10 to 50 minutes and at 400 ppm in 4 to 15 minutes.^{7 8 18} Kidde's sheet warns that its alarm won't fully protect people with certain medical conditions.³³

So a quiet alarm doesn't prove the air is safe. If anyone at home has headaches, nausea or dizziness that could be CO, get outside to fresh air right away and call for help. If someone in the home has heart disease or is pregnant, ask a doctor whether a separate low-level monitor is worth adding.^{31 35} National cases peak in December and January, so test your alarms before heating season.³⁶

Check the marks and the recall list

Look for UL 217 and UL 2034, or an ETL mark to the same standards, on the alarm itself.^{31 37} Register your alarm so that a recall notice reaches you, and check the exact model against CPSC's list before you buy.³⁸

Several combos have been recalled for failing to alarm, and known brands aren't exempt. CPSC warned consumers to stop using BQQZHZ combos in 2023 (about 25,000 units) and recalled CHZHVAN combos in 2024 (about 6,800) and Treatlife combos in 2026 (about 20), all sold on Amazon. Recalls also reached Apollo America combos sold by Vivint in 2025 (about 50,000), Universal Security Instruments in 2022 (about 8,000) and Kidde TruSense in 2021 (about 226,000 smoke and combo alarms). Kidde also recalled about 3.6 million NightHawk KN-COSM-IB combos made from 2004 to 2010, because they can stop chirping at end of life once the batteries are changed.^{38 39}

If you own a BQQZHZ, CHZHVAN or recalled KN-COSM-IB combo, stop using it now and install a working alarm. For the others, contact the company for a replacement or refund, and keep the old alarm up until the new one is installed.^{38 39}

Hang it in the right spot

CPSC says not to put CO alarms in kitchens, above fuel-burning appliances, near heating vents or where furniture or drapes can cover them.³¹ NIST advises keeping smoke alarms at least 6 feet, and ideally 20 feet, from the cooking area.⁴⁰

Replace it on the date, not the test button

First Alert and Kidde say 10 years, though Kidde's footnote says its warranty isn't a battery performance claim. CDC says to follow the maker's instructions or replace detectors every 5 years.^{7 25 33} The test button checks the circuit and not the sensor's accuracy, so follow the date on your alarm, and replace it when the end-of-life chirp starts at the latest.³¹