Best Smoke & Carbon Monoxide Detectors

The letter tells you what it puts out — A (ordinary combustibles like wood and paper), B (flammable liquids), C (energized electrical). The number in front of A and B is a relative size rating from independent UL testing: a 2-A extinguisher can handle roughly twice the wood/paper fire of a 1-A. A basic home extinguisher is usually 1-A:10-B:C; step up to 2-A:10-B:C or higher for a garage, workshop or kitchen with more to burn.

Yes. Sealed-battery smoke/CO combo alarms are rated for about 10 years and should be replaced, not just re-batteried, at the end of that window — the sensor itself degrades. Hardwired units with a separate battery backup follow the same 10-year sensor life even though you can swap the backup battery sooner.

US fire codes and CPSC guidance call for both smoke and carbon monoxide detection on every level of a home and outside sleeping areas. A combination unit satisfies both with one device; the tradeoff is that if either sensor fails you lose both functions, so some homes still choose separate units for redundancy.

NFPA guidance is one per level, easily reachable near an exit — not hidden under a sink or behind a door. Kitchen units should be near the doorway rather than beside the stove, so you are not reaching past the fire to grab it.