The short answer: smoke alarms catch fire, CO detectors catch poisoning from incomplete combustion. A house can have a CO problem with no fire (a cracked furnace heat exchanger, a blocked flue, a generator running too close to the house, a car left running in an attached garage). A house can have a fire without enough CO to trigger a CO alarm. Each device protects against a separate failure mode. Combination units exist and are fine, but understanding what each is doing helps with placement, testing, and replacement.
The two devices side by side
| Smoke alarm | CO detector | |
|---|---|---|
| What it detects | Visible smoke particles from fire. | Carbon monoxide gas (invisible, odorless). |
| What it warns about | Fire. | Fuel-burning equipment releasing CO into living space. |
| Sensor type | Ionization or photoelectric (or both). | Electrochemical. |
| Service life | 10 years. | 7 to 10 years depending on model. |
| Test cadence | Monthly — press the test button. | Monthly — press the test button. |
| Battery (user-replaceable) | Yearly, often at daylight saving in fall. | Yearly, often at daylight saving in fall. |
| Code requirement | Every level, near every sleeping area. | Every level and near sleeping areas; required for homes with fuel-burning appliances or attached garages. |
Why CO is different
Carbon monoxide is the byproduct of incomplete combustion. Sources in a home:
- Furnace, boiler, or water heater with a cracked heat exchanger.
- Blocked or damaged flue or chimney.
- Generator running too close to the house (CDC: at least 20 feet, exhaust pointing away).
- Car running in an attached garage, even with the door open.
- Gas range used as a heater.
- Charcoal grill used indoors.
You can't see CO. You can't smell it. Symptoms (headache, dizziness, nausea, confusion) can be confused with the flu. CO detectors are the only practical way most homes catch it before it reaches dangerous levels. The CDC reports ~400 US deaths a year from accidental CO poisoning.
Where each goes
Smoke alarms (NFPA)
- Inside every bedroom.
- Outside every separate sleeping area.
- On every level, including basement and finished attic.
- For ceiling mounts, at least 4 inches from any wall. For wall mounts, 4 to 12 inches below the ceiling.
- Not in kitchens (too many false alarms from cooking). Photoelectric is less prone to cooking false alarms if you want one near the kitchen.
- Not in bathrooms (steam causes false alarms).
CO detectors
- Outside every separate sleeping area (so the alarm wakes sleeping people).
- On every level of the home.
- At least 15 feet from cooking appliances to reduce false alarms.
- Not in garages (CO levels there can be high transiently and trigger false alarms; for garage protection, use a CO detector inside the house near the door to the garage).
- Not in unconditioned spaces unless the unit is specifically rated for those temperatures.
Combination units: worth it?
Combination smoke+CO alarms are practical for many homes. They cover both threats in one device, simplify placement (one less alarm per bedroom/hallway), and are required in some jurisdictions.
Tradeoffs:
- Pro: fewer units to test, fewer batteries to manage, simpler installation.
- Pro: usually code-compliant for both threats in one device.
- Con: when the CO sensor wears out at 7 years, you replace the whole unit even if the smoke sensor is good for another 3 years.
- Con: single point of failure for both sensors.
Either approach is safe. Pick what fits your house and budget. Don't mix and match individual units in a way that leaves any room or level uncovered.
Hardwired vs battery
Most new construction has hardwired alarms with battery backup. Existing homes often have battery-only. Both are NFPA-acceptable. Hardwired units (interconnected so when one sounds, all sound) are the gold standard for whole-house warning. Battery-only is fine when correctly placed and maintained.
10-year sealed battery alarms (smoke or CO) eliminate the yearly battery swap. When they reach end-of-life, you replace the whole unit. This matches the unit's sensor life and is the simplest schedule.
Testing both
- Monthly: press the test button on every alarm. This tests the battery and speaker, not the sensor.
- To test the sensor: smoke alarms can be tested with smoke alarm test spray (sold at hardware stores). CO detectors are harder to test the sensor on; trust the manufacturer's life rating and replace on schedule.
- End-of-life chirps: different from low-battery chirps. End-of-life means replace the unit, not the battery.
Common mistakes
- Assuming a CO detector is just a different version of a smoke alarm. They're different sensors for different threats.
- Skipping CO detectors in homes with electric appliances only. Attached garages are still a CO risk.
- Replacing batteries on a 12-year-old unit. The sensor is past life; the battery doesn't fix that.
- Hush-buttoning a CO alarm and going back to sleep. CO can build to fatal levels in minutes during sleep.
- Putting either alarm in a corner where dead air pockets sit. Follow placement guidelines.
Good maintenance rhythm
- Monthly: press the test button on every smoke and CO alarm.
- Yearly in fall: replace user-replaceable batteries when clocks change.
- Every 7 to 10 years: replace each CO detector per its date.
- Every 10 years: replace each smoke alarm per NFPA.
- For combination units: replace at the shorter (CO) interval.
- When moving into a new home: check dates on every unit. Replace any without a clear date.
- If any alarm sounds: treat as real. Get out, call 911, don't go back in until cleared.