Smoke + CO Detectors Reference
Why this matters
Smoke + CO detectors save lives. Properly placed + maintained = early warning saves families. Missing OR failed = preventable death + lawsuit. The tech who installs + maintains correctly delivers safety + builds customer trust. This is the field card.
Code requirements (IRC R314 + R315)
Smoke alarms required:
- Every sleeping room
- Outside each sleeping area (typical hallway)
- On every level including basement
- Inter-connected new construction
- Battery OR hardwired
CO alarms required:
- Outside each sleeping area
- On every level (some jurisdictions)
- Near attached garage (CO can migrate from car)
- Near fuel-burning appliances
Modern requirements: 10-year sealed battery OR hardwired with battery backup.
Smoke detector technology
Ionization: small radioactive source ionizes air; smoke disrupts; alarm sounds.
- Fast at flame fires (kitchen, paper)
- Slower at smoldering fires
- Common in older installations
Photoelectric: LED + sensor; smoke scatters light; alarm.
- Better at smoldering fires (mattress, upholstery)
- Slower at flame fires
- Preferred modern choice
Dual sensor (combo ionization + photoelectric): best of both; recommended.
Air sampling (commercial): continuously samples air; very early warning.
CO detector technology
Electrochemical sensor: most common residential; reliable + accurate.
Metal oxide semiconductor: older type; less accurate.
Lifespan: 7 - 10 years; sensor degrades over time.
Replacement: by date (typically printed on unit) OR every 7 years max.
Combo smoke + CO detector
Single unit detects both: cost-effective for code compliance.
Common brands: First Alert, Kidde, Nest Protect.
Same install location: hallway outside sleeping rooms.
End-of-life: when sensor expires, replace entire unit.
Detector placement (smoke)
Inside sleeping rooms: where you sleep; closest to bed.
Outside sleeping areas: hallway near bedrooms.
Each level: including basement + finished attic.
4 - 12 inches from ceiling (typical residential; manufacturer spec).
Away from:
- Bath fans (humidity nuisance)
- Kitchen (cooking nuisance; place 10+ ft from stove)
- Garage (heat + exhaust nuisance)
- HVAC vents (drafts disturb)
Vaulted ceilings: peak placement; or on inclined surface 4-12" from peak.
Stairs / hallways: include junction areas.
Detector placement (CO)
Outside sleeping areas (same as smoke).
Each level (some jurisdictions; check local code).
Near attached garage (CO from car).
Near fuel-burning appliances (furnace, water heater, fireplace).
5+ ft above floor (CO disperses; not floor-level).
Don't place in: kitchen, bathroom, near combustion equipment exhaust outlet (false alarm).
Hardwired interconnect
3-wire interconnection (typical):
- Black: hot
- White: neutral
- Red OR Yellow: interconnect (carries alarm signal to other detectors)
Compatible models: same brand interconnects easily; mixing brands risky.
Battery backup: 9V battery OR sealed Li battery; replaces yearly OR lifetime.
Junction box mount: standard 4" round OR 4-square.
Wireless interconnect
Newer technology: detectors communicate wirelessly.
Programming: pair detectors per manufacturer.
Range: typical 1,000 sq ft residential.
Brands: First Alert SC9120LB, Kidde KN-COSM-PE, Google Nest Protect.
Battery-powered (legacy + supplemental)
9V battery: legacy; annual replacement.
AA OR AAA battery: smoke-only OR combo; lifecycle 1 year.
Sealed lithium 10-year: no battery swap; replace entire unit at 10 years.
Preferred modern: sealed 10-year units (no annual battery management).
Testing + maintenance
Test monthly: press test button; alarm sounds.
Replace batteries (where applicable) annually.
Vacuum / dust: every 6 months; dust degrades sensor.
Replace entire unit:
- Smoke: 10 years from manufacture date
- CO: 7 years from manufacture date (varies by brand)
- Combo: shorter of the two
Common detector mistakes
Skipping interconnect on new construction (code violation). Wrong placement (kitchen, bath, garage = false alarms). Painting over (blocks sensor). Removing battery (no protection). Not replacing at end of life (sensor degraded; doesn't alarm). Installing only one (every level + sleeping area required). Skipping CO near attached garage.
False alarm causes
Smoke detectors:
- Cooking (kitchen too close)
- Steam from bath fan
- Dust buildup in unit
- Insect crawled into unit
- Battery low signal misread as alarm
CO detectors:
- Sensor reaching end-of-life
- Backdraft from chimney
- Vehicle exhaust nearby
- Welding / gas appliance startup briefly
Wiring procedure (hardwired)
- Power off circuit + verify dead
- Box install (J-box at detector location)
- Run 3-wire cable from breaker panel (15A typical) to first detector
- Daisy-chain to additional detectors (interconnect wire)
- Wire each detector: hot, neutral, interconnect, ground
- Mount detector to base
- Install backup battery (if required)
- Restore power; test all units
Common service-call scenarios
Chirping detector: low battery; replace.
Constant alarm: detector OR battery issue; replace OR clean.
Unit not testing: bad detector; replace.
False alarm during cooking: relocate detector OR change cooking habit.
Inter-connect not working: re-pair; check wiring.
End-of-life beep (different cadence): replace unit entirely.
CO levels + response
0 - 9 ppm: normal; no concern.
10 - 35 ppm: prolonged exposure can cause headache.
36 - 99 ppm: investigate; possible HVAC OR appliance issue.
100+ ppm: evacuate; call fire department; possible CO leak.
400+ ppm: lethal exposure within hours.
1600+ ppm: lethal within 1 hour.
Common sources: cracked heat exchanger, blocked flue, atmospheric water heater backdrafting, attached garage emissions.
Smart detectors
Nest Protect: app integration, voice alerts, multiple sensors.
First Alert OneLink (Apple HomeKit): HomeKit integration.
Smart features: smartphone alerts, smart-home integration, where-detector-tripped status.
Same code coverage: hardwired OR battery-powered.
Specific scenarios
Detector keeps chirping after battery replace: end-of-life; replace unit.
Detector chirps but no battery low signal: end-of-life; replace.
Detector tested but no alarm sounds: bad unit; replace.
Old (pre-2012) detector: likely ionization-only; replace with combo OR photoelectric.
Apartment / rental: landlord responsibility per code + insurance.
Customer education
Test monthly: walk customer through test.
Replace batteries annually (where applicable; many newer are sealed).
References
- IRC R314 (smoke alarm requirements)
- IRC R315 (CO alarm requirements)
- NFPA 72 (National Fire Alarm + Signaling Code)
- UL 217 (smoke alarms), UL 2034 (CO alarms)
- Manufacturer technical data (First Alert, Kidde, Nest)
- Manuall internal: Combustion Fundamentals, Common Codes Overview