Smoke Detectors
Photoelectric, Ionization & Beyond
The two sensing technologies, where each one works best, spacing rules from NFPA 72, and why getting this wrong is the number-one cause of nuisance alarms.
Two Technologies, Very Different Strengths
Photoelectric
A light source (usually an LED) shines across a chamber. A photodiode sits at 90° to the beam. In clean air, no light reaches the photodiode. When smoke particles enter, they scatter light into the sensor. The controller reads the scatter signal and triggers alarm when it crosses threshold.
Best at: smoldering fires, soft furnishings, slow-burning fabrics, electrical insulation. Common in bedrooms, hallways, healthcare.
Ionization
A tiny radioactive source (americium-241) ionizes air molecules inside a sealed chamber, creating a small measurable current between two plates. Smoke particles attach to the ions and reduce the current. The controller reads the current drop and triggers alarm.
Best at: fast-flaming fires (paper, wood, gasoline). Historically common in residential; less common in new commercial installations due to nuisance-alarm rates.
Spacing Rules
NFPA 72 §17.7 sets spot-type smoke detector spacing for smooth, level ceilings up to 10 feet:
- Maximum 30 feet between detectors (listed spacing for most photoelectric detectors).
- Maximum 15 feet from a wall (half of listed spacing).
- No more than 900 sq ft of coverage per detector on a smooth flat ceiling.
- Beams, waffle ceilings, joists, and high ceilings trigger specific adjustments in §17.7.3.
- For corridors less than 15 feet wide: 41 feet maximum between detectors, 20.5 feet from ends.
Where airflow exceeds one air change per minute, the listed spacing must be reduced. High-airflow environments — data centers, air-handling units, laboratory fume hoods — often require either a tighter spot-type spacing or a dedicated air-sampling (VESDA-style) detector.
Placement Don'ts
- Within 3 feet of an HVAC supply register (wind blows smoke away from the detector).
- Within 3 feet of a bathroom door (steam causes nuisance alarms).
- In the apex of a peaked ceiling deeper than 4 inches without special listing (dead air pocket).
- In kitchens within 20 feet of cooking equipment (a heat detector is typically used instead).
- In unconditioned attics or outdoor locations (not listed for environment).
Sensitivity Testing
NFPA 72 §14.4.5 requires the sensitivity of every smoke detector to be tested after the first year in service and every two years thereafter. If sensitivity has remained within the listed range on two successive tests, the interval can be extended to every five years until sensitivity begins to drift. Sensitivity is measured by aerosol test method or by the panel reading for addressable systems with on-board self-diagnostics.
References
1. NFPA 72 (2022), §17.7 — Smoke detector spacing and placement.
2. NFPA 72 (2022), §14.4.5 — Sensitivity testing intervals.
3. UL 268 — Smoke Detectors for Fire Alarm Signaling Systems (2019 revision added polyurethane foam test and cooking nuisance immunity).
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Discussion (2)
Great breakdown of the technical details. The NFPA 25 maintenance table is exactly what I needed for my ITM schedule.
Really clear explanation. Would love to see a companion video walkthrough of the inspection process.