Initiating Devices and Automatic Detection
Match the technology to the fire
Ionization detectors respond fastest to flaming fires with small particles; photoelectric detectors respond best to smoldering, smoky fires; combination units cover both. Heat detectors (fixed temperature, rate-of-rise, or combination) are slower but immune to smoke and are used where smoke detection causes nuisance alarms - and they are life-safety devices only when the code permits them. Projected beam, air-aspirating, and flame (UV/IR) detectors serve large or special spaces. Duct smoke detectors are required on air handling units above the threshold flow (commonly 2,000 cfm) and are never a substitute for area detection. Elevator lobbies, machine rooms, and hoistways have their own detection rules tied to ASME A17.1.
Spacing rules you must be able to use
For smooth ceilings, the classic baseline is 30 ft spacing / 900 sq ft per smoke detector and 50 ft spacing / 2,500 sq ft per heat detector, with the first device no more than half the spacing from any wall (15 ft for smoke, 25 ft for heat). The 0.7 rule says no point on the ceiling may be more than 0.7 times the listed spacing from a detector (21 ft for a 30-ft smoke spacing). Keep detectors at least 4 in. from the wall/ceiling junction to stay out of dead air, reduce spacing for high ceilings and for joists/beams deeper than 18 in., and keep spot detectors clear of supply diffusers, door drafts, and ceiling fan tips (roughly 3 ft).
Worked example
A 60 ft × 40 ft smooth-ceiling room = 2,400 sq ft. By area: 2,400 ÷ 900 = 2.67, so 3 smoke detectors minimum. Now check geometry: three detectors spaced across the 60-ft dimension at 15, 30, and 45 ft - every point is within 21 ft of a detector and within 15 ft of the 40-ft walls. Passes.
Traps
Using square footage alone and ignoring the 0.7 rule; forgetting ceiling-height reduction; treating duct detectors as area coverage.