LVWave · Independent educational reference

Fire Detector Atlas

Educational reference based on NFPA 72 (2022 reference edition, incl. TIA 22-4), NFPA 90A, product listings and manufacturer documentation. Requirements vary by adopted code edition, AHJ, application and listed equipment. Verify the requirements applicable to each project. Not an NFPA, UL or AHJ publication or code interpretation.

CODENFPA 72 requirement CH. 29household (dwellings, Ch. 29) requirement ANNEXexplanatory, not mandatory NFPA 90AHVAC standard LISTING / MFRlisting or manufacturer instructions EXAMPLEillustrative value PHYSICSphysical principle

Spot-type spacing geometry

The S / S/2 / 0.7S geometry appears in several detector-spacing applications, but the value of S and additional installation requirements depend on detector type, listing and application.

Plan view, smoke detectors, smooth ceiling. S/2 is measured at right angles from walls or partitions extending to within the top 15 % of the ceiling height.

30ftnominal spot smoke spacing, smooth ceiling, absent performance-based designCODE17.7.4.2.3.1

S/2max from walls/partitions to a smoke detector (15 ft at S = 30 ft)CODE17.7.4.2.3.1

0.7Salternative: every ceiling point within 0.7 S (≈ 21 ft at S = 30 ft)CODE17.7.4.2.3.1

40.8ftderived geometry for a 10 ft corridor: 2·√(21² − 5²). Not a prescribed spacing.EXAMPLE

32–100 °F · ≤ 93 % RH · ≤ 300 ft/minenvironmental limits for smoke detectors unless the detector is specifically designed/listed for other conditionsCODE17.7 LISTING / MFR

Conceptual. Peaked/sloped, beam and high-ceiling rules differ for smoke and heat detectors — see the detector sections.

Why “≤ S/2 from the wall”?

Section along the ceiling, smoke detectors. The wall acts like a mirror: a detector S/2 from the wall covers the wall zone the same way two detectors cover their midpoint.

Maximum, not target. For spot smoke detectors the Code sets the S/2 maximum; closer placement follows the manufacturer's instructions. Heat detectors have their own rule: on the ceiling ≥ 4 in from the sidewall (see 07).
How to measure. At right angles from walls or partitions extending to within the top 15 % of the ceiling height, to the detector center. Check every wall.
HEAT DETECTOR EXAMPLE. Adjust S first, then halve: 50 ft listed spacing on a 19 ft ceiling → 50 × 0.64 = 32 ft → ≤ 16 ft to the wall.
S/2 is the simple rule. The 0.7 S alternative (every ceiling point within 0.7 S) can allow more than S/2 along a narrow room. The corridor numbers below are derived geometry, not prescribed spacings.

Check a room EXAMPLE

Length, ft Width, ft Spacing S, ft Smooth flat ceiling, geometry only. 30 ft = nominal smoke spacing; for heat use listed spacing × height factor.

Detectors along length Rows across width Detectors are spread evenly: equal gaps, half a gap to each wall.

Ionization smoke detector

Tends to respond more readily to the smaller combustion particles commonly produced by fast-flaming fires · NFPA 72 17.7 · UL 268 (system detectors) / UL 217 (smoke alarms)

Section — shared by spot smoke types 01, 02, 03 (when used as a smoke detector).

30ftnominal spacing, smooth ceilingCODE17.7.4.2.3.1

≤ 12insidewall mount: top of detector below ceilingCODE17.7.4.2.1

Airflowavoid direct HVAC airflow; account for air movement per the Code and manufacturer instructionsCODE17.7 LISTING / MFR

36inhousehold: from forced-air supply registers (and outside their direct airflow); from ceiling-fan blade tips (unless room configuration prevents); from door of bathroom with tub/shower (unless listed for such proximity)CH. 2929.11.3.4

Light fixturesseparation from fixtures (especially where fluorescent/electrical interference is a concern) — per manufacturer instructions; no universal NFPA distanceLISTING / MFR

How it alarms

Am-241 ionizes air in the chamber
Small current flows between plates
Smoke particles attach to ions, current drops
Change past the alarm threshold — alarm

Common components

Am-241 sourcesmall sealed source

Sensing chamberopen to smoke

Reference chambercompensation (many designs)

Measuring electronicsdesign-specific

Avoid household cooking zones — see Ch. 29 diagram direct HVAC airflow exhaust fumes, dust outside listed environmental range

Photoelectric smoke detector

Generally more responsive to the larger smoke particles commonly associated with smoldering fires · NFPA 72 17.7 · UL 268 / UL 217

30ftnominal spacing, smooth ceilingCODE17.7.4.2.3.1

≤ 15ftto walls/partitions (S/2)CODE17.7.4.2.3.1

Airflowavoid direct HVAC airflow; per Code and manufacturer instructionsCODE17.7 LISTING / MFR

36inhousehold: supply registers, ceiling-fan blade tips, bathroom door — same conditions as 01CH. 2929.11.3.4

How it alarms (light scattering)

Light source pulses past the receiver
Smoke scatters the light
Scattered light reaches the receiver
Signal past the alarm threshold — alarm

Common components (design examples)

Light sourcetypically LED; wavelength and number of optical channels are model-specific

Photodiodeoff-axis receiver

Optical chamber / labyrinthadmits smoke, blocks ambient light

Insect screenmany designs

Processordrift compensation on many models

Avoid household cooking zones — see Ch. 29 diagram steam, showers aerosols, dust air velocity above the listed limit

Multi-sensor / multi-criteria detector

Combines two or more sensing technologies. Sensor combinations and alarm algorithms are model-specific.

Per listingspacing and use follow the detector listing, the detection function relied on, NFPA application requirements and manufacturer instructions. Individual sensor channels do not automatically receive independent spacing credit.CODE17.7 / 17.6 LISTING / MFR

2+sensing technologies combinedLISTING / MFR

COCO sensor service life — model-specificLISTING / MFR

Logic EXAMPLE

Smoke signal alone: evaluated with more caution
Smoke + supporting signal (e.g., heat or CO): confirmation
Faster alarm decision

EXAMPLEConceptual example — actual decision algorithms are manufacturer- and listing-specific.

Common components / example architecture

Optical smoke sensingcommon

Thermal sensingcommon

CO sensingsome models

Processorlisted algorithm

Projected-beam smoke detector

Detects smoke along a light path across the space: atriums, warehouses, plants · NFPA 72 17.7.4.7 · installed per manufacturer's published instructions

Plan view, smooth ceiling, end-to-end units. Spacing values are Annex guidance; actual layout follows the listing, manufacturer instructions, ceiling conditions, airflow and performance objective.

≤ 60ftsmooth-ceiling guidance between parallel beamsANNEXA.17.7.4.7

≤ 30ft≈ one-half of the selected beam spacing to a parallel sidewallANNEXA.17.7.4.7

Beam lengthmaximum per equipment listing — must not be exceededCODE17.7.4.7 LISTING / MFR

Sensitivityobscuration settings — per listing and manufacturerLISTING / MFR

Stable mountingdetector and reflector on stable surfaces — building movement can misalign the beamCODE17.7.4.7 LISTING / MFR

How it alarms

Light beam to the receiver (end-to-end) or to a reflector and back (single-ended)
Smoke along the path reduces the received signal
Reduction reaches the listed alarm setting — alarm
Sudden total blockage — typically a trouble signal (listed detector behavior)

Common components

End-to-endseparate transmitter + receiver

Single-endedcombined transceiver + prism reflector

Alignment aidssome models: automatic alignment

Avoid cranes, racks, signage in the beam path strong light into the receiver unstable / flexing mounting surfaces

Air-sampling smoke detector (ASD)

A pipe network with sampling ports draws air to a central detector · NFPA 72 17.7.4.6

Section. Port spacing shown is a prescriptive smooth-ceiling example, not an inherent ASD specification.

≤ 120smaximum sample transport time from the farthest sampling port to the detectorCODE17.7.4.6.2.1

Port = spotin the absence of specific performance-based design criteria, each sampling port is treated as a spot-type smoke detector for location and spacingCODE17.7.4.6

Sensitivitydetector- and application-specificLISTING / MFR

Port sizesize and quantity from the listed pipe-network design calculations/software and manufacturer instructionsLISTING / MFR

How it alarms

Aspirator draws air through the ports
Filtering, where used
Optical sensing chamber measures smoke
Multiple programmable alarm thresholds may be available depending on the listed detector

EXAMPLESome VESDA/Xtralis systems use Alert, Action, Fire 1 and Fire 2.

Common components

Aspiratorfan

Optical sensing chambersensing technology varies by product

Filterwhere used

Airflow monitoringblocked or broken pipe

Pipe networkpressure, airflow, transport time and sampling-point performance are calculated with the listed design method/software

Duct smoke detector

Smoke detection for control of smoke spread in HVAC systems · NFPA 72 17.7.6 (location/installation 17.7.6.5) · where required by NFPA 90A. Duct smoke detection is not a substitute for required open-area detection.

Supply side example: downstream of filters (NFPA 90A). Follow the detector's listed installation instructions and verify proper sampling airflow.

> 2,000cfmsupply systems: detector downstream of air filters and ahead of any branch connectionsNFPA 90A6.4.2.1

> 15,000cfmreturn systems serving more than one story: at each story, before connection to the common return and before any recirculation or fresh-air inlet connection. Exceptions exist — check the adopted NFPA 90A / code for the specific system.NFPA 90A6.4.2.1

Representative sampleinstalled to obtain a representative sample of the airstreamCODE17.7.6.5

≥ 6 widthstypical manufacturer/application guidance for avoiding turbulent locations (elbows, dampers)LISTING / MFR

Remote indicator

Provides indication for a detector whose status or location is otherwise not readily identifiable — where required, subject to system/code configuration.

Remote test station

A separate testing accessory/capability. Its use depends on the detector/system, manufacturer and project requirements. Not interchangeable with a remote indicator.

Tube sizing and orientation depend on the listing, manufacturer and system design.

How it works

Sampling tube in the duct picks up a sample
Duct pressure differential drives it through the sensing chamber
Smoke detected → HVAC / fire-alarm control sequence

Common functions can include fan shutdown and/or damper operation. Engineered smoke-control sequences can operate differently.

Avoid turbulent locations right after elbows or dampers air velocity outside the listed range humidifier discharge

Spot-type heat detector

Fixed-temperature, rate-of-rise or rate-compensated operation · NFPA 72 17.6 · UL 521

Section. S = listed spacing (UL 521).

Slisted spacing — e.g. 25, 30 or 50 ft (examples, not a complete set)CODE17.6.3 EXAMPLE

≥ 4 in · 4–12 inceiling: ≥ 4 in from sidewall; sidewall: 4–12 in below ceilingCODE17.6.3.1.3.1

½ Sperpendicular to solid joistsCODE17.6.3

⅔ Sperpendicular to beams more than 4 in deepCODE17.6.3

+20°Frating at least 20 °F above the maximum expected ceiling temperatureCODE17.6.2.3

0.4× Hhigh-ceiling reduction need not result in spacing below 0.4 × ceiling heightCODE17.6.3.5.2–17.6.3.5.4

EXAMPLE 50 ft listing, 19 ft ceiling → 50 × 0.64 = 32 ft. Above 30 ft the table gives no prescriptive multiplier: use the appropriate listing, manufacturer, performance-based/engineered and AHJ-approved approach.

EXAMPLE 135 °F is a common fixed rating, not the only one; 15 °F/min is a typical rate-of-rise example. Actual operating characteristics per listing/manufacturer.

Operating modes

Fixed temperature: element reaches its rating
Rate-of-rise: temperature rises faster than the listed rate (typical example 15 °F/min)
Rate-compensated: accounts for thermal lag

Common sensing technologies

Thermistorelectronic / addressable models

Bimetaloften self-restoring

Eutectic elementnon-restorable

Pneumatic rate-of-riseair chamber with vent

A given detector uses one or more of these — not all of them.

Linear / line-type heat detector

Heat sensing along a cable or tube: cable trays, conveyors, tunnels, parking garages · NFPA 72 17.6

Section. Special applications (trays, conveyors, tanks) — per manufacturer's layout.

≤ 20inon the ceiling or on the sidewall not more than 20 in below the ceilingCODE17.6.3.1.3.2

S · S/2S per listing; S/2 wall-spacing concept where applicableCODE17.6.3 LISTING / MFR

Alarm temp.alarm temperature / operating range — per product listingLISTING / MFR

High ceilings: exceptionTable 17.6.3.5.1 derating does not apply in the same way to line-type electrical-conductivity detectors and pneumatic rate-of-rise tubing, which rely on integration effect — manufacturer's published instructions govern alarm point and spacing.CODE17.6.3.5.2–17.6.3.5.4 LISTING / MFR

Digital fixed-temperature electrical-conductivity cable only.

How digital fixed-temperature LHD works

Two conductors separated by heat-sensitive polymer
Polymer yields at the rated temperature
Conductors contact — alarm; compatible interface/location equipment can identify the distance to the actuated point

Types — different mechanisms

Digital / electrical-conductivityheat-sensitive cable; operating principle depends on product

Analogtemperature-dependent electrical characteristics monitored along the cable

Fiber-optic DTSdistributed optical temperature measurement along fiber

Avoid tension, sharp bends crushing by fasteners unlisted splices

Flame / radiant-energy fire detector

Placement is not based on a universal spacing grid · NFPA 72 17.8

Placement is performance-based CODE 17.8

expected fire size

fuel

detector sensitivity / listing

field of view

distance

atmospheric attenuation

interfering radiation

obstruction / line of sight

purpose of detection

required response

Every area requiring detection must be within the field of view of a detector, without obstruction.

PHYSICS Idealized inverse-square explanation. Actual detection distance also depends on fuel, flame size, atmosphere, viewing angle and detector listing.

1/d²received radiant energy decreases approximately with the inverse square of distancePHYSICS

FOVfield of view — per detector listing (example: many products ≈ 90°)LISTING / MFR EXAMPLE

≈ 4.3–4.5 µmimportant CO₂ emission region used by many hydrocarbon IR flame detectorsPHYSICS

Line of sightclear line of sight required to the protected areaCODE17.8

Technologies

UV: senses ultraviolet radiation (typical product band shown as example)
IR / multispectrum: many analyze temporal flame/flicker characteristics to distinguish flame from steady background radiation
Multi-channel detectors correlate two or more spectral signals; exact alarm logic is manufacturer- and listing-specific

Common components — technology dependent

UV sensorUV-type detectors

IR sensor(s) + filtersIR / multi-IR types

Optical windowmust stay clean

Optical self-checksome models

Signal processorlisted algorithm

Potential interference / nuisance sources welding lightning strong / hot radiation sources reflected radiation other UV/IR interference

Optical obstruction / loss of sensitivity dust grease / oil paint water / ice physical obstruction

Manual fire alarm box (pull station)

Where manual fire alarm boxes are required by the applicable code or system design, NFPA 72 governs their installation and location · NFPA 72 17.15

Wall elevation at an exit doorway.

42–48infinished floor to operable partCODE17.15.6

≤ 5ftfrom each exit doorway on each floorCODE17.15.9.4

≤ 200fthorizontal travel distance to the nearest box on the same floorCODE17.15.9.5

> 40ftgrouped openings wider than 40 ft: a box on both sides, within 5 ft of each sideCODE17.15.9.6

Visibleconspicuous, unobstructed, accessibleCODE17.15.9.2

Rednormally red unless the installation environment precludes redCODE17.15.9.3

How it works CODE 17.15.7

Single-action: one motion. Double-action: two separate motions
Mechanism latches, contact/electronics change state
Alarm at the control unit · reset per product design

Common construction examples

Operating mechanismmechanical

Electrical contact / switchdesign-specific

Addressable electronicson applicable products

Reset mechanismdesign-specific

CODE 17.15.8 · Optional accessory: a listed protective cover is permitted over single- or double-action boxes as applicable.

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Clearance summary

Show table for all types

Type	Spacing	Walls	Mounting	Other
Spot smoke (01–02)	30 ft CODE	≤ S/2 CODE	sidewall: top ≤ 12 in below ceiling CODE	direct airflow: avoid CODELISTING / MFR · 36 in registers / fan / bath door CH. 29 · cooking: 10 ft exclusion, 6–10 ft exception, 10–20 ft listed CH. 29 · light fixtures LISTING / MFR
Multi-sensor	per listing & function LISTING / MFR	—	—	no automatic per-channel credit
Projected beam	≤ 60 ft ANNEX	≈ ½ spacing (≤ 30 ft) ANNEX	per listing; stable surfaces LISTING / MFR	beam length ≤ listed max CODE
Air sampling	port = spot detector CODE	≤ S/2 EXAMPLE	—	transport ≤ 120 s CODE · port size per listed design LISTING / MFR
Duct	—	—	representative airstream sample CODE	supply > 2,000 cfm: after filters, before branches; return > 15,000 cfm, multi-story: each story NFPA 90A · ≥ 6 widths from elbow LISTING / MFR
Spot heat	listed S × height factor (10–30 ft) CODE	≤ S/2 · ≥ 4 in CODE	sidewall 4–12 in below ceiling CODE	rating ≥ max ceiling temp + 20 °F; ≥ 0.4 × H floor on reduction CODE
Line-type heat	per listing LISTING / MFR	≤ S/2 CODE	sidewall ≤ 20 in below ceiling CODE	high-ceiling derating exception for integration-effect types CODE
Flame	performance-based CODE	—	—	clear line of sight; FOV per listing CODELISTING / MFR
Pull station	travel ≤ 200 ft CODE	≤ 5 ft from exit doorway CODE	42–48 in CODE	openings > 40 ft: both sides CODE