A smoke detector responds to smoke particles; a heat detector responds to temperature. Smoke detectors catch most fires much earlier, so they are the standard for life safety in commercial buildings, while heat detectors cover spaces where smoke detectors would false alarm. Commercial smoke detectors come in six main types: photoelectric, ionization, multi-criteria, projected beam, aspirating (air sampling) and duct. Photoelectric is the most common spot type today. This guide explains how each type works and where it fits.
Smoke detector vs heat detector
A fire usually produces visible smoke well before it produces enough heat at the ceiling to operate a heat detector. That head start is what gives occupants time to leave, which is why codes call for smoke detection in many occupied areas and escape routes. Heat detectors are for property protection and for rooms where smoke, steam, dust or temperature would make a smoke detector unreliable.
| Smoke detector | Heat detector | |
|---|---|---|
| Responds to | Smoke particles (optical or ionization) | Temperature, or rate of temperature rise |
| Speed | Early, often before flames are large | Later, once ceiling temperature rises |
| Main purpose | Life safety and early warning | Property protection |
| Nuisance sources | Cooking, steam, dust, exhaust, insects | Very few; rate-of-rise can trip on rapid normal temperature changes |
| Ambient limits | NFPA 72: 32–100°F, up to 93% RH, under 300 ft/min, unless listed otherwise | Rated by temperature class; works in hot, cold, dirty spaces |
| Typical spacing | 30 ft nominal on smooth flat ceilings | Listed spacing, often 50 ft for mechanical units |
| Typical locations | Corridors, offices, sleeping areas, electrical rooms, elevator lobbies | Kitchens, garages, boiler rooms, attics, dusty or humid areas |
For heat detection, browse our heat detectors: fixed temperature, rate-of-rise, rate-compensated and addressable heat sensors.
Photoelectric vs ionization smoke detectors
Photoelectric (optical)
A photoelectric detector has a light source and a light sensor inside a dark chamber, arranged so the sensor normally sees no light. Smoke entering the chamber scatters light onto the sensor and the detector alarms. NFPA describes photoelectric detection as generally more responsive to smoldering fires, the slow, smoky fires typical of overheated wiring, upholstery and paper. Photoelectric is also less prone to nuisance alarms from small combustion particles. Nearly every commercial brand's standard addressable head is photoelectric, for example the Notifier FSP-951, Simplex 4098-9714 TrueAlarm and Edwards SIGA-OSD.
Ionization
An ionization detector uses a small radioactive source to ionize air between two charged plates, creating a small current. Smoke particles disrupt the current and the detector alarms. NFPA describes ionization as generally more responsive to fast flaming fires, which produce many small particles. It is also more sensitive to cooking particles and drafts. Because of the radioactive source, follow the manufacturer's instructions when disposing of old ionization detectors.
| Photoelectric | Ionization | |
|---|---|---|
| Sensing method | Light scattering in an optical chamber | Ionized air current between plates |
| Best response | Smoldering, smoky fires | Fast flaming fires |
| Nuisance risk | Lower from cooking particles; can react to steam and dust | Higher from cooking, drafts and high air velocity |
| Commercial use today | Standard choice for spot detectors and duct heads | Limited; mostly existing systems and special applications |
Types of commercial smoke detectors
Spot-type photoelectric detectors
The ceiling-mounted head you see in most buildings. They come in conventional versions for zone panels, such as the System Sensor i3 2W-B 2-wire (see our 2-wire vs 4-wire guide), and addressable versions that must match the panel brand. Many are available with a built-in fixed heat element, like the 2WT-B smoke and heat detector.
Multi-criteria detectors
A multi-criteria (multi-sensor) detector combines a photoelectric chamber with one or more other sensors, such as heat, carbon monoxide or infrared, and uses the combination to decide whether there is a fire. This improves response to different fire types and reduces nuisance alarms from things like steam or cooking. Examples include the Fire-Lite AD365 and the photo/heat/CO models in our multi-criteria detectors range. Smoke/CO combination detectors can also cover carbon monoxide detection where it is required; see carbon monoxide detectors.
Projected beam detectors
A beam detector sends an infrared beam across a space, either to a separate receiver or to a reflector that returns it, and alarms when smoke obscures the beam. One beam can cover a long, wide area, so beams are used in warehouses, atriums, gyms, arenas and other high-ceiling spaces where spot detectors are hard to reach. Example: the System Sensor OSI-R-SS reflected beam. Browse all beam detectors.
Aspirating (air sampling) smoke detectors
An aspirating system uses a fan to draw air through a network of small pipes with sampling holes back to a central detector. The detector can be far more sensitive than a spot head and can give several alert levels before a full alarm. It is used for very early warning in data centers, telecom rooms, clean rooms and cold storage, and where the protected area is hard to access for maintenance, since the detector itself sits in an accessible spot. Examples include the Notifier FSA-8000 FAAST XM and VESDA units in our aspirating smoke detectors collection.
Duct smoke detectors
Duct detectors sample air in HVAC ducts and shut down the air handler to keep smoke from spreading. They are required by airflow under the mechanical code and are not a substitute for area detection. See our guide to choosing a duct smoke detector and the duct detectors range.
UL 268 7th Edition
Smoke detectors used with commercial fire alarm panels are listed to UL 268. The 7th Edition added new fire tests using polyurethane foam, for smoldering and flaming fires, along with a cooking nuisance test, and the compliance date for newly manufactured detectors was June 30, 2024. That is why many manufacturers have released new heads or multi-criteria models in recent years. The standard governs products being made and listed; it does not by itself require replacing detectors already installed, but when you replace or add detectors, use the current listed model for that panel and confirm compatibility in the panel's documents.
How to choose a commercial smoke detector
- Match the panel. Addressable heads must use the panel's protocol. Conventional heads must be listed as compatible with the panel or zone module, including 2-wire versus 4-wire.
- Check the environment. Stay within the listed temperature, humidity and air velocity range. Kitchens, garages and dusty rooms usually call for heat or multi-criteria detection.
- Consider ceiling height and area. Large open spaces favor beam detectors; critical rooms favor aspirating systems.
- Plan the base. Standard, relay, isolator and sounder bases change what the detector does on alarm.
- Follow NFPA 72 spacing. Start from 30 ft nominal spacing on smooth flat ceilings and adjust for beams, slopes and airflow.
Shop all commercial smoke detectors by brand and type.
FAQ
Which is better for commercial buildings, photoelectric or ionization?
Photoelectric is the standard choice for most commercial spot detectors. It responds well to smoldering fires and gives fewer nuisance alarms. Multi-criteria detectors add heat or CO sensing to improve response to flaming fires.
Can I use a heat detector instead of a smoke detector?
Only where the code and the design allow heat detection for that space. Where smoke detection is required for life safety, a heat detector is not a substitute.
Where should smoke detectors not be installed?
Outside their listed environmental range: NFPA 72 sets default limits of 32°F to 100°F, 93% relative humidity and 300 ft/min air velocity unless the detector is listed for other conditions. Avoid kitchens, showers, dusty areas and spots next to supply diffusers.
What is a multi-criteria smoke detector?
A detector that combines a smoke sensor with heat, CO or infrared sensors and uses all of them to decide on an alarm, which reduces nuisance alarms and improves response to different fires.
Do I have to replace my detectors because of UL 268 7th Edition?
Not automatically. The standard applies to new detectors. Replacements you buy now will be 7th Edition models; confirm they are compatible with your panel.
How far apart should smoke detectors be?
NFPA 72 uses 30 ft nominal spacing on smooth flat ceilings, with adjustments for ceiling height, beams, slopes and air movement. Follow the detector's installation sheet and your AHJ.