Types of Fire Detector Explained: Optical, Heat, Multi-Sensor, CO and Aspirating
By the DC Fire & Security engineering team — installing and maintaining fire and security systems since 2010. Updated June 2026.
Quick answer
Optical smoke detectors sense visible smoke and suit escape routes and bedrooms; heat detectors trigger on temperature and belong in kitchens and dusty areas where smoke sensing false-alarms; multi-sensors combine both and cut false alarms in mixed-use spaces; CO fire detectors sense smouldering fires' carbon monoxide; aspirating systems sample air continuously for the earliest possible warning in critical spaces.
The main detector types and where each belongs
| Type | Detects | Best locations | Avoid |
|---|---|---|---|
| Optical (photoelectric) | Visible smoke particles | Escape routes, bedrooms, offices, lounges | Kitchens, bathrooms, dusty workshops |
| Heat (fixed temp / rate-of-rise) | Temperature threshold or rapid rise | Kitchens, boiler rooms, garages, dusty areas | As the only protection on escape routes — slower response |
| Multi-sensor | Smoke + heat (+CO on some) | Mixed-use rooms, hotels, HMOs, false-alarm-prone areas | Cost-sensitive simple spaces where optical suffices |
| CO fire detector | Carbon monoxide from smouldering fires | Sleeping risks, deep-seated fire risks | Fast-flaming fire risks — pairs with, not replaces, smoke |
| Aspirating (ASD) | Continuous air sampling, very early smoke | Server rooms, heritage interiors, cold stores, atria | Routine spaces — cost rarely justified |
Why detector choice drives false alarm rates
Most false alarms are the right detector in the wrong place: an optical detector near a kitchen door reads toast as fire; a smoke detector in a dusty loft cries wolf until it's ignored. BS 5839 expects the designer to match sensing technology to each room's normal environment — that's the design skill. On addressable systems, multi-sensors can also be programmed by time of day (heat-biased during service hours in a commercial kitchen area, full sensitivity overnight).
What about detectors in dwellings?
Domestic and HMO systems under BS 5839-6 use the same physics in Grade D1 form: mains-powered interlinked optical alarms in hallways and living rooms, heat alarms in kitchens, all sounding together. Carbon monoxide alarms — required wherever there's a fixed combustion appliance — are a separate life-safety device for poisoning, even though CO sensing also exists inside some fire detectors. The two jobs shouldn't be confused when planning coverage.
Frequently Asked Questions
Are ionisation smoke detectors still used?
What detector should be in a commercial kitchen?
How far apart should detectors be?
Can one detector type cover a whole small business?
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