Every smoke detector is a compromise between responding early and responding to the wrong thing. The types differ in where they sit on that line, and choosing between them is a question about the room rather than about the technology. A detector that suits a hotel corridor is a poor choice for a loading bay.
Optical and ionisation
Optical detectors sense light scattered by smoke particles, and respond well to the larger particles of a smouldering fire, which is the fire that fills a house with smoke while people sleep. Ionisation detectors respond faster to the small particles of a fast flaming fire. Optical is the general purpose choice today, and ionisation has become less common in new domestic installations.
Multi-sensor heads
A multi-sensor combines optical smoke detection with heat sensing, and sometimes carbon monoxide sensing, in one head, using the combination to distinguish a real fire from steam, dust or cooking. They cost more and they are the sensible default in spaces with a history of unwanted alarms. They are not a licence to site a detector badly: a multi-sensor over a kettle will still be triggered.
Beam and aspirating
Optical beam detectors project a beam across a large open volume and detect the obscuration smoke causes, which suits atria, warehouses and sports halls where ceiling mounted point detectors would be unreachable and slow. Aspirating systems draw air through a pipe network to a very sensitive central sensor, and are used where the earliest possible detection matters or where the ceiling is genuinely inaccessible.
Choosing per room, not per building
The mark of a good design is a device schedule that names the type for each space and the reason. Corridors and offices take optical. Kitchens and plant rooms take heat. Areas with steam, dust or vehicle movement take multi-sensor. Tall open volumes take beam. A schedule with one type throughout is either a very simple building or a design nobody did.
Questions people ask about smoke detector types
Which smoke detector type is best?
There is no single best type. Optical suits most general spaces, heat suits kitchens and plant rooms, multi-sensor suits areas prone to false alarms, and beam or aspirating suit large open volumes.
Are multi-sensor detectors worth the extra cost?
In spaces with a record of unwanted alarms, usually yes, because the alternative cost is repeated callouts and eventually somebody disabling the device. In a quiet office they add little.
Should I still use ionisation detectors?
They respond well to fast flaming fires, but optical has become the general purpose choice and ionisation is less common in new domestic work. Follow the device schedule your designer produced rather than swapping types on site.