Detectors differ in what they sense, and the difference decides where each belongs. A fire that smoulders for an hour produces large smoke particles and little heat. A fire in a pan of oil produces heat and small particles almost immediately. No single sensing technology is first to both, which is why a building holds more than one type.
Optical, also called photoelectric
A light source and a sensor at an angle to each other. Large smoke particles scatter light onto the sensor and trigger the alarm. Optical detectors respond fastest to smouldering fires, which are the ones that kill people in their sleep, and they are the general purpose choice for circulation spaces and bedrooms. They are more prone to nuisance alarms from steam and dust than from cooking fumes.
Ionisation
A small radioactive source ionises air in a chamber and smoke particles disrupt the current. Ionisation detectors respond fastest to fast flaming fires with small particles. They are more prone to nuisance alarms from cooking, which is why they have largely been displaced by optical devices in domestic circulation spaces, and they carry disposal considerations because of the source.
Heat
A thermistor or similar element responds either to a fixed temperature or to a rate of rise. Heat detectors ignore smoke entirely, which makes them the right choice in kitchens, garages and dusty workshops where any smoke detector will be triggered by normal activity. They are slower than smoke detection by design, so they protect a space rather than provide early warning.
Multi sensor and where to put what
Multi sensor detectors combine optical and heat sensing and weigh the two signals, which reduces nuisance alarms while keeping early response. As a rule: optical in circulation spaces and bedrooms, heat in kitchens and garages, multi sensor where nuisance alarms have been a problem, and specialist detection where the risk is unusual.
Questions people ask about smoke detector alarm
Which type should go in a hallway?
Optical, because the fire most likely to threaten sleeping occupants is a smouldering one and optical finds those first.
Why not put a smoke alarm in the kitchen?
Cooking produces the particles a smoke detector is looking for. A heat alarm in the kitchen and an optical alarm in the adjoining circulation space is the arrangement that works.
Are ionisation alarms being phased out?
They remain available and are less used in dwellings than they were, because optical devices handle the dominant domestic risk better and generate fewer nuisance alarms.