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.
A smoke heat detector in one head, and when it is the right choice
Combined heads carry both sensing elements and report either, which sounds like it removes the choice and mostly moves it. Where they earn their place is a space that is sometimes smoky for ordinary reasons and sometimes not, a utility room or an open plan kitchen and living area, because the head can be configured to weight the two elements differently rather than being a smoke alarm somebody eventually disables. Where they do not is a plain bedroom or a hallway, which has no reason to see smoke at all and is better served by the cheaper single sensing head.
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.
Is a wifi smoke detector better than a standard one?
It is a standard alarm with a notification added, and the question is whether you need the notification. The safety function is the same: it has to meet the same standard and be sited and interlinked the same way. What the connection adds is a message to a phone when nobody is home, which matters for an empty property, a holiday let or a relative living alone, and a log of tests and battery states. What it also adds is a dependency on the router and an account, so the alarm must still work and still interlink with the mains and the network down, and any unit that does not is not a fire alarm with an app, it is an app.
What is an industrial smoke detector, and why is it different?
It is a detector chosen for an environment that would defeat an ordinary one: dust, fumes, steam, vibration, extremes of temperature, or a space too large for a point device. In practice that means one of three things, and a specification should say which: a sealed or filtered optical unit in a rated enclosure, a beam detector across a large span, or aspirating pipework sampling the space. The reason it matters commercially is false alarms: an ordinary detector in a workshop gets dusty, alarms, and gets disabled, which is how a building ends up with no detection at all.
Does a smoke detector tester actually prove anything?
It proves the sensor and the path to the panel, which is exactly what the panel's own test button does not. The aerosol or the smoke capsule in a tester pole introduces the thing the detector is meant to see, so a device that fails a tester is either dirty, at the end of its life or wired wrong, and all three are findings. What it does not prove is coverage: a detector that operates correctly in the wrong place is still in the wrong place. So the tester belongs to the service regime and the siting belongs to the design.