A smoke alarm buys time. Everything about specifying them comes down to putting a detector where smoke will reach it early, choosing a type that will not cry wolf in that room, and making sure that one going off wakes everybody rather than only the person standing next to it.
Where each type goes
Optical detectors respond faster to the smouldering, smoky fires that start in furniture and bedding, which is why they suit circulation spaces and living areas. Ionisation detectors respond faster to fast flaming fires with small particles. Heat alarms do not respond to smoke at all, which is exactly why they go in kitchens and garages, where cooking and exhaust would set off a smoke alarm daily until somebody removed the battery.
Why the kitchen gets a heat alarm
Because a smoke alarm in a kitchen is a disabled smoke alarm within a month. The pattern is always the same: false alarms while cooking, the alarm is silenced, then the battery is taken out, then the house has no detection at all. A heat alarm in the kitchen and a smoke alarm in the hall, interlinked, is the arrangement that survives contact with people actually cooking.
What interlinking achieves
One alarm sounding means all of them sound. In a house that means the person asleep upstairs hears a fire that started downstairs, which is the whole point. Interlinking is done by wiring or by radio, and radio interlinked alarms are what make retrofitting practical in an occupied property. Scotland's domestic standard requires interlinked alarms throughout, which is why the question comes up more there.
Mains powered, battery and sealed units
Mains powered alarms with a battery backup are the specification for new work and for most rented property. Sealed long-life battery units removed the annual battery change and its main failure mode, which was the battery never being replaced. Whatever the type, alarms have a service life and are replaced rather than tested indefinitely, and the date is on the unit.
Testing them
Weekly with the test button, which checks the sounder and the electronics rather than the sensor. In rented property that is usually the tenant's routine and the landlord's duty to have provided and to check at the start of a tenancy. An alarm that has been painted over, covered or removed fails silently, which is why the visual check matters as much as the button.
Questions people ask about smoke alarm
Where should a heat alarm go instead of a smoke alarm?
In kitchens and garages, where cooking fumes or vehicle exhaust would cause repeated false alarms and lead to a smoke alarm being disabled.
What does interlinking smoke alarms do?
When one alarm detects, all of them sound, so people in other parts of the building are warned. It can be wired or radio linked.
What is the difference between optical and ionisation detectors?
Optical detectors respond faster to smouldering, smoky fires; ionisation detectors respond faster to fast flaming fires. Optical is the usual choice for circulation and living spaces.
How often should a smoke alarm be tested?
Weekly with the test button, with a visual check that it has not been covered, painted or removed. Alarms have a service life and are replaced at the end of it.