An alarm that sounds only where the smoke is will not wake somebody two floors up with the doors closed, which is the exact situation domestic fires kill people in. Interlinking is what turns a set of separate alarms into a system: one detects, all sound. It is the single most valuable upgrade in most homes and it is cheaper than people expect.
What having fire alarms interlinked does
When any alarm in the set detects smoke or heat, every alarm in the property sounds. The person furthest from the fire, behind the most doors, gets the same warning as the person standing next to it. Since closed doors are what buy escape time, and closed doors are also what stop sound, interlinking is not a refinement: it is what makes the rest of the protection work.
Radio against cable: wired interlinked smoke alarms
Cable linked alarms are wired together during installation or a rewire and are the natural choice when the wiring is accessible. Radio linked alarms pair to each other and need no cable between them, which makes retrofitting an occupied home straightforward. Both are recognised methods. Radio units still need a mains supply where the specification calls for mains powered alarms, because the link and the power are separate questions.
The Scottish standard
Scotland moved to a single standard for every home, owned and rented alike, requiring interlinked alarms: smoke alarms in the main living room and circulation spaces on each storey, a heat alarm in the kitchen, all interlinked, plus carbon monoxide alarms where there is a fuel burning appliance. If you are reading advice written for England and your property is in Scotland, this is the difference that matters.
Retrofitting a set
Mixing brands generally does not work, because the radio protocols are proprietary. Plan the whole set as one purchase, count the storeys and rooms first, and check whether the property needs mains powered units. Replacing three alarms with three interlinked ones is a morning's work and it changes the protection more than adding a fourth unlinked one ever would.
Radio interlinked smoke alarms, and what to check before buying
Radio interlinking solves the retrofit problem: alarms talk to each other over a wireless link so that one detecting smoke sounds them all, without lifting floors to run an interconnect cable. Four things to check before ordering. Compatibility, because the link is proprietary: alarms interlink with their own family and usually their own brand, so mixing a heat alarm from one maker with smoke alarms from another does not work however similar the packaging. The standard mark on each unit, smoke alarms to BS EN 14604 and heat alarms to the heat alarm standard. The power arrangement, either mains with a backup or a sealed ten year cell, and not a replaceable battery if you can avoid it. And range in your building, since thick stone walls and foil backed insulation cut a radio link more than an open plan flat does, which is why the pairing should be tested from the furthest head before the fixings go in.
Questions people ask about linked fire alarms
Do smoke alarms have to be interlinked?
In Scotland, yes, for every home under the standard that applies to owned and rented property alike. In England the statutory minimum for rented homes does not require it, but the design standard and many licence conditions do.
Can I mix brands of interlinked alarms?
Usually not. Radio interlink protocols are proprietary, so a set needs to be from one compatible range. Plan the whole set together rather than adding to it later.
Is radio interlinking as good as wired?
Both are recognised. Radio suits retrofit in an occupied home; wired suits a rewire or new build. The link method is separate from whether the alarms must be mains powered.
Do linked smoke alarms have to be the same make?
Within one system, effectively yes for radio interlinking, because the radio protocols are proprietary and alarms from different makers will not talk to each other. Wired interlinking is more forgiving but still wants a consistent specification.
Are radio linked smoke alarms as reliable as wired ones?
In a domestic installation, yes, and they solve the problem that matters more: getting alarms interlinked at all in a house where cabling between floors would mean lifting carpets and chasing walls. The devices talk to each other on a radio link so one sounding sets off all of them, which is the whole point of interlinking, and the standards accept them. The trade offs are battery management, the pairing being a job somebody has to do properly, and a practical limit on how many devices sit on one link. In a rented property they are usually the proportionate answer; in new build, mains powered with a battery backup and hard wired interlink is still the norm.