Short answer
Interconnected smoke detectors are linked so that when one senses smoke, every alarm in the home sounds. The link is a dedicated wire in hardwired systems or a radio signal in wireless ones. Building codes generally require interconnection in new homes, because a fire far from the bedrooms may otherwise go unheard.
In this article
How do interconnected smoke detectors work?
Interconnected smoke detectors are linked so that when one senses smoke, it sends a signal that makes every other alarm in the home sound as well. In hardwired systems the signal travels over a dedicated interconnect wire in the same cable that carries power. In wireless systems it travels by radio between alarms paired to each other.
The alarm that detected smoke usually behaves differently from the rest: many models flash an indicator light on the originating unit so that someone checking the house can tell where the alarm started. The others simply sound. When the smoke clears from the originating alarm, the signal stops and the others go quiet.
Interconnection does not change how each alarm senses smoke. Each unit still has its own sensor, its own horn, and its own power. What changes is that the warning no longer depends on a person being close to the alarm that detects the fire.
Hardwired vs. battery smoke detectors: what is the difference?
Hardwired smoke detectors run on household power with a backup battery for outages, and are usually interconnected by wire. Battery smoke detectors run on a replaceable or sealed long-life battery and need no wiring. Battery models can still be interconnected wirelessly, so the practical choice is between running cable and relying on batteries and radio links.
Each approach has a cost. Hardwired alarms need a circuit and cable to every location, which is simple in new construction and disruptive in finished walls. They still depend on their backup batteries during an outage, and those batteries need attention. Battery-only alarms install in minutes anywhere, but every unit is only as reliable as its battery, and wireless interconnection adds radio range and pairing to the things that can go wrong.
Many replacement units are designed to fit an existing hardwired base and wiring harness from the same manufacturer, which keeps the wiring in place while the alarms themselves are swapped.
Why do building codes require interconnection?
A fire that starts in a basement or a far bedroom can grow for minutes before its smoke reaches the alarms where people are sleeping. Interconnection helps shorten that delay by sounding every connected alarm at once. Building codes generally require hardwired, interconnected smoke alarms with battery backup in new construction and in many substantial renovations.
In existing homes, codes generally allow wireless interconnection where running new wire would mean opening finished walls and ceilings. Whether a given renovation triggers the requirement, and whether wireless alarms are accepted in place of wired ones, is decided by the local building or fire official. That decision is worth confirming before work starts rather than at the final inspection.
Interconnection does not change how many alarms are required or where they go. The baseline of one per bedroom, one outside each sleeping area, and one per level is explained in our guide to how many smoke detectors a house needs.
Can different smoke detector brands be interconnected?
Generally not. Interconnect signals are not standardized between manufacturers, so alarms are designed to interconnect only with compatible models from the same manufacturer. Mixing brands on one interconnect wire, or pairing wireless alarms from different lines, can leave some alarms silent when another detects smoke. Check the manufacturer's compatibility list before replacing a single unit.
This matters most when one alarm fails years after installation. The easiest replacement to find may fit the ceiling base and look right while no longer sharing a signal with the others. Replacing all the interconnected alarms in a home at the same time avoids that mismatch, and since alarms installed together reach their replacement date together, it is usually the sensible point to do it.
What does interconnection mean for a monitored system?
Interconnected alarms only make noise inside the home; they do not notify anyone outside it. A monitored system sends the signal from a detector to a control panel and then to a central station. A home can have both, but interconnection alone is not monitoring, and an empty house with interconnected alarms is still an unmonitored house.
In practice, the code-required interconnected alarms protect the people asleep inside, and monitored detectors cover the hours when nobody is home. Some panels can also listen for a standalone alarm and report it, which ties the two layers together without rewiring. How that reporting is set up, and what the station does with it, is described on our fire alarm monitoring page. For the wider picture of how smoke, heat, and carbon monoxide devices work together, see our overview of smoke and carbon monoxide detectors.