Smoke and CO alarm

Two sensors, two jobs

A smoke sensor detects particles in the air. A carbon monoxide sensor detects a gas that is colourless, odourless, and produced by anything burning fuel incompletely — a furnace, a water heater, a car left running in an attached garage.

CO gives no warning a person can perceive. That is the entire reason the detector exists: unlike smoke, there is nothing to see or smell before it affects you.

Interconnection

Interconnected alarms all sound when any one of them detects something. The difference this makes is not marginal: a fire starting in a basement at three in the morning sets off the alarm outside the bedrooms upstairs, which is the one that will actually wake you.

On a hardwired system this is a third conductor running between the alarms. Wireless interconnected units achieve the same thing over radio.

They expire

Both sensor types degrade, and the units carry a manufacture date and an end-of-life date for that reason. An expired alarm still powers up, still tests, and still fails to detect reliably.

This is also the answer to the chirp that will not stop after a battery change: many units chirp a distinct pattern at end of life, and no battery will silence it because the battery is not the problem.

Hardwired with battery backup

Mains power runs the unit and the interconnection; the battery covers a power cut. Both matter. An alarm that only had mains power would be useless in exactly the scenario where a fault has killed the circuit.