We built our house about 5 years ago, and as per the then building code we had smoke detectors put in every bedroom and in the hallways. We bought fairly run-of-the-mill detectors and I have been diligent in changing the batteries.
It’s time again to swap out the batteries and I just noticed that they are wired into the house’s AC power. I guess the idea is that they can rely on AC power when it’s available, but if the AC goes down, which happens now and then, the batteries will be available.
If the batteries get low the units start to beep loudly so I know to immediately change the battery.
So what’s the point of having them connected to AC? It’s not like there’s a trickle charge keeping the batteries fresh, and they make such a racket when the batteries get low that you would be hard pressed to ignore it.
So what value does having them tied into the house power add? There must be a good reason.
I am not an expert, but I was told by our contractor that having all of the detectors hard wired lets them operate as a system. If one goes off, they all go off. Independent battery powered only detectors may go off way over on the other end of the house, a 2nd story or basement, and the others won’t deploy until the conditions tell them to. If you’re asleep in the upstairs bedroom the fire may be well along before the upstairs alarm goes off.
AC powered detectors in modern times are also networked, all detectors will alarm when any detects an alarm situation. by tying to the house power allows them to network.
a battery only non-networked detector will last a year on a battery.
an AC powered battery backup networked detector will last a month on a battery.
also detectors have about a 7 year lifetime, especially the carbon monoxide (CO) detectors. you should replace dual (smoke/CO) detectors with the same. modern networked detectors can also be speaking along with alarm, that is very useful in helping you realize the hazard (or the low battery) being alarmed.
I am the service and repair manager for a fire alarm company. Our friends Beauregard and johnpost have it right: the AC detectors, by code, are networked together and the signal will sound from all of them.
This is similar to AC-powered clocks with a 9v battery backup. If the power never goes off, the battery is never used, and its life is equal to shelf life, probably longer than the detector.