Smoke Alarm confusion

[Scoot]

The other night one of the smoke alarms on our upstairs level starting "chirping". I removed it from the ceiling/plug in and pulled the battery. All five alarms on this level started to chirp over the next few minutes. So, I pulled them all and removed the batteries.

I've now replaced the batteries in all five units. When I test them (batteries in, not plugged in to the ceiling), I find that one of the five sounds the alarm at the decible (spelling?) level it should-- nice and loud, roughly the equivalent to a person shouting at the top of their lungs. The other four aren't nearly as loud as they are supposed to be- more like a person talking with a normal voice kinda level. I'm totally confused!

Are the four that aren't too loud the "good" ones, because they aren't supposed to be too loud when they're only battery run? Are they all shot for some reason? Is the loud one the only one that's still good? What the heck should I do to make sure I've got good, functional units that'll work and not keep me up all night while they "chirp"?

Any help appreciated.

[JohnMickish]

Plug them all in to their hardwired harness and put the new batteries in. Push the test button and all five should be alarming at the same time at the same level. If you still have 4 that are quiet, replace all 5. You can probably get away with just replacing the 4 but if you do that make sure they are compatable with the old one. 5 is just cheap insurance. Cudos for testing them.

[CAMAN]

Many new smoke alarms have a hearing safe test alarm that sounds the alarm at a lower level when pushing the button to save the eardrums. So you could have that one that is not the same as the others and it doesn't have that function.

If you suspect the detectors are acting goofy and you have everything installed correctly with them it's cheap insurance just to replace them all.

It is recommended to replace all smoke detectors and CO2 detectors every 10 years. So check the date of manufacture on the back of each alarm and if it is past 10 years replace them all. If there is no date of manufacture stamped on it, and you have no idea the age of the units replace them and write down the installation date on the back of each of the new ones.