Key takeaways
- The overnight timing is physics, not bad luck. A weak battery's internal resistance rises as the house cools overnight, which is what pushes it over the threshold into low-battery chirping.
- Work in order: battery, then dust, then age. Most chirps stop at step one, and the ones that do not are usually the third.
- Smoke alarms have a service life, commonly ten years from the date of manufacture printed on the back. An alarm past that chirps for reasons a new battery cannot fix.
- On interconnected hardwired alarms, one failing unit can make the whole set sound. Finding the one is the job, and the manufacture dates usually point at it.
It is always the middle of the night. That is a real pattern with a real cause, and the cause tells you what to do.
The 3am thing is physics
A battery near the end of its life develops internal resistance, which limits how much current it can deliver. That resistance is temperature-dependent and it rises as things get colder.
A house is at its coolest in the hours before dawn. A battery that was marginal at 6pm and holding is not marginal at 3am, and the alarm's low-battery detection crosses its threshold and starts chirping. As the house warms the following morning the resistance falls again and the chirping can stop, which is exactly the behaviour that persuades people it fixed itself.
It did not fix itself. It went back to being marginal, and it will be back tomorrow night.
The order to work through
There are three causes and they are worth trying in this sequence, because most chirps end at the first one and the ones that do not are usually the third.
- Battery. Replace it, including on hardwired units, where the battery is the backup and still chirps when it is low. Use the chemistry the manufacturer specifies rather than whatever is in the drawer.
- Reset. After changing the battery, most units want the test button held for around fifteen seconds with the battery out, to clear residual charge that keeps the low-battery state latched.
- Dust. Vacuum the unit, gently, around the vents and the chamber. Dust interferes with the sensor and with battery contacts, and San Antonio houses run their air handlers hard for most of the year.
- Age. Read the date of manufacture on the back. Past roughly ten years from that date, the alarm is at end of life and the chirp is a designed behaviour telling you to replace it.
The date on the back is the one that counts
Every alarm carries a manufacture date, and that date rather than your install date starts the ten-year clock. A unit can sit in a warehouse and on a shelf for a long time before anyone screws it to a ceiling.
This is also the diagnostic shortcut in a house where alarms are going off together. Pull them, read the dates, and the oldest one is where to start.
When it is all of them at once
Hardwired alarms in a modern house are interconnected on purpose. An alarm triggered in the garage sounds the one in the back bedroom, because the point is waking a person who is not near the fire.
The cost of that design is that a single failing unit can set off the entire set, and the noise gives you no information about which one started it. Some units have an indicator light identifying the originating alarm. Where they do not, dates and elimination are the method.
When it stops being a battery
A hardwired alarm that chirps with a fresh battery and a clean chamber and a recent manufacture date is telling you about its supply rather than about itself. Intermittent power on the alarm circuit, a loose connection in a daisy chain, or a failing interconnect wire all produce the same symptom, and that is electrical work rather than a battery. So is a set of alarms that is out of date across the house, which is a replacement job rather than four separate errands. Either way that is smoke and CO detector work.
If you are replacing them anyway, that is the right moment to check where they actually are against where the code puts them, because houses frequently have the right number of alarms in the wrong places. That is its own subject: where smoke alarms are required.