The 10-year rule
Most manufacturers recommend replacing smoke detectors every 10 years. The sensors wear out, and an older unit may not respond reliably even when the test button still makes it beep. The manufacture date is printed on the back.
Where they belong
Texas code calls for a smoke detector inside every bedroom, outside each sleeping area, and on every level of the home. Homes with gas appliances or an attached garage also need carbon monoxide detectors outside the sleeping areas.
Hardwired and interconnected
Hardwired detectors run on your home's power, with a battery backup. When they're interconnected, one alarm sets off all of them, so a fire in the garage wakes the upstairs bedrooms.
Combination units
Combination smoke and CO detectors cover both hazards with one device per location, which keeps ceilings cleaner and replacement simpler.
Test them monthly
Press the test button once a month. If an alarm chirps, replace the backup battery, and if it keeps chirping, replace the detector.