Replacing a hard wired smoke detector in your house is similar to replacing a light fixture. If it is time to replace the detector (the general rule is if it is over 10 years old), it may be as easy as plugging it in, or you may have to do a little wiring.
Generally, if you buy the same brand of smoke detector, you can pull out the plug on the back of the old one and push it into the new one. During that time, if the plug style has changed or if you are buying a different brand, you will need to wire the plug into the house wiring.
Turn off the breaker that feeds the smoke detector, and replace the wires of the old plug with the wires of the new plug. There will be wire nuts that hold the wires together. Unscrew them one-by-one and twist the wire nuts on until snug. You may or may not have the interconnect wire attached. If the old smoke detector plug had the interconnect wire attached, then secure it as well, but if you see the interconnect wire (usually yellow in color) not connected then don’t connect it. The interconnect wire makes it so that if one smoke detector goes off, then they all go off. Some houses have the detectors installed connected and some do not.
A customer just called saying he wanted to replace his 17 year old smoke detectors. That's about 7 years late, as you should replace them when they reach 10 years old. He was going to try it himself, but he couldn't make the electrical connections work.
Replacing a smoke detector is a pretty basic job. If you don't have to replace the electrical plug, the job will take about 5 minutes. There may be different plug configurations if the old detector is very old, or if you are switching manufacturers. But even then, it's no different than a basic fixture or switch.
Turn the power off to the detector at the main panel and test to make sure the power is off. Remove the wire nuts where the plug connects to the house wiring in the ceiling. Connect the black ceiling wire to the black plug wire, connect the white ceiling wire to the white plug wire, and if you have a 3rd wire (an interconnect wire), connect it to the 3rd plug wire. Use wire nuts for tight connections. The interconnect wire is for systems that are interconnected. That means that if one smoke detector senses smoke, all of the alarms go off instead of just the one that senses the smoke.
Once the plug is wired, install a 9-volt battery in the alarm and plug the alarm into the ceiling plug. Then just mount the alarm to the base plate on the ceiling.
How many times have you been awakened by a chirping smoke detector? A few times I'll bet. So you replace the battery only to find that it still chirps? Try using a vacuum with a hose attachment and hold the hose up to the smoke detector. After you suck out a few years worth of dust, plug the unit back in to see if it works. If not, you may consider replacing it.
It's recommended that smoke detector batteries be changed when you set your clock for daylight savings time and again when you set it back to standard time. It's also recommended that the entire unit be replaced if it's more than 10 years old, because the circuits wear out and eventually stop working. Also, years of dust and clumsy painting can interfere with the sensor.
Smoke detectors are inexpensive. You can buy a direct-wired unit with a battery backup for about $20. And, if you have an area where you want to add one, install a battery-operated unit, which is about $15.
To replace your direct-wired smoke detector, first turn off the power at the main panel. The top of the old smoke detector will either pop off or will twist off with a slight turning motion. On the old units, you have to remove the body of the detector to access the wires that connect it to the electricity. The body is held in place by two screws. Remove these screws and the detector will dangle from the ceiling. Test the wires to make sure the power is off, then untwist the wire connections.
Screw the new smoke detector mounting plate to the electrical box with the two screws that you removed from the old unit.
Your new smoke detector will come with a connector plug that attaches to the electricity. After you make the electrical connections, simply plug this connector into the detector.
The electrical connections are the same as replacing a light fixture, except for what is called an interconnect wire. If you have multiple smoke detectors that are interconnected, when one goes off, they all go off.
You can tell if your detector is interconnected if it has a third colored wire. Connect the white neutral wire from the ceiling to the white wire on the connector plug. Connect the black hot wire from the ceiling to the black wire on the connector plug. If your old smoke detector had a third colored wire connected to it, then connect this wire to the yellow interconnect wire on the connector plug. New smoke detectors are wired for either single or interconnected use, so just connect the new detector with the same number of wires as the one you are replacing. Use wire nuts for tight connections.
Once the wiring is done, plug the connector into the new smoke detector and twist it onto the mounting plate.
This is really a simple job. However, it's a job that is vitally important. If this one smoke detector is old, the other ones in your home probably are old, too. Do yourself a favor and spend the money to change them all.