Replace a hardwired smoke detector

How a hardwired smoke alarm is connected

A hardwired smoke alarm runs on household power from a 120-volt circuit, with a battery as backup during outages. In the ceiling or wall there's an electrical box with three wires: black (hot), white (neutral) and, in most homes with more than one alarm, a red or other colored interconnect wire that links the alarms. When one alarm senses smoke, it signals the others through that wire so they all sound. The house wires are joined to a short plug-in harness, and the alarm plugs into that harness, then twists onto a mounting plate screwed to the box.

Harness connectors are specific to each brand and sometimes to a product line, so a new alarm of the same brand often plugs straight into the old harness, while another brand needs its own harness wired in. Alarms of different brands generally can't signal each other over the interconnect wire, which is why replacing them all with one brand matters. Smoke sensors lose sensitivity with age, so the National Fire Protection Association (NFPA) says to replace smoke alarms 10 years from the manufacture date, and many alarms start a distinct end-of-life chirp around then.

When to replace a hardwired alarm

Let's fix it

1. Check the date and buy compatible alarmsTwist the alarm counterclockwise off its base and look at the label on the back for the manufacture date. If it's 10 years old or older, or the label is missing, replace it. Note the brand and model, and look at the plug where the harness connects. For the easiest swap, buy hardwired alarms from the same brand with a compatible connector; the package or the maker's website lists which old models it matches. If you're switching brands, make sure every new alarm comes with its own harness. Consider smoke and carbon monoxide combination alarms, especially near bedrooms. Buy enough to replace all the linked alarms.
   Done

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2. Turn off the powerFind the breaker for the smoke alarm circuit in your electrical panel and switch it off. It's often labeled smoke detectors, hall, or bedrooms, or shared with a lighting circuit. Then check the alarm: its green power light should be off. The alarm may still chirp from its backup battery, which is normal. If you can't tell which breaker it is, turn off the main breaker, but then everything in the house is off. You only need to flip a breaker, not open the panel. Bring a flashlight and a stable step ladder.
   Done

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3. Remove the old alarm and harnessTwist the old alarm counterclockwise to release it from the mounting plate, and let it hang by the harness. Squeeze the tab on the connector and pull it straight out of the back of the alarm. Before touching any wires, hold a non-contact voltage tester to the wires in the box to confirm they're dead; a dark green light alone doesn't prove it. Look at the harness wires in the box: there are usually black, white and red wires joined to the house wires with wire nuts. If your new alarm uses the same connector, leave the harness in place and skip to Step 4. If not, unscrew the wire nuts, separate the harness from the house wires, and remove the old mounting plate.
   Done

   Something went wrong?

   • The alarm won't twist off Look for a small locking tab or pin on the edge, and press it in or remove it as the label shows. Paint can also glue it on; score around the edge carefully with a utility knife.

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4. Plug in a same-brand alarmIf the new alarm's connector fits the existing harness, this is the simple path. Check that the harness wire nuts are tight and no copper shows below them. If the new alarm came with its own mounting plate and the old one doesn't fit, swap the plate: unscrew the old one and screw the new plate to the box, feeding the harness through. Plug the harness firmly into the back of the new alarm until it clicks. Pull the backup battery tab or install the battery if the alarm uses one. Then go to Step 6.
   Done

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5. Wire in a new harnessFor a different brand, screw the new alarm's mounting plate to the box, feeding the house wires through. Connect the new harness to the house wires with the wire nuts provided: black to black (hot), white to white (neutral), and the harness's interconnect wire, usually red or orange, to the house's red interconnect wire. Twist each wire nut on clockwise until snug, and tug each wire to make sure it holds. If there's no red house wire, cap the harness interconnect wire alone with a wire nut. Fold the wires neatly into the box and plug the harness into the alarm.
   Done

   Something went wrong?

   • House wire colors don't match Stop if you see more wires than black, white, red and bare ground, or the colors are unclear. Have an electrician confirm the wiring before connecting.
   • A wire pulls out of the wire nut Remove the nut, line the wire ends up evenly, and twist it back on until no bare copper shows below it.

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6. Mount it and test every alarmLine the alarm up with the mounting plate and twist it clockwise until it locks. Turn the breaker back on. The green power light should come on. Press and hold the test button until the alarm sounds; all interconnected alarms in the house should sound too, which confirms the red wire is connected. Some models take a short time to start up after power returns, so check the manual. Write the installation date on the alarm with a marker. Test every alarm monthly, and replace the batteries as the manual says if they aren't sealed.
   Done

   Something went wrong?

   • Only this alarm sounds The interconnect wire may not be connected, or the brands are different. Turn the breaker off and check the red wire nut, and make sure all alarms are compatible.

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Watch it done

How to replace old hard wired smoke detectors - easy DIYBy Platoon Garage on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it comes back

If a new alarm chirps, it's usually the backup battery, a battery tab that wasn't pulled, or an alarm that wasn't fully twisted onto its plate. See the guide for a chirping smoke alarm.

If it goes off while cooking or showering, it may be too close to the kitchen or bathroom. See the guide for a smoke alarm going off for no reason for placement tips.

If your home has fuel-burning appliances or an attached garage, make sure you also have working carbon monoxide alarms. See the guide for a beeping carbon monoxide alarm.