How to Replace a Hardwired Smoke Detector Like a DIY Pro

Most modern smoke detectors are hardwired to your home’s electrical system with a backup battery. Less commonly, alarms are powered only by battery. This step-by-step guide focuses on how to replace a hardwired smoke detector.

Before you buy replacement hardwired smoke detectors, there are a few things to do:

• Test existing devices: Even if your alarm isn’t due for immediate replacement, test detectors regularly. And if the alarm is making a warning chirp, this is another indicator it needs replacing—or at least the backup batteries do.

• Pick an appropriate new model: To avoid having to replace the mount or doing complicated rewiring, purchase a new smoke detector compatible with the old version. Alternatively, buy a wire adapter to apply while fitting. 

• Check manufacturer’s instructions: Read the instructions that come with the model you have to ensure there isn't any bespoke installation guidance.

Always turn off the power on the linked circuit breaker before replacing a smoke detector. It might be marked “fire alarm” or could be on the same circuit as the room lighting.

Most of the time, you can simply twist the old alarm out of the mounting bracket. It’s ready to pull free when you hear a click. If the old mounting bracket isn’t compatible with the new detector, you might need to loosen the screws with your screwdriver and remove the bracket to replace it with a new one.

If you don’t have a compatible wiring harness or adapter, disconnect the old detector wiring first. There will be two or three wires. A 120-volt black wire, a neutral white wire, and there might be a red or yellow wire that is used if you have a multi-alarm system that is interconnected.

To release the wires, squeeze the side of the harness plug. You may need to remove electrical tape or wire caps before untwisting and removing the harness to expose the metal ends of the wires.

If you have a new detector that’s compatible with your wiring, connect the new alarm to the wiring harness. Don’t forget to add or activate the backup battery if the detector has this feature. If the wiring harness isn’t a match, you might be able to plug it into an adapter to connect to your new smoke detector and save you from fiddling around with the wiring.

If you have some experience with electrical work, you can carefully match up and twist the wires of the new alarm together with the exposed ones of the same color before replacing the wiring harness and adding new electrical tape or putting the wire caps back on.

Pair the black-to-black wire, white-to-white, and red-to-red or yellow-to-yellow. If you have a yellow wire on the new alarm, but the old wiring is red, twist these two together. You might need to use a wire stripper and wire nuts to expose enough wire and secure the connection.

Note: Do not play with the wires if you’re not confident in what you’re doing. Electrical work can be dangerous, so call in a local electrician to help you if you need it.

You can now click the smoke detector back into the mounting bracket. Alternatively, if the bracket isn’t compatible with the new alarm, you might need to screw in the new one.

Once the wiring is securely connected, turn the power back on at the circuit breaker. Your new alarm might beep, and an LED light could indicate everything is good to go. Warn everyone at home before pressing the test button on the new smoke detector to check it's functioning properly.

If the alarm doesn’t work, check if you flipped the right breaker switch. If it still doesn’t work, you can turn the power off again and check all the wire connections. Still not working? It might be worth asking for a replacement from the retailer before calling the pros.