This page is intended to help you replace hardwired 110-120-volt
electric smoke detectors. These residential smoke alarms are required
for fire protection by building codes in cities in the United States,
and are typically installed during the construction phase of a new
Different types of smoke detectors can be easily identified by removing the unit from the base plate or wall mounting bracket. Look for these indicators:
Replacing a 110-volt fire alarm isn’t difficult, and can be done in 30 minutes or less by following the instructions included with the new detector.
Changing out multiple electric smoke alarms could take a
few hours or more, depending on how many you have and how hard they are
I’m replacing an old smoke detector in my own house for this project. When the home was built, just one wired-in electric smoke alarm was installed; I added the low-voltage combination smoke and heat detector in the same location when I installed my monitored home security system.
If you have a similar situation, be sure to call the alarm monitoring station and put the system in “Test” mode while you’re working on the change-out. I did this just to be safe because, even though I wouldn’t be working on the monitored detector, any dust or vibration I might cause near the unit could possibly set it off.
Since the existing System Sensor low voltage unit is a photoelectric smoke alarm, I decided to use an ionization type smoke alarm for the new unit.
The new smoke detector being installed here is a First Alert smoke alarm, Model SA9120BPCN. It is a 110volt ionization type with battery backup, and is a good general purpose hardwired detector for most homes.
If you have gas appliances, or for some other reason want CO (carbon monoxide) protection, you might consider the First Alert Model SC9120B combination smoke and carbon monoxide alarm.
1) Turn off power at the circuit breaker panel
Most 110-volt hardwired smoke detectors have a light that stays lit constantly. Have a helpful assistant turn off breakers, one by one, and watch for the light to go out.
If you aren't sure which circuit breaker controls your smoke detectors, turn off the "Main" breaker. This will likely cause clocks in some appliances like microwaves and ovens to lose their time settings, so do this when you have time to reset them.
Also, plan on doing this project during daylight hours, since throwing the main breaker will kill all inside lighting.
2) Remove the old unit from the base plate
Electric smoke detectors can consist of a main unit with a removable cover, or a main unit and a detachable base plate.
For the first type, the cover simply twists off or can be popped off by pulling on it.
Electric smoke detectors with a detachable base plate
will come off when twisted, usually to the left or counterclockwise.
you can’t get the unit apart easily, check the plastic housing for an
indication of how to remove the unit. Some electric smoke detectors have
a tab that must be pressed before the housing can be released or
Once the unit is apart, look for two mounting screws holding the main unit or the baseplate to the wall or junction box. In most cases, these only need to be loosened; the housings and base plates usually have slotted holes that allow them to be removed while the screws are still attached to the junction box.
3) Compare the wiring connected to your old electric smoke detector to the smoke alarm wiring diagram supplied with the new one
Pull the house wiring out of the electrical junction box until there is enough slack to work with. There should be 2 or 3 wires of the old smoke detector that correspond to the wires on the new unit.
Electric smoke detectors usually have a black “hot” wire, a white “neutral” wire, and a third colored wire (usually red, orange, or yellow) for other interconnected smoke alarms. If your house has only one electric smoke detector, the third wire is not used.
If there is a bare copper “Ground” wire, it is not needed and it can be tucked out of the way.
4) Disconnect the house wires from the old smoke
There will probably be a plug-in connector tied to the house wiring. If this connector matches one of the adapters included with the new detector, connect it now.
If none of the adapters fits the existing connector, remove it by taking off the wire nuts; just twist them counter-clockwise to remove.
Thread the house wiring through the center opening of the new base plate, then mount it to the junction box. Fasten the base plate using the existing screws or the new hardware supplied.
Connect the “Hot”, “Neutral”, and, if used, "Interconnect" house wires to the matching wires on the new detector pigtail harness. Refer to the supplied smoke detector wiring diagram and, if needed, your labels and/or photo to match up the correct wires.
The First Alert smoke detector shown here came with 2 adapters that will fit many older Kidde, Fyrnetics, Lifesaver, and Firex detector harnesses. As mentioned above, if your new smoke has an adapter that happens to fit your existing harness, just snap the connectors together and skip dealing with the wire nuts.
If you need to use wire nuts, hold the wires to be joined so that their ends are even. Then, screw on the wire nut by twisting to the right or clockwise.
When wiring smoke alarm connections with stranded wire, twist the strands clockwise into a tight bundle to make installing the wire nut easier.
If you’re not using the third interconnect wire, make sure it is factory insulated. If it's not, wrap the
end with electrical tape or screw on a wire nut to insulate it.
If you are using the third wire, be aware that these are usually factory insulated. Use wire strippers to expose the bare copper wire on the new harness, then use a wire nut to connect it to the house third wire.
6) If the new unit only allows access to the backup battery from the back, install it now
7) Push the smoke detector wiring back into the junction box, and install the smoke alarm onto the base plate
8) Install the backup smoke detector battery, if you didn’t do so in Step 7
This detector has a convenient swing-out door on the battery compartment, making battery replacement easy. This might be a feature you want to look for when shopping for replacement smoke detectors.
9) If you are replacing multiple hardwired smoke alarms, proceed to the next detector and repeat the removal and installation steps
10) When all smoke detector installation is complete, turn power back on at the electrical breaker box
11) Test the new smoke detector(s) according to the smoke alarm manufacturer’s instructions
For most units, you’ll need to press and hold a smoke detector “Test” button until the detector (and other interconnected smoke detectors) sound an alarm.
If any unit doesn’t test
properly, turn off the power and check that the connections are all
wired correctly and are secure.
See First Alert SA9120BCN Hardwire Smoke Alarm at Amazon.com
See First Alert SC9120B Hardwire Combination Smoke/Carbon Monoxide Alarm at Amazon.com
More info on hardwired smoke detectors
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