First Alert SC9120B Hardwired Smoke and CO Alarm 2 Pack
- 2 units, hardwired with battery backup
- Smoke and CO combination alarm
- Ionization smoke sensor
- Electrochemical CO sensor
- Interconnects with compatible BRK or First Alert hardwired units
- 1.3 pounds item weight
Last refreshed Sep 8, 2026 · Based on the Amazon listing and published specifications, not hands-on testing.
A two pack of hardwired combination smoke and carbon monoxide alarms with battery backup. They link only to compatible First Alert or BRK hardwired units.
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ASIN: B078HW9GY4Model: SC9120BCategory: Smoke Detectors & Fire Alarms
Overview
A bedroom hallway or living area wired into household power is where these First Alert alarms belong. Each unit pairs an ionization smoke sensor with an electrochemical carbon monoxide sensor, and a battery takes over if power fails.
The reason to buy is two combination alarms that tie into a wired, interconnected system in a single family home. The limit is the sensor type, which uses ionization smoke sensing, so some homes may prefer a photoelectric model.
Who it's for
A single family residence with a hardwired alarm setup is the situation, including linking to compatible units. Renters without a wired circuit should choose a battery alarm, because these need household wiring.
Pros and cons
- Smoke and CO in one alarm
- Battery backup if power fails
- Interconnects with compatible units
- Two alarms in the pack
- Needs existing hardwired wiring
- Ionization smoke sensing only
- Interconnect limited to compatible brands
Performance notes
The ionization sensor reacts to fast flaming fires, and the electrochemical sensor measures carbon monoxide gas in the air. Hardwiring supplies constant power, with the battery as a backup. Interconnecting lets all linked alarms sound together.
Where it stands among smoke detectors & fire alarms
- #1 most-searched brand and model of the 33 smoke detectors & fire alarms lines we track (by search demand, not sales).
From the specs of the 45 smoke detectors & fire alarms we list; not a test result.
Specifications
| Brand | First Alert |
|---|---|
| Model | SC9120B |
| Dimensions & fit | |
| Item weight | 1.3 pounds |
| Power & efficiency | |
| Power Source | Battery Powered, Corded Electric |
| Included components | |
| Unit Count | 2 Units |
| Other details | |
| Sensor Type | Electrochemical, Ionization |
| Product Style | Hardwired Alarm |
| Mfr Part Number | SC9120B |
Frequently asked questions
Is the First Alert SC9120B a battery only alarm?
No. It is hardwired with battery backup, so it connects to household wiring.
Can I link it to other brands?
The listing says it connects to other compatible BRK or First Alert hardwired units within a single family residence.
How many alarms come in the pack?
The listing says 2 units.