First Alert SC9120B Hardwired Smoke and CO Alarm 2 Pack
- Power Source: Battery, Corded Electric
- Sensor Type: Electrochemical, Ionization
- Product Style: Hardwired Alarm
- Item Weight: 1.3 pounds
Last refreshed Sep 8, 2026 · Based on the Amazon listing and published specifications, not hands-on testing.
A solid choice for hardwired, interconnected smoke and CO coverage across several rooms, sold in a practical 2 pack for that job.
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ASIN: B078HW9GY4Model: SC9120BCategory: Smoke Detectors & Fire Alarms
Overview
The First Alert SC-9120B is a hardwired smoke and carbon monoxide alarm sold as a 2 pack, wired into household power with a battery backup so it keeps monitoring through an outage. It uses ionization smoke sensing paired with electrochemical CO sensing in one unit.
Because it runs on house wiring rather than a coin cell, this alarm is built for whole-home interconnection: wire several together (BRK or First Alert compatible units) and any one alarm triggering sounds all of them. That makes it a better fit for a full electrical retrofit or new construction than for a quick drop-in swap.
- Hardwired with battery backup
- Combination ionization smoke and electrochemical CO sensing
- Interconnects with compatible alarms
- 2 pack
Who it's for
Suits a homeowner or electrician wiring multiple alarms into one interconnected circuit across a house. Not the right pick for someone who just wants a battery unit to stick on a ceiling with no wiring involved.
Pros and cons
- Hardwired with battery backup for outages
- Interconnects with compatible BRK or First Alert units
- Separate lights show smoke vs CO trigger
- Sold as a 2 pack for multi room coverage
- Requires household wiring, not a plug and play battery unit
- Interconnection limited to compatible BRK or First Alert models
Performance notes
The unit combines an ionization smoke sensor with an electrochemical carbon monoxide sensor, and separate lights indicate which hazard triggered the alarm. A single button handles both testing and silencing, and the battery compartment loads from the side for easier access once the unit is mounted.
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, Corded Electric |
| Construction & materials | |
| Color | 2 Units (Sc-9120b) |
| Other details | |
| Sensor Type | Electrochemical, Ionization |
| Product Style | Hardwired Alarm |
| Number Of Items | 1 |
| Mfr Part Number | SC9120B |
Closest rivals on specs
X-Sense SC07 Smoke and Carbon Monoxide Detector Combo- Item Weight 1.3 vs 1 pounds (+30%)
Kidde 20SDR Battery Smoke Detector- Item Weight 1.3 vs 0.84 pounds (+55%)
Kidde 20SD10 10-Year Battery Smoke Detector- Item Weight 1.3 vs 0.6 pounds (+117%)
Frequently asked questions
Is the First Alert SC-9120B hardwired or battery powered?
It is hardwired to household electrical power with a battery backup that keeps it running during a power outage.
Can the SC-9120B connect to other alarms in the house?
Yes, it can interconnect with other compatible BRK or First Alert hardwired CO smoke detectors in a single family residence.
Does the SC-9120B tell you whether it detected smoke or carbon monoxide?
Yes, indicator lights show which hazard triggered the alarm, and one button handles both test and silence functions.