First Alert SC-9120B Hardwired Smoke and Carbon Monoxide Alarm with Battery Backup 2 Pack
- Smoke Detectors & Fire Alarm
- Battery, Corded electric
- Electrochemical, Ionization
- 1
- Better rated than 80%
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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How it ranks among the 10 smoke detectors & fire alarms we track
4.6★ across 574 Amazon ratings, Bayesian-smoothed toward the category average.
574 Amazon ratings, weighted on a log scale so huge counts don't drown everything.
2,000+ bought on Amazon last month.
Brand identity, imagery, description and spec coverage.
The ProductDome score is separate from the Amazon rating shown above: it combines the Bayesian-smoothed Amazon rating with evidence volume, tracked demand, and data quality across the 10 smoke detectors & fire alarms we track - how the scorecard works · full smoke detectors & fire alarms data report. Confidence: high - strong rating volume and complete data.Updated 7 days ago. Data-computed, not a hands-on test. Compiled by the ProductDome Research Team.
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.
Strengths and trade-offs
Drawn from this product's published specifications.
- 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.
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 |
| Number of Items | 1 |
| Mfr Part Number | SC9120B |
| Product Style | Hardwired Alarm |
Other First Alert smoke detectors & fire alarms
How it compares
This product against the closest alternatives we track in smoke detectors & fire alarms.
| Product | Amazon rating | Ratings | ProductDome score |
|---|---|---|---|
| First Alert SC-9120B Hardwired Smoke and Carbon Monoxide Alarm with Battery Backup 2 PackThis product | 4.6 / 5 | 574 | 4.5 / 5 |
| X-Sense Smoke Detector Carbon Monoxide Detector ComboX-Sense | 4.5 / 5 | 4,200 | 4.5 / 5 |
| Kidde Compact Smoke DetectorKidde | 4.5 / 5 | 940 | 4.3 / 5 |
| Kidde 21007584 I4618 Firex Hardwire Ionization Smoke Detector with Battery BackupwhiteKidde | 4.3 / 5 | 398 | 4.2 / 5 |
| Kidde Smoke DetectorKidde | 4.0 / 5 | 1,400 | 4.2 / 5 |
Similar smoke detectors & fire alarms from other brands
Questions about the First Alert SC9120B
Common buyer questions answered straight from the product spec data - no fluff, no "it depends".
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.