First Alert SC9120B Combination Smoke and Carbon Monoxide Alarm - Easy

In this review of the First Alert SC9120B, the bottom line is simple: this unit is for homeowners who need a straightforward replacement for an existing hardwired alarm and want long-term peace of mind. The biggest reason to buy is the combination of easy swap-in installation using adapter plugs and a 10-year battery backup that guarantees continuous protection even during power loss. The reviewer found it particularly useful when upgrading older First Alert hardwired systems without rewiring, while still offering reliable smoke and CO sensing for daily home safety.

Key Features

• Adapter plugs and quick-connect: Makes replacing an existing hardwired alarm fast and avoids the need to rewire, saving time on an upgrade.
• 10-year battery backup: Provides constant monitoring for the life of the alarm so it still works during power failures and reduces maintenance frequency.
• Combination sensing: Electrochemical CO sensing and ionization smoke sensing together detect both threats from one unit, simplifying coverage.
• Intelligent smoke sensing: Reduces nuisance alarms while still responding to real threats, with a single test/silence button to manage alerts.
• Loud 85 dB siren and indicators: Clear visual indicator lights and a loud alarm ensure occupants are alerted to smoke or carbon monoxide events.

Who It's For

This alarm is best for homeowners or landlords replacing older hardwired First Alert units who want a drop-in solution that keeps existing wiring intact. It is also a good choice for people who prefer combination detectors to cover both smoke and carbon monoxide with one device and value a built-in 10-year battery.

It may be less appropriate for someone wanting a battery-only portable detector, a different sensing technology mix, or smart/connected features, since this model focuses on hardwired reliability and long-life backup rather than Wi-Fi connectivity or app alerts.

Pros & Cons

Pros

• Easy installation using adapter plugs makes replacing older hardwired alarms quick and tool-friendly.
• 10-year lithium battery backup is built in and locked to prevent tampering, reducing long-term maintenance.
• Combination of electrochemical CO sensing and ionization smoke chamber covers two hazard types in one unit.
• Intelligent sensing and a single test/silence button make daily use straightforward.

Cons

• Some users report mixed experiences with battery condition on arrival, so check the battery and test on installation.
• Not a smart or connected alarm, so it will not send remote notifications.

Specifications

Brand	First Alert
Model	SC9120B
Power	Hardwired with 10-year lithium battery backup
Sensors	Ionization smoke chamber and electrochemical CO sensor
Alarm level	85 dB siren
Installation	Adapter plugs and quick-connect wiring harness
Warranty	10-year limited warranty with end-of-life signal

Our Verdict

The SC9120B is a practical, value-minded choice for anyone replacing an existing hardwired alarm who wants dependable smoke and carbon monoxide protection without rewiring. Its 10-year battery backup and combination sensing make it low-maintenance and broadly protective, though those seeking smart home features should look elsewhere.

Frequently Asked Questions

Can this replace older hardwired First Alert units?
Yes; the included adapter plugs and quick-connect harness are designed to allow easy replacement of existing hardwired alarms without rewiring.

Will it work during a power outage?
Yes; the unit includes a locked 10-year lithium battery backup to keep monitoring even when AC power is unavailable.

Does it detect both smoke and carbon monoxide?
Yes; it combines an ionization smoke sensing chamber with an electrochemical CO sensor to detect both hazards.