First Alert hardwired smoke and carbon monoxide alarm 2-pack — SC-9120B with battery backup, ionization and electrochemical sensor

The First Alert SC-9120B is a hardwired combination smoke and carbon monoxide alarm with electrochemical CO sensing, ionization smoke detection, and a battery backup for continuous protection.

This two-pack combines two detection technologies into a single, interconnected device designed for homes wired for hardwired alarms. The SC-9120B pairs an ionization smoke sensor—responsive to fast-flaming fires—with an electrochemical carbon monoxide sensor that minimizes false readings from household gases. Hardwiring gives you a constant power source while the included battery backup keeps the unit operational during power outages. LED indicators and a single test/silence button make operation straightforward for both homeowners and installers.

How does the SC-9120B detect both smoke and carbon monoxide?

The unit uses two distinct sensing technologies tailored to different hazards. An ionization chamber detects small combustion particles common to fast, flaming fires, while an electrochemical cell measures CO levels to identify dangerous, odorless gas buildup. Each hazard has a dedicated LED so you can quickly tell which sensor triggered.

• Ionization sensor for rapid flame/particle detection
• Electrochemical CO sensor for stable, accurate carbon monoxide readings
• Separate LED status lights to distinguish smoke from CO events
• Audible alarm patterns that differentiate hazards

Can I connect these alarms to other units in my home?

Yes — the SC-9120B is designed to interconnect with compatible BRK and First Alert hardwired alarms. When one interconnected unit detects a danger, all linked alarms sound, alerting occupants throughout the property. This feature is especially valuable in larger homes or multi-level layouts.

• Compatible with BRK and First Alert hardwired detectors
• Interconnected triggering for whole-home alerts
• Wiring follows standard hardwired alarm installation practices
• Ideal for multi-floor or large single-family residences

What are the power and usability features I should know about?

The SC-9120B runs on household hardwiring and includes a side-load battery compartment for backup power, ensuring operation during outages. A single test/silence button lets you check functionality or temporarily quiet nuisance alarms without removing the unit. LEDs provide quick visual confirmation of normal, alarm, or fault states.

• Hardwired primary power with replaceable battery backup
• Side-load battery compartment for easy replacement
• Single test/silence button for convenient operation
• Visual LED indicators for smoke vs. CO and unit status

How should I maintain and install the SC-9120B for best performance?

Install on ceilings or high on walls, away from drafts and kitchen steam. Test the alarms monthly using the test button, replace backup batteries at least once a year, and follow local electrical codes for hardwired connections. Regular cleaning prevents dust buildup that can degrade sensor performance.

• Mount on ceiling or high wall, away from HVAC and kitchens
• Monthly function tests via the single button
• Replace backup battery yearly or as needed
• Clean gently to avoid dust accumulation on sensors

Who is this for: The First Alert SC-9120B 2-pack is aimed at homeowners, landlords, and contractors who need a reliable, code-friendly hardwired smoke and carbon monoxide solution with battery backup. It fits homes that require interconnected alarms, those who want both ionization and electrochemical sensing in one unit, and anyone seeking straightforward operation with clear visual alerts.