First Alert BRK First Alert 9120BFF/ SMI100-AC Smoke Detector, Hardwired Alarm with Battery Backup, White, 1-Pack

Why I chose the First Alert 9120B

I needed to refresh a set of aging, interconnected smoke alarms in a hardwired system. My priorities were straightforward: a dependable, UL-listed unit that would drop into an existing 120V setup, include a battery backup for outages, and avoid complicating the job with proprietary quirks. The First Alert 9120B checked those boxes on paper, and in practice it made the upgrade easy, reliable, and predictably unremarkable—in the best way a life-safety device can be.

Installation and setup

Swapping in the 9120B was uneventful, which is exactly what you want. The detector ships with a universal mounting bracket, screws/anchors, and a three-wire pigtail for the hot, neutral, and interconnect conductors. In locations where I was replacing older First Alert/BRK units, the existing bracket and plug were identical, so the new head clicked into place in a couple of minutes. In spots that previously used another brand, I spent a few extra minutes replacing the pigtail. Either way, the wiring is standard, and the included hardware is better than typical big-box fare.

The side-load 9V battery drawer is a thoughtful touch. You don’t have to take the alarm off the ceiling for a battery swap, and the drawer’s tamper-resist clip helps prevent curious hands from defeating the backup. The test/silence button is large, positive, and easy to reach with a broom handle.

A steady green LED indicates AC power. It’s there if you look for it but doesn’t light up a bedroom at night. After power-up, I ran through a quick check: local test using the button, then a network test via the interconnect. With the interconnect wire tied properly, all units sounded together as they should.

Design and build

This is a conventional, low-profile mains-powered smoke detector in a clean, matte white housing. It’s not a design statement; it’s a practical, durable appliance meant to fade into the ceiling. The housing feels solid, the bayonet mount engages securely, and the labeling on the back is legible and complete (model, date code, wiring legend, and replacement interval guidance). Nothing fancy, nothing flimsy.

The alarm is rated at 85 dB. Using a phone meter as a rough check, I measured the sound in the mid-80s dB at about 10 feet, which squares with the spec. It’s piercing enough to wake a deep sleeper without being painfully shrill at close range.

Detection performance

The 9120B uses an ionization sensor, which is generally faster to react to fast-flaming fires. In practice, with canned smoke for testing and a few “real-world” provocations (think overzealous toast), the unit responded promptly. Ionization sensors can be more sensitive to cooking aerosols; the hush function handled that gracefully. A single tap muted a nuisance alarm while the detector continued to monitor. The silence period is long enough to clear the air but short enough to fail-safe, and pressing the button again will rearm immediately when the air clears.

I ran an interconnected test by holding smoke near one unit and verified that the rest of the network sounded in short order. That’s the big advantage of a hardwired system: if a fire starts in a remote corner of the house, every alarm can alert simultaneously.

A caveat worth noting: ionization detectors are not as quick to pick up slow, smoldering fires as photoelectric sensors. My recommendation is to pair ionization detectors like this one with photoelectric units elsewhere in the home, or to use combination smoke alarms where code and wiring allow. The 9120B does its job well, but it’s one half of a complete detection strategy.

Power and reliability

This is a 120V AC device with a 9V battery backup. I verified failover by tripping the branch circuit while watching for the LED state and listening for low-power chirps—no surprises. The alarm keeps running on battery and resumes normal operation as soon as power returns.

Maintenance on a unit like this is simple:
- Test monthly using the button.
- Vacuum the vents every six months to reduce dust that can trigger nuisance alarms.
- Replace the 9V battery annually or when you hear chirping.
- Replace the entire unit at 10 years (check the date stamp on the back).

The chirp patterns are distinct enough to tell you whether you’re hearing a low-battery, a fault, or an end-of-life signal. That clarity matters at 3 a.m., when the temptation is to pull a battery and forget about it. The side-drawer design lowers that risk.

Interconnect and compatibility

Wired interconnect remains the most dependable way to make all alarms speak at once. The 9120B’s third conductor ties into other compatible First Alert/BRK devices. If you’re mixing generations or adding CO or combo units, check the manual for device limits and compatibility. In most residential scenarios there’s ample headroom to wire a full set across a floor plan, and I had no issues linking smoke-only units in a daisy-chain.

If you’re replacing a mixed-brand lineup, expect to change pigtails—the 9120B includes the correct connector. It’s a small upfront investment in time that pays off with uniform behavior across the system.

What it’s not

A few boundaries are important to set:
- This is not a smart alarm. There’s no Wi‑Fi, app notifications, or voice guidance. If you want mobile alerts or self-testing, look to smart models and be ready for higher cost and, often, battery-only power.
- It’s ionization-only. For the most comprehensive coverage, pair it with photoelectric detectors or use combo units where appropriate.
- It’s not a CO detector. If you have fuel-burning appliances or an attached garage, add CO alarms (a hardwired smoke/CO combo like First Alert’s SC-series can coexist on the same network in many systems).
- It does not have a sealed, 10-year lithium battery. You’ll be replacing a 9V annually.

None of these are faults—they’re design choices that keep the 9120B straightforward, affordable, and easy to service. But you should match the device to your needs and local code.

Everyday use

Living with the 9120B has been pleasantly uneventful. The green LED is unobtrusive, the housing blends into the ceiling, and the test/silence button is easy to find without a step stool. Monthly tests are quick; interconnected testing confirms the network is healthy. In the kitchen-adjacent hallway I did get a couple of predictable nuisance triggers during high-heat searing with poor ventilation; the hush button handled them, and better ventilation eliminated repeats.

What stands out is the consistency. The alarms behave the same way every time, across every room, and that predictability builds trust.

Alternatives and buying advice

• If you want smart features: Consider a Wi‑Fi–enabled detector with app alerts and self-testing. Just confirm code compliance and power requirements; many smart units are battery-only.
• If you need both smoke and CO on one head: Look at a hardwired combination unit compatible with your interconnect line.
• If you prefer photoelectric detection (or have a history of nuisance alarms near kitchens and baths): Use a photoelectric model in those locations and keep ionization units in bedrooms and larger common areas. A mixed-technology approach is often best.

Regardless of the model, plan your layout: one on each level, in every bedroom, and outside sleeping areas, with attention to manufacturer clearances from bathrooms and cooking appliances. Interconnect where possible.

The bottom line

The First Alert 9120B is a workhorse hardwired smoke alarm that emphasizes the basics: dependable ionization detection, easy installation, unobtrusive design, and a simple test/silence workflow. It drops into existing BRK/First Alert wiring cleanly, includes a battery backup that’s easy to service, and provides the reliability and uniform behavior I want in a life-safety system.

Recommendation: I recommend the 9120B for anyone maintaining or upgrading a conventional hardwired network who values reliability and straightforward operation over smart features. Pair it with photoelectric units (or combination detectors where needed) for broader coverage, follow the usual maintenance schedule, and it should serve quietly and effectively for its 10-year service life.