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First Alert · Hardwired smoke alarm

First Alert 9120B: age, expiry and what replaces it

Quick answer

The 9120B is a First Alert (BRK) ionization smoke alarm wired to house power with a 9-volt backup battery. First Alert lists it as discontinued and names the SMI100-AC as its replacement model. Its manual says to replace it ten years on, and it only links with the First Alert and BRK models the manual lists.

Last checked

At a glance

Status
Discontinued; First Alert says it is no longer for sale1
Sensor type
Ionization1
Power
Wired to 120 V AC, with a replaceable 9 V backup battery1
Backup battery
One standard 9 V cell. The manual lists acceptable ones (for example Duracell MN1604 or Energizer 522) and also allows a named 9 V lithium battery2
Where the date is
First Alert says its alarms carry the manufacture date on the back. The 9120B manual also asks you to write the purchase date in a space on the back3
When to replace
The manual says to replace the alarm 10 years after it was bought; First Alert's general advice is to replace any alarm 10 years or more past its manufacture date2
Low-battery signal
About one chirp a minute when the backup battery gets weak. The manual says that warning should last 7 days, but to change the battery straight away2
During a power cut
If the house power fails and the backup battery is dead or missing, the manual says the alarm cannot operate2
Interconnect limits
Up to 18 compatible units on one loop, no more than 12 of them smoke alarms, all on the same breaker, with under 1,000 feet of interconnect wire2
What it links with
Only the First Alert and BRK smoke, CO and heat alarms and accessories listed in its manual (for example SA4120, SC9120B, 7010B and CO5120BN); the manual warns that connecting anything else may damage it or stop it working2
Warranty
10-year limited1

Replacing it

Replacement named by First Alert
SMI100-AC, listed as the replacement model in the 9120B's specifications1
Replacing a Kidde alarm with a 9120-series
First Alert's ADK-12 adapter guide lists the BRK 9120 series among the alarms that can plug into certain older Kidde harnesses (Kidde models made on or before January 1, 2004)4
Replacing a Firex alarm with a 9120-series
For older Firex harnesses First Alert uses a different plug, the ADF; its guide also lists the 9120 series5
Recalls
Search CPSC's recall list for your exact model before reusing or reselling an alarm6

Related models

Common questions

Where is the date on a First Alert 9120B?

First Alert says its alarms carry the manufacture date on the back. The 9120B manual also asks you to write the purchase date in a space on the back3

When should a First Alert 9120B be replaced?

The manual says to replace the alarm 10 years after it was bought; First Alert's general advice is to replace any alarm 10 years or more past its manufacture date2

What replaces the First Alert 9120B?

SMI100-AC, listed as the replacement model in the 9120B's specifications1

Which adapter guide lists the First Alert 9120B?

First Alert's ADK-12 adapter guide lists the BRK 9120 series among the alarms that can plug into certain older Kidde harnesses (Kidde models made on or before January 1, 2004)4

Sources

  1. First Alert — Hardwired Ionization Smoke Alarm with Battery Backup 9120B (product page) checked 2026-11-04
  2. First Alert / BRK — 9120 / 9120B User's Manual (PDF) checked 2026-11-04
  3. First Alert — 10-Year Battery Alarms (How to check your alarms) checked 2026-11-04
  4. First Alert / BRK — Installation of the ADK Adapter Plug (PDF) checked 2026-11-04
  5. First Alert / BRK — Installation of the ADF Adapter Plug (PDF) checked 2026-11-04
  6. CPSC — Recalls & Product Safety Warnings checked 2026-11-04

Facts are restated in our own words. If a source page changes, the "checked" date tells you how fresh our reading is. Spot something out of date? Open an issue on GitHub.

Published November 4, 2026. Part of the First Alert / BRK brand hub.