First Alert has been making residential safety gear for decades, and the SC-9120B is a workhorse update to the standard hardwired alarm. What separates this unit from a basic smoke detector is its dual-sensor approach. It uses an ionization sensor for fast-flaming fires and an electrochemical sensor for carbon monoxide. That means one device handles two distinct threats without needing separate units on your ceiling.

Installation is straightforward if you already have a hardwired system. The unit connects directly to 120V AC power and includes a battery backup. In real-world terms, that battery keeps it running during a power outage when CO risks often rise from generators or furnaces. The interconnect feature lets you link multiple units so when one alarm triggers, every connected alarm sounds. For a two-story home, that means hearing a basement alarm from the second-floor bedroom.

The trade-off is sensor type. Ionization sensors are faster at detecting open flames but slower at smoldering fires compared to photoelectric alternatives. If you have a kitchen or garage prone to false alarms from cooking, this may trigger more often. It is a solid, code-compliant choice for most homes, but pairing it with a photoelectric unit in key areas gives better overall coverage.