The BRK SC9120B hardwire combination smoke and carbon monoxide alarm detects two hazards with one simple unit. Carbon monoxide (CO) is a colorless, odorless, deadly gas. It can be emitted by sources including gas or oil furnaces, gas clothes dryers, water heaters, fireplaces, wood stoves, charcoal grills, gas ranges, and space heaters. A clogged chimney, improper venting, and attached garages can also cause carbon monoxide accumulation. The BRK SC9120 hardwire combination smoke and CO alarm with battery backup includes these features: Quick plug in power connector for easy installation, 120 volt AC inter connectable (with maximum 18 compatible devices 12 other compatible smoke alarms + 6 compatible devices like bells, horns, repeaters and door closers. If one unit triggers an alarm, all smoke alarms in the series will sound. Single test/silence button which both silences a non-threatening alarm or low battery warning and allows you to test the unit's functionality. Automatically performs internal test of functionality. Latching alarm indicator which automatically identifies which unit initiated the alarm. Battery drawer lock and mounting bracket lock for tamper resistance. AC power indicator and alarm indicator light. Low battery warning ''chirp'' and missing battery tab. 9 volt battery backup for peace of mind during power outages. All BRK Electronics carbon monoxide alarms are listed to UL Safety Standard 2034 for residential carbon monoxide alarms. Includes 9 volt backup battery. 10 year limited warranty. Includes: One combination smoke and CO alarm, one 9 volt battery, and users manual.Hardwired combination CO smoke alarm features battery backup for constant monitoring.