DescriptionSize:1 Pack smoke detector with co battery backup Amazon.com The BRK 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. According to the Consumer Product Safety Commission (CPSC), at least one CO alarm should be installed near a home's sleeping area. 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). The alarm can be interconnected with up to 12 other compatible smoke alarms and six compatible devices like bells, horns, repeaters and door closers. If one unit triggers an alarm, all smoke alarms in the series will sound. Power indicators show which unit in the series initiated the alarm. Ionization sensor which is more effective at detecting flaming fires which consume combustible materials rapidly and spread quickly. Microprocessor controlled for fewer nuisance alarms. 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. Separate smoke and carbon monoxide visual and audible warnings. 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. 5-year limited warranty.-Bree NorlanderWhat's in the Box One combination smoke and CO alarm, one 9-volt battery, and user's manual