FireX Hardwire Smoke and Carbon Monoxide Combination Detector with 9-Volt Battery Backup and Voice Alarm (3-Pack)This Kidde FireX Smoke and Carbon Monoxide detector can warn clearly of any type of hazard. It is a photoelectric combination alarm built into one unit. It uses a voice and beeping tone to warm of any danger. The 9 Volt battery is included. Smoke and carbon monoxide detectors should be installed in a central location outside each sleeping area, on every level of the home, and in other locations where required by law 9 Volt battery backup provides