135° F Heat Detector, 120V AC, Wire-In, Size: 5" Diameter, 2" Depth, Color: White, Includes: 9V Battery Back-Up. Key Features: Interconnectable – Interconnects with up to 24 Kidde devices of which 18 can be initiating. Test Button – Tests unit’s electronic circuitry, horn and battery function. 9V Battery Back Up – Provides continuous protection even during power outages. Red LED – Flashes every 30-40 seconds to indicate proper operation. Low Battery Indicator – Alerts user in the event of alow or missing battery. Quick Connect Power Harness – Makes installation fast and easy.