The Kidde FireX HD135F is a 120-Volt AC Wire In Heat Alarm that responds to temperature rather than smoke. The alarm will sound if temperatures rise above 135 degree F. The Kidde HD135 degree F is inter connectable. The interconnect system must not exceed the NFPA (National Fire Protection Association) limit of 18 initiating devices (Smoke, heat, CO, etc.) interconnected, it is still possible to interconnect 6 strobe lights and or relay modules. This unit includes a 5-year limited warranty and is UL Listed. Receive an instant alert when temperatures exceed 135 degree F. Test button assures proper operation of the sensor's electronic circuitry, horn and battery function. Interconnects with up to 24 Kidde devices (of which 18 can be initiating), including smoke and carbon monoxide alarms. 9-Volt battery backup provides continuous protection even during power outages. Red LED flashes every 30 - 40 seconds to indicate proper operation. Quick-connect power harness for fast and easy installation. Tamper-proof pin helps deter tampering and theft. Replacement 9-Volt battery Kidde model 21025830. UL/CSFM Ref. HD135F. Not a smoke detector. Compatible with Kidde, FireX, Code One and Sentinel products. Kidde offers a full range of 120-Volt AC wire-in life safety products with interconnect capability to provide the most immediate and complete system available; when any 1 interconnected alarm is triggered, all interconnected alarms within the home sound an immediate warning. UL/CSFM Ref. Kidde Part HD135.