FeaturesTest button tests units electronic circuitry, horn and battery function9 VDC battery backup provides continuous protection even during power outagesRed LED flashes every 30 to 40 s to indicate proper operationLow battery indicator alerts user in the event of a low or missing batteryQuick connect power harness makes installation fast and easyInterconnects with up to 24 Kidde® devices of which 18 can be initiatingThe Kidde HD135F 120 VAC wire in heat alarm gives you and your family an early warning to dangerous temperatures of 135 deg F or moreThis unit also includes a tamper-resistant pin that helps to deter thief and tamperingThis unit can be used as a single station unit or it can be interconnected to up to 24 Kidde devices in a multiple station arrangement