The NAVIEN TANKLESS WATER HEATERS 30024849A VID Sensor is a Vent Installation Detector designed for compatible Navien tankless water heaters and combination boiler systems. Installed near the vent connection, it detects whether the exhaust vent pipe has been inserted to the required depth and sends feedback to the unit's control system. If the vent is improperly positioned, the sensor... https://www.partshnc.com/30024849a-navien-tankless-water-heaters-vid-sensor-npe-2-and-ncb-h