A HVAC occupancy sensor is a device that detects the presence of people in a given space. It is typically used in commercial buildings to control the heating, ventilation, and air conditioning (HVAC) system in order to reduce energy consumption. The sensor is usually installed in the ceiling and uses infrared or ultrasonic technology to detect motion. When the sensor detects motion, it sends a signal to the HVAC system, which then adjusts the temperature and ventilation accordingly. The sensor can also be programmed to detect the number of people in the room, so that the HVAC system can adjust the temperature and ventilation to accommodate the number of occupants. HVAC occupancy sensors are an important part of any energy-efficient building, as they help to reduce energy consumption and save money on energy bills.