The Duct Smoke Detector provides early warning of smoke by continually sampling air movement within heating and ventilation ducts in industrial or commercial premises. The duct smoke detector provides switching facilities to shut down associated fans to prevent the spread of smoke into other areas. DDH series has been designed to detect smoke in ventilation and combines an optical intelligent or conventional smoke detector, housing and sampling tube. DDH series has been designed to allow optimum airflow through the optical smoke detector. DDH series is recommended for installations in ducts with low airflow and will operate in air speed of 0.5m/s to 20m/s.