The DH-99 is designed and built to meet all local requirements, as well as the NFPA regulations regarding duct smoke detectors. The DH-99-AR is provided with output terminals for remote accessories such as a remote status indicators and reset key switches or push buttons. Air sampling is accomplished by two tubes which protrude into the duct. An exhaust tube of one standard length (7.5") is supplied in the installation kit with the smoke duct unit. Once the duct width has been determined the air intake sampling tubes must be ordered. Sampling tubes are supplied in three standard lengths 2.5 ft., 5 ft. and 10 ft. and cut to size to fit the duct.