Duct detectors are installed through the wall of a duct in order to sense whether smoke is present in the air-circulation system. They have three main functional parts: a sampling tube, the detector itself and an exhaust tube. The length of the sampling tube is chosen to sense across a cross-section of the duct. Insertion of the tube increases the air pressure at its opening, as compared to the overall duct pressure, which is the pressure at the exhaust tube. This positive pressure differential causes duct air to enter the sampling tube, which directs it to the detecting device. The air exits through the exhaust tube.

Two main detecting technologies exist: ionization and photoelectric. According to the System Sensor, St. Charles, Ill., application note, “Duct Application Smoke Detectors,” smoke consists of varying kinds of airborne particles. Each detection technology responds better to certain kinds of particles and different environments.

Ionization-type detectors are best at sensing smoke that contains small, submicron, invisible particles, which generally are found in smoke close to a flaming fire. Photoelectric detectors that use light-scattering technology are best with larger particles, typical of smoldering fires.

Per National Fire Protection Association’s 72, photoelectric detection is the recommended sensor type for duct smoke detectors. NFPA 72, section A.5-16.4.2, states, “In almost every fire scenario in an air handling system, the point of detection will be some distance from the fire source, therefore, the smoke will be cooler and more visible because of the growth of sub-micron particles into larger particles due to agglomeration and recombination. For these reasons, photoelectric detection technology has advantages over ionization detection technology in air duct system applications.”

The detectors should be carefully set up for the average normal conditions existing at their location in the heating, ventilating and air conditioning (HVAC) system. The unit is set for the normal background particle level and then triggers when the air particles reach a preset threshold level. The detectors are quite sensitive to small changes in particle concentration. In addition, one should consider the airflow patterns in a particular duct system. For example, the concentration of smoke particles can be reduced by clean air from nearby returns in other rooms.

Duct sensors can protect infrastructure and people. For example, they can shut down an overheated fan or, if placed in cooling system output ducts, can save large computer or data storage systems from destruction. However, duct detectors also have their limits. They should not be used as the primary means of fire detection; that should be the job of open-area, spot and projected-beam detectors, as well as heat detectors or software-based video detectors. Primarily, duct smoke detectors are designed to sense smoke and protect building occupants and equipment from its spread. In some cases, they may be used to help evacuate building occupants in the case of a fire, but this is a secondary use.

Duct detectors only can sense smoke when it is being circulated through the forced air-circulation system, but there are times when the fans may not be running. Because they sample large volumes of air from different areas of the building, their placement in the system must be planned carefully. They also must be tested regularly, as contaminated filters can restrict their effectiveness. The NFPA 90A and the International Mechanical Code have specific requirements for placing the detectors that differ in some cases, so consult the legal codes of the locality where the system is being installed.

Alarming requirements

The primary function of duct detectors is to monitor the HVAC system to minimize smoke circulation. Their secondary function is to send an alarm—secondary only because the area detectors are the ones designed to be on the front lines of fire sensing. But things do not always work as designed, so with a function as vital as fire detection, redundancy is a major design principle. For this reason, the codes call for duct detectors to generate an alarm in addition to performing their control functions. There are two basic scenarios; the first is an integrated fire alarm system controlled by a fire alarm control panel (FACP). In that case, the duct detector should signal the FACP that it has gone into alarm mode. This is usually hardwired from an auxiliary contact closure in the detector. If there is no FACP, the duct detector must connect to its own audible and visible alarm, which also must include a trouble light to warn if the detector is not in proper operating condition.

A stand-alone detector typically is integrated into an air-handling device, such as a rooftop exhaust fan, by a manufacturer, who then sells it to a mechanical contractor as a complete unit, whereas a fire alarm system typically is sold to an electrical contractor. So duct detectors often are present in both the electrical and mechanical specs.

Finally, it is important to do regular maintenance, guided both by the manufacturers’ instructions and by codes. Smoke detection systems can be lifesavers, but they must be cleaned and serviced regularly to work reliably.

BROWN is an electrical engineer, technical writer and editor. He serves as managing editor for SECURITY + LIFE SAFETY SYSTEMS magazine. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at ebeditor@gmail.com.