When is a duct detector required

In occupancies not required to be equipped with a fire alarm system, actuation of a smoke detector shall activate a visible and audible signal in an approved location. Duct smoke detector trouble conditions shall activate a visible or audible signal in an approved location and shall be identified as air duct detector trouble. Protection provided by the installation of smoke detectors and related requirements is intended to prevent the distribution of smoke through the supply air duct system and, preferably, to exhaust a significant quantity of smoke to the outside.

Neither function, however, guarantees either the early detection of fire or the detection of smoke concentrations prior to dangerous smoke conditions where smoke movement is other than through the supply air system.

All automatic shutdown devices shall be tested at least annually. Sprinklers are often tested weekly. Where it is desirable to prevent the accompanying automatic shutdown of fan system s reference 6. Supply or return sources larger than those commonly found in residential and small commercial establishments can require greater clearance to smoke detectors. Similarly, smoke detectors should be located farther away from high velocity air supplies. In addition, the research showed that a smoke dilution effect occurred near air returns.

An air return pulls air up from levels in the room that are beneath the ceiling jet, which has the effect of diluting smoke concentration near the air return grille. Consequently, the designer should arrange the detection so that detectors are not adjacent to either air supplies or air returns. Situations may exist where even a 3 ft 0. This situation would depend on the air velocity supply air and return air and the throw characteristics of the supply diffuser and diffuser size.

Unfortunately, because the research did not address wide variations in HVAC flow rates, the minimum distance between a detector and the HVAC system supply or return recommended in A.

Where in doubt, airflow in the vicinity of the detector should be mapped with a velometer or anemometer.

Detector spacings and locations shall be selected on the basis of anticipated airflow patterns and fire type. Conversely, heating a room sometimes requires introducing extremely hot air into a room. Consequently, HVAC plenums usually have ambient conditions that are far more extreme than the spaces they support. Smoke detectors are electronic sensors. Ambient temperature, the relative humidity, and, especially in the case of spot-type ionization detectors, the velocity of the air around the detector all affect detector operation.

Not all smoke detectors are listed for the range of conditions found in HVAC plenums or in under-floor or above-ceiling spaces. See also Where detectors are used for the control of smoke spread, the requirements of Where open area protection is required, Commentary from the National Fire Alarm and Signaling Code Handbook : In most buildings, there are times when the HVAC system is not moving significant quantities of air from the compartments it serves.

This is typical of variable air volume VAV systems. Consequently, the fire detection system cannot be designed to rely on the HVAC system operation for the transport of smoke to smoke detectors. Furthermore, when the ventilating system is operating, the detector s can be less responsive to a fire condition in the room of fire origin due to dilution by clean air.

Commentary from the National Fire Alarm and Signaling Code Handbook : Between and , several fires in high-rise buildings demonstrated the difficulty of trying to evacuate an entire building. Not only did occupants incur injuries during the evacuation, but also the means of egress often became untenable due to heavy smoke concentrations.

As improved building codes resulted in structures that could maintain their integrity in spite of the complete combustion of the interior fire load through passive fire-resistive construction and compartmentation, defending occupants in place became a viable option. Strategies for establishing smoke compartments and areas of refuge and for managing the flow of smoke by directing it away from the occupants were developed.

Experiences with high-rise fires indicate that the proactive control of smoke with either automatic smoke detectors and HVAC systems or engineered smoke control systems is a viable strategy for occupant protection in high-rise buildings.

Subsection The purpose of Smoke detectors installed and used to prevent smoke spread by initiating control of fans, dampers, doors, and other equipment shall be classified in the following manner:.

This technology permits the use of area detection without the incremental cost of large numbers of detector relays, as was the case decades ago.

Both projected beam smoke detectors and video image smoke detectors are also used as area detection and can be used as an input signal for the control of the HVAC system serving the related smoke compartments. Smoke might not be drawn from open areas if air-conditioning systems or ventilating systems are shut down. Commentary from the National Fire Alarm and Signaling Code Handbook : All too often, uninformed designers attempt to use air duct—type smoke detectors to provide open area protection.

This strategy does not address the potential for a fire during those times when the HVAC system is not running, nor does it address the delay in detection due to smoke dilution. Paragraph Area smoke detectors within smoke compartments shall be permitted to be used to control the spread of smoke by initiating operation of doors, dampers, and other equipment. Existing detectors can perform double duty through the programming of the fire alarm control unit. When area smoke detectors are used, smoke detectors are needed where they can identify the presence of smoke at a particular location or the movement of smoke past a particular location.

The locations for area smoke detectors are a function of building geometry, anticipated fire locations, and intended goals of smoke control functions. The tube is plugged on the other end to force the air it captures through the smoke detector and then back out into the duct through a second, non-perforated return tube. Duct detectors are wired to a circuit board. When smoke is detected, the device sends a signal to the circuit board.

Most circuit boards are equipped with several different relays, which are used to send signals to various other systems, including the fire alarm control panel to trigger the alarm and the HVAC control system to close the dampers and shut down the fans.

There will also be a relay for signaling the supervisory panel. Most duct detectors are equipped with two alarms -- a fire condition alarm triggered when smoke is detected and a trouble condition, which is indicated on the supervisory panel. A trouble condition could be triggered when someone has removed the cover from the detector or if the power to the detector is lost or malfunctioning somehow, all of which help reduce the occurrence of false alarms. Duct smoke detectors are required for HVAC systems with airflows exceeding 2, cubic feet per minute cfm , including settings where multiple air handling systems share ducts if the combined design capacity is greater than 2, cfm.

However, where they are installed depends on the governing code s in your jurisdiction. This can be a complicated question because of the two most common codes governing their installation present conflicting requirements:. NFPA 90A requires duct detectors to be installed on the supply side of air handler units with airflows of more than 2, cubic feet per minute cfm. The standard also requires a duct detector on the return side of any unit with air flows exceeding 15, cfm and which serve more than one floor of a building.

In contrast, the IMC requires smoke detectors to be installed in return air systems of more than 2, cfm and upstream of any filters or exhaust or outdoor air connections. Both also have exceptions that allow the deletion of the return duct smoke detectors if the area served is supplied with area smoke detectors and they are configured to control the HVAC system in the same way that the duct detectors would have.

NFPA 72 also states that a duct smoke detector is not allowed to replace an area smoke detector, mainly because if the air-handling unit AHU is not running, the detector would not detect smoke. Therefore, duct smoke detectors are typically considered mechanical equipment devices instead of life safety devices. Both the IMC and NFPA 90A state that, if multiple air handlers serve the same area and the combined air volume is greater than 2, cfm, detectors are required.

Both also have exceptions that would not require the duct smoke detectors if the air handler is only serving one room, since smoke could not be distributed to other areas. So why is there such a difference in requirements? There are two schools of thought concerning the purpose of duct detectors. The primary purpose of the duct detector on the supply side is to shut down the air handler if fire is detected in the fan or filter.

This would protect the equipment and keep smoke from being distributed to the air conditioned space. Some would argue that detectors on the supply side are susceptible to outside air or smoke, which could affect the operation of the supply side smoke detector.

Locations of Duct Detectors are documented in the installation and acceptance documentation, and is kept at the protected premise in the document box, by having this information on site and in records, the testing and maintenance of Duct Detectors is completed at the time of service with no return trips needed for lifts and access.

Facebook Twitter LinkedIn Instagram. What is a Duct Smoke Detector and why are they required? Challenge: The installation of Duct Detectors can be a main cause of nuisance alarms and supervisory alerts. In some applications, Duct Smoke Detectors can be used for the control of ventilation dampers.

Where are Duct Smoke Detectors required to be installed: The International Mechanical Code requires a duct smoke detector in the return for units over 2, cfm and requires a detector in the supply duct for systems over 15, cfm. Results of Best Practice: Accessibility of Duct Detectors is done by diligence during the installation, record of location in project and acceptance documents, and notes on inspection reports during the Inspection, Testing and Maintenance of the device.