As with many aspects of fire detection, three codes apply when dealing with duct-type smoke detectors: the International Mechanical Code (IMC); NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems; and NFPA 72, National Fire Alarm and Signaling Code.
There are multiple ways to establish smoke control. NFPA 92, Standard for Smoke Control Systems, covers both smoke control/management systems and smoke containment systems. Smoke containment systems are the most common and are automatically activated. They keep smoke from entering specific areas such as closing a smoke damper, shutting down fans or releasing smoke doors. Smoke control/management systems actively move air during a fire event to maintain tenable environments. These systems have more robust installation, power, circuit, detector connection and supervision requirements since they must be operational during a fire. These smoke control systems use UULK listings from Underwriters Laboratories.
With smoke control systems, you will need to coordinate with the HVAC contractor and possibly a fire protection engineer. Developing this coordination effort early and will in avoiding several costly changes that you will be responsible for making due to installation mistakes.
For the rest of this article, I will focus on the duct smoke detector/smoke containment system requirements from the International Building Code (IBC), NFPA 90A and NFPA 72.
The IBC requires in Section 606 that all HVAC air distribution systems be equipped with duct smoke detectors listed and labeled for installation in these systems and comply with UL 268A. When estimating a fire alarm system that will include duct smoke detectors, it is important to review the HVAC system serving the building to ensure you understand how it is put together and how it works so the installation does not include too many duct smoke detectors. Duct smoke detectors are not required where air distribution systems are incapable of spreading smoke beyond the enclosing walls, floors and ceilings of the room or space in which the smoke is generated. This includes ducts serving air exhaust only.
The duct smoke detectors must be connected to shut down all operational capabilities of the HVAC distribution system in accordance with the listing and labeling of the appliances used in the system. It is important to know that the codes now require that duct smoke detectors transmit a supervisory signal, not an alarm signal. Another issue is to ensure the relays used for the shutdown operation are listed and designed to handle the current draw of the fans.
In general, the IBC and NFPA 90A require duct smoke detection to be installed in all return air systems that have a capacity greater than 2,000 cfm and the detectors must be installed in the return air ducts upstream of any filters, exhaust air connections, outdoor air connections or decontamination equipment and appliances.
There is one exception to the duct smoke detection requirements—where the area served by the HVAC system is provided with complete area smoke detection as defined in NFPA 72, duct smoke detectors are not required. In the instance where this type of complete coverage is in place, the shutdown operation would be by addressable control modules connected to the relay used for fan shutdown. The control modules are generally not capable of handling the high current load of the fans and should not be directly connected to the fan power.
There are several options for how the duct smoke detectors should be installed. For example, in occupancies not required to be equipped with a fire alarm system, actuation of a duct smoke detector must activate a visible and audible signal in an approved location. Duct smoke detector trouble conditions must activate a visible or audible signal in an approved location and must be identified as air duct detector trouble.
Additionally, the IBC states, “where a smoke damper is installed within a duct, a smoke detector shall be installed inside the duct or outside the duct with sampling tubes protruding into the duct. The detector or tubes within the duct shall be within 5 feet of the damper. Air outlets and inlets shall not be located between the detector or tubes and the damper.”
Although you may be using a UL 2368A duct detector, it is important to ensure that the detector is listed for the air velocity, temperature and humidity anticipated at the point where it is installed.
The IBC also requires the following:
“1. Where a smoke damper is installed above smoke barrier doors in a smoke barrier, a spot-type detector shall be installed on either side of the smoke barrier door opening. The detector shall be listed for releasing service if used for direct interface with the damper.
“2. Where a smoke damper is installed within an unducted opening in a wall, a spot-type detector shall be installed within 5 feet horizontally of the damper. The detector shall be listed for releasing service if used for direct interface with the damper.
“3. Where a smoke damper is installed in a corridor wall or ceiling, the damper shall be permitted to be controlled by a smoke detection system installed in the corridor.
“4. Where a smoke detection system is installed in all areas served by the duct in which the damper will be located, the smoke dampers shall be permitted to be controlled by the smoke detection system.”
The IBC also requires that a service opening be provided in air ducts adjacent to each fire damper, smoke damper, combination fire/smoke damper and any smoke detectors that need access for installation, cleaning, maintenance, inspection and testing. However, you will not be providing the service opening needed to ensure the HVAC contractor is aware of and complies with the requirement.
NFPA 90A has similar requirements to the IBC with a few additional requirements.
One of the more important additions is that duct smoke detectors must be installed “at each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 15,000 ft3/min and serving more than one story.”
Of course, both the IBC and NFPA 90A require smoke detectors be installed, tested and maintained in accordance with NFPA 72.
One final exception to the requirements is found in NFPA 90A, which states that smoke detectors powered separately from the fire alarm system for the sole function of stopping fans do not require standby power.
Coordination with the HVAC contractor is paramount to successfully passing the fire alarm acceptance test, and knowing the above code requirements will ensure a both a code-compliant system and a profitable installation.