My last column detailed duct smoke detectors and some of the conflicting requirements we experience. I discussed the basic location requirements for duct detectors—on either the supply or return air side of the air handler unit—depending on whether the International Mechanical Code or NFPA 90A is used.
Before we can determine where and when duct smoke detectors must be used in conjunction with smoke dampers, one must determine where smoke dampers are required. A variety of documents may cover this.
Let’s start with NFPA 101, the Life Safety Code, 2009 edition. Although you will find some requirements in the occupancy chapters, most of that information will tell you when the dampers are not required. Chapter 8, Features of Fire Protection, covers the smoke detection requirements. Section 126.96.36.199 states, “Dampers in air-transfer openings shall close upon detection of smoke by approved smoke detectors installed in accordance with NFPA 72, National Fire Alarm Code.” NFPA 90 defines an air transfer opening as “an opening designed to allow the movement of environmental air between two contiguous spaces.”
Does this mean we must have a smoke damper in every wall? No, we provide them in barriers separating smoke compartments. To find where smoke barriers and smoke compartments are required, refer to either the International Building Code or NFPA 101, the Life Safety Code. Keep in mind that there are many sprinkler trade-offs in place in the codes that may reduce the number of smoke compartments required.
When smoke compartmentation is required and smoke dampers will be installed, the fire alarm contractor must determine where to install the duct smoke detector. One source is the International Building Code, 2009 edition, Section 7188.8.131.52, which states, “Where a smoke damper is installed within a duct, a smoke detector shall be installed in the duct within 5 feet (1,524 mm) of the damper with no air outlets or inlets between the detector and the damper.” This is typical of the requirements for installation. As indicated in the previous paragraph, the detectors are installed per NFPA 72. Paragraph 184.108.40.206 (2007 edition) gives the installer four choices of mounting detectors in air duct systems. These include using a duct detector with sampling tubes protruding into the duct; spot-type detectors listed for the velocity, temperature and humidity suspended in the center of the duct or mounted to the inside of the duct or using a beam smoke detector.
To recap, first determine the occupancy. Then find smoke barrier or smoke compartment requirements. Next, find where the air ducts penetrate the smoke barriers to determine damper locations by looking at mechanical plans.
Next, let’s discuss testing duct smoke detectors. NFPA 72 was a little vague on this until the 2010 edition. Basically, a duct detector is a system smoke detector that performs a specific function, so you need to test the duct detector as you would any other system smoke detector as well as test to ensure smoke can get to the detector. It is a two-step process. One step is to ensure proper air movement from the duct to the detector through the sampling tubes. This can be verified with an instrument that will measure the differential pressure between the inlet and outlet sampling tubes, such as a manometer.
Secondly, you must test for smoke entry into the detector by introducing a smoke or aerosol acceptable to the smoke detector manufacturer. Both of these tests must be conducted to meet the testing requirements of NFPA 72. The use of magnets or remote test switches does not meet the intent of NFPA 72 for initial and annual testing. They may be used for more frequent testing than required, but cannot be used in place of the required annual test verifying smoke entry into the detector.
This can be a challenge due to the locations of the detectors. Sometimes, it is almost impossible to get to them, but that should be considered during installation. Remember that NFPA 72 requires all fire alarm equipment to be accessible for periodic maintenance. Communicating with the general and mechanical contractors ahead of time can help a great deal.
HAMMERBERG is the president/executive director of the Automatic Fire Alarm Association Inc. headquartered in Jasper, Ga. He serves on a number of NFPA committees, including the NFPA 72 Technical Correlating Committee and the Protected Premises Technical Committee. He can be reached at TomHammerberg@afaa.org.