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Author(s): Wayne Moore. Published on November 2, 2015.

Smart Design

Knowing where smoke detectors should go and where they shouldn't. BY WAYNE D. MOORE

MANY CONVERSATIONS WITH FIRE ALARM SYSTEM designers and installers begin with the complaint that they designed the system to meet the requirements of NFPA 72®, National Fire Alarm and Signaling Code, but the system still produces false or nuisance alarms.

Most users of NFPA 72 understand that the code does not address specific design issues. Instead, it contains minimum requirements for the installation of fire alarm equipment. It also provides information on the impact that the installed environment may have on a device or appliance. The responsibility for proper system design belongs to the designer, who is required to understand how a particular detection device integrates with their design and with the fire protection goals of the owner.

The designer must also understand the code’s intent when the building code or NFPA 101®, Life Safety Code®, requires the installation of a fire alarm system. In most cases, the building codes or the Life Safety Code will state where the designer should place detection devices. But rarely will any code dictate all of the locations where devices must be placed. Typically, where the owner sets early warning as a goal, a designer assumes that the design should protect all spaces with smoke detectors. But, as many designers find out, smoke detectors do not always represent the best detection choice for all areas of the building, based on the particular installed environment.

A recent example of this comes from a conversation on NFPA’s LinkedIn site. The issue involved a smoke detector in a space that met environmental parameters for acceptable temperature and humidity. But because the space opened to the outside via louvers, sand would enter the space and cause false alarms. This offers a perfect example of why a designer needs to understand how smoke detectors operate, and why they shouldn’t be installed in certain areas. Whenever an owner wants a smoke detector in an environmentally unfriendly location, the designer must explain why certain environments would adversely affect the reliability of the detector.

The NFPA technical committee has included some helpful design guidance information in Annex A. Table A.17.7.1.9(a) lists contaminants unfriendly to smoke detectors. While it might not be possible to isolate all environmental factors, an awareness of the listed contaminants will certainly help the designer during system layout and design.

In the example cited, the table describes dust as an atmospheric contaminant that can adversely affect the operation of a smoke detector. Based on this information, the designer can then inform the owner that good design practice suggests not installing a smoke detector in that space. When presented with a smoke detector already installed in such a space, the designer must determine what type of detection should take its place. The answer will always depend primarily on the owner’s fire protection goals. If an automatic sprinkler system protects the space, then removing the smoke detector is certainly an option. If the space is not protected by an automatic sprinkler, then replacing the smoke detector with a heat detector that will accommodate the hostile environment is a possible solution. But if the owner’s goals include detecting a hostile fire as soon as possible, the designer should review the availability of smoke detection devices that will both meet the fire protection goal and operate reliably in the installed environment.

Regardless of the variables, it’s the designer’s responsibility to know what options exist—the designer should make this determination and not expect a solution to be spelled out in the code.

WAYNE D. MOORE, P.E., FSFPE, is vice president at JENSEN HUGHES.