One of the most common problems I find when reviewing installed fire alarm designs is the misapplication of smoke detectors. Some installers don’t know what smoke detectors do or why simply installing spot-type smoke detectors on a 40-foot ceiling or installing one detector in a 60-foot-long corridor does not achieve early warning.
Most designers assume that, if they space smoke detectors in accordance with NFPA 72 2016, National Fire Alarm and Signaling Code, they have properly completed their job. Also, designers who believe they have a complete grasp of the code requirements and a full understanding of the products they specify think they know everything necessary about smoke detector placement. Sadly, they are wrong.
I see too many fire-alarm systems installed where an owner with no fire alarm or fire protection background has requested the addition of smoke detection. In response, the salesperson only provides one or two detectors for coverage and does not come close to the proper application of smoke detection.
The code provides initial guidance in the annex of Chapter 17: “The person designing an installation should keep in mind that, in order for a smoke detector to respond, the smoke has to travel from the point of origin to the detector. In evaluating any particular building or location, likely fire locations should be determined first. From each of these points of origin, paths of smoke travel should be determined.”
Because of this, the testing laboratories do not assign a specific spacing to smoke detectors. Additionally, as stated in the code: “If there are no detectors in the room or area of fire origin, the fire could exceed the design objectives before being detected by remotely located detectors.”
Of course, on occasion, smoke detection provides protection for something other than early warning. In these cases, the code provides guidance by allowing “partial or selective coverage” in Section 188.8.131.52. In these applications, a designer can place smoke detectors to detect fires in a specified area or equipment location and not use these detectors for general alarm and evacuation. Selective coverage intends to address only a specific hazard and does not intend to provide early warning. An example of selective coverage would include detection used for some form of fire-suppression-system release.
I sometimes hear the excuse that, because the code does not actually require smoke detectors for the application presented, the designer need not adhere to a specific spacing of the detectors. The code addresses this concept in 184.108.40.206 and does allow exceptions to the prescriptive spacing criteria of Chapter 17. However, it cautions the designer to notify both the owner and the authority having jurisdiction (AHJ) of the specific design concept.
The design narrative supplied by the designer should capture this kind of information. As stated in the annex of Chapter 17: “The performance objective statement should describe the purpose of the detector placement and the intended response of the fire alarm control unit to the detector activation. This statement can include a narrative description of the required response time of the detectors, a narrative of the sequence of operations, a tabular list of programming requirements, or some other method.”
Furthermore, the designer should make the owner aware that, once the installation of smoke detectors takes place, the code requires acceptance testing, annual testing and ongoing maintenance.
The code discusses a number of detector types that a designer can use, including linear projected beam smoke detectors, duct smoke detectors and aspirating smoke detectors. The designer must evaluate a number of considerations to determine the appropriate type of smoke detector for the particular application.
Designers need to understand the limitations of spot-type detectors in challenging environments, such as high air flow, extreme temperatures (hot or cold), dirty environments and environments with radio frequency issues, to name a few. These environments could all interfere with meeting the smoke detection goal.
An owner may well have a protection goal for smoke detection to provide a “very early warning.” Data centers, clean rooms, telephone switching centers, other high-value electronic equipment facilities and other mission-critical areas may all have such a goal. When a designer encounters this very early detection requirement, he or she should know immediately that a spot-type smoke detector will not meet the goal.
When designing smoke detection for a building such as a high-rise hotel, the designer must remember that nature and occupancy of some spaces, such as atria and mechanical spaces, will make smoke detection difficult.
Any time we know an area has high airflow, high ceilings or beam pockets, we can expect challenges to the detection of smoke. These challenges can come in the form of smoke dilution and stratification.
When the designer encounters these areas, he or she should give special care to the choice of detection for the space. In fact, in most atria and other restricted areas, such as transformer vaults, the additional challenge of how to safely complete the annual testing and maintenance access will affect the design. These example spaces would lend themselves to the use of beam smoke detection in the atria and aspirating smoke detection in the restricted access areas.
Beamed ceilings with many beam pockets present a design challenge; the design could use the same two types of detectors to provide a more economical approach than using spot-type smoke detectors in each pocket, while meeting the same detection goal. When judging the economics of required smoke detection, designers must look beyond the initial installation costs and include the life cycle costs.
As a general rule, anytime designers have concerns about maintenance access, they should evaluate which type of smoke detection to use. For example, meeting the installation requirements for duct smoke detectors may put them in ducts that become virtually inaccessible after the completion of construction.
This means that, without a doubt, these detectors will never receive proper testing or maintenance.
Designers should also remain aware that duct detectors may not substitute for area smoke detection. Due to smoke dilution in the ducts, the duct detectors will respond more slowly than an area detector.
In the areas where smoke detectors become inaccessible, designers should include smoke detection controls remote from the inaccessible area. The use of aspirating smoke detection can accomplish this goal. In some cases, the designer can locate beam smoke detectors so that they remain accessible for testing and maintenance.
The maintenance of smoke detectors helps ensure the operational reliability in the space.
As a goal, the designer must make sure the smoke detection provided will not become prone to nuisance alarms. Paying attention to the smoke detectors installed in inaccessible spaces, and ensuring access for both annual testing and for maintenance as required, will help to alleviate nuisance alarm problems. Additionally, avoiding placement in dirty spaces will also reduce or eliminate nuisance alarms. If these spaces must have smoke detection, then the designer must consider using something other than spot-type smoke detection.
The bottom line is designers must understand smoke detector applications and placement. There is more to smoke detection than just following the code and the manufacturer’s guidelines.
Don’t assume all designers have equal knowledge and experience. If a designer has a strong understanding of fire protection principles you will find their designs more reliable, more nuisance-alarm-free and more economical.