Do you ever wonder if standard smoke detector spacing is sufficient for high ceiling areas? At what ceiling heights should you consider the possibility of smoke stratification? What device layout is best in high ceiling spaces? The NFPA Fire Protection Research Foundation (FPRF) just released their latest Phase II report “Smoke Detector Spacing for High Ceiling Spaces” to help answer these commonly asked questions. Jensen Hughes engineering firm was commissioned to conduct the research and publish findings. The Phase II report recommends new smoke detector spacing guidelines at ceiling heights of 15 ft and above (this is expected to impact the next NFPA 72 edition). Recommendations were provided for spot type and beam type smoke detectors. Both were analyzed in the researchers’ simulation models. This document is Fireray’s summary of the report and its potential impacts for system designers. The full report number FPRF-2023-08 can be found on the NFPA.org website here: Full Report. Note that “beam type smoke detector”, “beam detector”, “photo-beam” and “beam” are all used interchangeably. The main takeaways are:
Phase I of the FPRF report (Written by ARUP in 2017) analyzed various codes from around the world covering smoke detectors in high ceiling spaces. It was determined that compared to other countries’ codes, NFPA 72 is severely lacking in prescriptive language to address smoke detection based on ceiling height. It pointed out the need for further research and testing to help provide designers and code enforcement officials with more clarity on the subject.
The new Phase II report utilizes data generated from simulations carried out in the latest FDS software. The simulations were modeled in a space with 67 ft Length x 67 ft Width footprint (constant). The ceiling varied from 10-60 ft height. All ceiling height test scenarios incorporated the same slow t^2 flaming type design fire which grows from 0 to 1.5 MW in 12 minutes.
Minimum detection time criteria was developed through modeling each detector type using standard NFPA 72 spacing (30 ft for spots, 60 ft for beams) in the 60 ft x 60 ft control space using a 10 ft high ceiling scenario. In this baseline situation with standard spacing, spot type detected smoke at 300 seconds (260 kW fire) while the beam type smoke detectors detected smoke at 145 seconds (60 kW fire). These detection times were used as the standard performance thresholds needed for each respective detector type in the higher ceiling scenarios. This means that beam smoke detectors were expected to detect smoke in less than half the time as spot type detectors for all test configurations.
(Download our technical pocket guide at www.ffeus.com for a visual breakdown of the NFPA 72 detector spacing guidelines)
As the ceiling height for the simulations increased incrementally, spot detectors with standard 30 ft spacing could not maintain the performance standard at 15 ft ceiling height and above. This required a reduction of spacing over 10 ft ceiling height. For spots to detect smoke within the standard 300 seconds (260 kW fire) at 40+ ceiling height, detector spacing would need to be reduced to under 10 ft apart. Due to this being impractical, performance-based design was recommended for ceiling heights over 40 ft when using spot detectors. Here is the recommended spacing for spot detectors:
Figure 55. Successful maximum spot type detector spacing for ceiling heights between 10 and 40 ft that meet the threshold optical density of 0.215 m-¹ for 260-kQ fire.
Table 13. Reduced smoke detector spacing for high ceilings based on a 260-kW fire and a detection threshold of 0.215m^-1
|Ceiling height (ft)||Spot type detector spacing (ft)|
Even though beam detectors were held to a much higher performance standard than spots (145 seconds detection time, 60 kW fire), the standard 60 ft spacing could be maintained up to ceiling heights of 20 ft. As the ceiling height increased to 40 ft, beam spacing needed to be tightened to 20 ft apart. Just like spot detectors, performance based design is recommended for ceiling heights above 40 ft. Here is the recommended spacing for beam detectors:
Figure 56. Successful maximum beam detector spacing for ceiling heights between 10 and 40 ft that meet the threshold obscuration of 90% for a 60-kW fire.
Table 15. Successful beam spacing based upon curve fit equation.
|Ceiling height (ft)||Beam type detector spacing (ft)|
The recommended spot and beam spacing tables might appear similar at first glance. However, keep in mind that the reduced spacing for spots needs to be applied on all sides of the device. For instance per the recommendation, spot detectors in a 30 ft high ceiling scenario would be spaced 20 ft apart providing 400 sqft of smoke coverage per device. This is a 55% reduction in coverage area compared to the standard 30 ft spot spacing (900 sqft). Beam detectors on the other hand can still reach their max beaming distance of 394 ft, for instance, regardless of the spacing. This means the reduced spacing only applies in the devices’ lateral or side-to-side direction. Using the same 30 ft high ceiling space, a Fireray beam detector would cover 394 ft x 40 ft reduced spacing giving you up to 15,760 sqft of coverage per device. This is only a 33% reduction in max coverage area. In this instance a beam type smoke detector provides up to 39 times more coverage than a spot detector in the same room. Here is a breakdown of both detector types at the different ceiling heights between 10-40 ft.
|Ceiling height (ft)||Recommended Spot Spacing (ft)||Recommended Beam Spacing (ft)||Max Spot Detector Coverage per Device (sqft)||Max Beam Detector Coverage per Device (sqft)||Max Number of Spots that can be Replaced with a single Beam Detector|
You might be wondering how smoke stratification played into all of this. Stratification is when the smoke is less buoyant than the warmer air above and stops rising. Using the 60 ft ceiling height scenario, the simulated smoke began to stratify at a 40 ft elevation (200 seconds, 120 kW fire). As the fire gained energy, the smoke broke through the 40 ft stratification layer shortly after but again became stagnant and stratified between 45 ft to 55 ft (400-500 seconds, 500-750 kW fire).
There is analysis of detection times by beam detectors at 30, 40, and 50 ft height in a 60 ft high ceiling space. It is acknowledged that mounting additional beam detectors at 40 ft elevation in a 60 ft high ceiling space could provide limited performance benefit however 30 ft and 50 ft heights do not. Tightening up the spacing of beams to 30 ft at ceiling height proved the shortest detection time of all the beam heights in this section. It does not explore the option of beam detectors angled upwards to cover multiple levels. NFPA-92 recommends angling beam detectors up or down as one methodology in cases where smoke stratification is expected. (see a visual breakdown of beam detectors in NFPA 92 here)
Keep in mind that the fire detection threshold for beams was set at a constant 90% obscuration or the point where only 10% of light passes through the smoke. UL recommends a much lower threshold setting at this 60 ft beaming distance: between 15% to 55% obscuration for Fireray depending on the model. Jensen Hughes recognizes this in the report and points out their adjustment factor of 1.43 that accounts for model bias. This would equate to about 63% obscuration threshold for the beam detectors in this simulation which is still more conservative than the real-world scenario.
The author acknowledges that the two different benchmark performance standards established for spot detectors and beam detectors are not tied to any specific life safety criteria. They are simply using the detection times of each detector type with standard spacing at 10 ft high ceilings for all higher ceiling height scenarios. They point out that “a standard for smoke sensing detection performance criteria needs to be developed that is independent of the test standard used for listing of the devices. The performance criteria should be quantifiable and versatile so that it can be applied in device testing and listing application, modeling and performance-based design, and device installation and commissioning.”
The report also points out that the spacing for beam smoke detectors is unclear in NFPA 72. The prescriptive requirement in NFPA 72 is to consider each beam like a row of spot type detectors and to use the spacing requirements specified for spots. This results in 30 ft lateral spacing for level ceilings. However, there is a non-enforceable annex section that allows beam spacing up to 60 ft. The report authors recommend re-evaluating the enforceable part of the code to align with the annex and manufacturer’s listing requirements.
It will be interesting to see how the NFPA Technical Committee uses these recommendations for the next NFPA 72 edition in 2025. If you have any questions or if Fireray can assist on an upcoming project, visit www.ffeus.com or email us at firstname.lastname@example.org.