Unique building spaces can defy the common approach to detector design, so that typical spacing, device type and installation does not work.
It is always the fire alarm system installer’s responsibility to install what a designer shows on the plans. But if the device is in a unique space, it should be brought to the owner and system designer’s attention.
High-ceiling areas
Detection in high-ceiling areas is the most common example. I arbitrarily consider anything over 15 feet a high-ceiling environment. Most spot-type smoke detection designed and installed in a high-ceiling environment will not provide any semblance of early warning, which is the usual design goal for smoke detection.
A high-ceiling spot-type smoke detector may not provide early warning because, for low-energy or smoldering fires, the effects of the environment (e.g., temperature of the space or airflow from the HVAC system) will be paramount. These represent common “barriers” to effective detection.
The obvious result is the smoke will not rise and push through the ambient temperature until the smoke is hotter. Airflow will dilute the smoke, thus keeping at bay the necessary amount of smoke to reach and actuate a smoke detector. Smoke detectors on high ceilings also increase maintenance costs and false alarms in the future, both of which negatively affect the owner’s budget.
As a rule of thumb, any detector that requires more than a 7-foot stepladder to install should be highlighted as a future problem and brought to the owner’s attention. Never assume the designer gave any thought to ceiling height when drawing circles and squares on a design. Ultimately, you will be the one called and blamed by the owner for the issues.
A table in NFPA 72, National Fire Alarm and Signaling Code, provides guidance on reducing the spacing of heat detectors on high ceilings. Table 17.6.3.5.1, Heat Detector Spacing Reduction Based on Ceiling Height, shows that for a ceiling height between 28 and 30 feet, the listed spacing of any heat detector used must be multiplied by 0.34, and resultant spacing used on that ceiling.
When pricing a bid for this space, use the most sensitive heat detector (some heat detectors on the market have a 50 foot on-center sensitivity) that can be used.
Assuming this was the detector you wanted for the installation at that height, you would need to design them at 17 foot on-center to obtain the same detection result as if the ceiling height was 10–12 feet and the devices were installed at 50 foot on-center.
No table for smoke detectors
Unfortunately, there is no similar table for smoke detectors. Intuitively, you would expect better performance of spot-type smoke detectors if they were spaced closer than 30 foot on-center, but there is nothing in the code to give you any guidance. Generally speaking, as the ceiling height increases above 15 feet, a linear beam smoke detector would serve the space better. But again, for spot-type heat and smoke detectors of any type, detection will be delayed until the fire gets larger.
Other ceiling types
For joisted and beam ceilings, the code provides guidance for installation. For spot type heat detectors, Section 17.6.3.3.1.2 requires that where the beams project more than 4 inches below the ceiling, the spacing of heat detectors at right angles to the direction of beam travel must not exceed more than two-thirds of the listed spacing. Where beams project more than 18 inches below the ceiling and more than 8 foot on-center, the code requires each bay formed by the beams to be treated as a separate area.
Smoke detection is affected by several issues besides ceiling height. The code requires the designer and installer to consider the following ambient conditions listed in Section 17.7.1.8:
- Temperature below 32°F
- Temperature above 100°F
- Relative humidity above 93%
- Air velocity greater than 300 ft./min.
The only exception is if the smoke detector is specifically designed and listed for the expected conditions.
Explosion-proof areas
Another common scenario, especially in industrial settings, is the need for a listed explosion-proof heat or smoke detector. All the above requirements also apply to each detection requirement in these areas.
The importance of knowing the areas where detection will be installed cannot be overstated. Annex A and the NFPA 72 Handbook provide valuable guidance to help install a code-compliant fire alarm system. I highly recommend reading both.
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About The Author
MOORE, a licensed fire protection engineer, was a principal member and chair of NFPA 72, Chapter 24, NFPA 909 and NFPA 914. He is president of the Fire Protection Alliance in Jamestown, R.I. Reach him at [email protected].
