In the 1970s, UL and NFPA 72E, Standard for Installation of Detection Devices (last published in 1990), recommended a guideline of 30 feet on-center for the spacing of spot-type smoke detectors installed on a smooth 10-foot-high ceiling. Currently, NFPA 72-2022, National Fire Alarm and Signaling Code, contains that same NFPA 72E guideline published 34 years ago! (Keep in mind that smoke detectors have never had a listed spacing.)

Does the fire protection community’s acceptance of this spacing guideline all these years crystallize it into good practice? Is there a record of acceptable performance of smoke detectors installed at this spacing? No published research proves this is the correct spacing for spot-type smoke detectors installed on smooth 10-foot ceilings.

Do smoke detectors installed as recommended have an impact on life safety? Yes, especially in the residential environment. But the exact time the detector will operate in a fire condition is unknown.

Smoldering, low-energy fires are the most difficult type to detect using a spot-type smoke detector on the ceiling. Smoke detection relies on the smoke plume (like an inverted cone with the tip starting at the fire source) produced by the fire to transport smoke upward and the ceiling jet to move the smoke across the ceiling to the spot-type detector located on the ceiling. 

As stated in the Annex A of Chapter 17: “A hot, energetic fire produces large plume velocities and temperatures and hot, fast ceiling jets. This minimizes the time it takes for the smoke to travel to the detector. A smoldering fire produces little, if any, plume, and no appreciable ceiling jet. Far more time elapses between ignition and detection under this circumstance.”

For the detector to alarm, the appropriate amount of smoke in the chamber must equal or exceed the concentration of the detector’s alarm setting. “Normal” sensitivity is usually 2% per foot obscuration measured in the UL testing apparatus. Where there are explicit design performance objectives for the response of the smoke detection system, use the performance-based design methods outlined in Annex B.

Since we have great difficulty in predicting smoke detector operation in a smoldering fire, “early warning” takes on a different meaning in this case. Early warning implies that as soon as the fire starts, we will get an alarm. In fact, detection time always depends on several factors, including what material is burning and whether the fire has enough “power” to move the smoke with the right concentration to the detector.

Exact measurements

However, many authorities having jurisdiction enforce the exact measurement of 30 feet between smoke detectors, as if moving a detector 1 or 2 feet in any direction on a 10-foot-high flat ceiling would cause the detector to fail to detect smoke. (Maybe the dictionary definition of “guideline” should be included in the code.)

I accept—and promote—that smoke detectors properly applied, installed and spaced will provide early warning of a fire (at least earlier than a heat detector). Answers to the following questions will guide the design process:

• Will the warning occur early enough in the fire development?
• If the smoke detectors are installed on a different spacing, can we accurately predict the detection of smoke at a specific stage of fire growth?
• What type of smoke detector will best serve each application?
• At what spacing should a detector be installed for optimum detection?

Unfortunately, there aren’t definitive answers. Technology has advanced to a point where designers could develop better-­defined smoke detector application and location (spacing). However, it appears the “minimum” requirement has become the “good practice” in smoke detector spacing. 

More research is obviously necessary to determine how smoke detectors react to various fires (type of combustibles) and how spacing relates to that detection. Remember how little is really known about smoke detection and the installed spacing of smoke detectors. And finally, it’s important to understand what the current installation spacing “good practice” really means. At the end of the day, the customer needs to understand the limitations of smoke detector placement, as well as the danger of installing too few smoke detectors and still expecting early warning of fire.

“An educated consumer is our best customer” is a slogan coined by Sy Syms, the former CEO of Syms Corp., in the 1980s. And it certainly applies to smoke detector installations.

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