As promised last month in the discussion on when the use of a heat detector should be considered, we now turn our attention to the basic requirements for the spacing of these detectors. There are two NFPA 72 sections within Chapter 17, Initiating Devices, that need to be consulted. They are 17.5, Requirements for Smoke and Heat Detectors, and 17.6, Heat-Sensing Fire Detectors. One may also consider Section 17.3, Performance-Based Design, but only if you are a professional engineer with a competency in fire protection.
While I am referencing the 2013 edition of NFPA 72 for this discussion, a number of its requirements were present all the way back in the first edition of NFPA 71-D, Thermo-Electrical Fire Alarms, published more than a century ago in 1899. At that time the devices were referred to as “thermostats.” Some of the requirements from the 1899 edition are shown in the sidebar to this article. So let’s get started reviewing what you need to know based on nearly 115 years of wisdom.
How and Where to Begin the Process
Section 17.5 covers requirements for both smoke and heat detectors. This section needs to be consulted first when starting the process of laying out the locations in which heat detectors for a project are to be mounted. The first rule to consider is that a detector is not to be recessed mounted unless it is listed for that use.
Partitions within a protected premises are to be looked at next. If the partitions extend to within 15% of the ceiling height, the spaces separated by the partitions shall be considered as separate rooms. For example, if you have a room with a typical ceiling height of 10 feet, any partition that exceeds 18 inches to the ceiling would invoke this rule.
Section 17.5.3 covers detector coverage. I will not go into that this time around, saving that area for another column. However, the designer or installer does need to be aware of the four categories: Total (Complete), Partial or Selective and Non-Required.
Section 17.6, Heat-Sensing Fire Detectors, contains the majority of the spacing requirements that are to be followed. When using heat detectors, the first item to consider is the expected performance objective of the system. This is different than performance-based design, and you should not confuse one for the other. In this case, the designer is to provide a simple narrative of what the expected results of the system with the heat detectors shall be. The same requirement is found in Section 17.7, which covers heat detectors.
The next item that needs to be considered is the ambient ceiling temperature of the space the detectors are to be installed and the temperature rating of the detectors. Depending on the detector temperature and type (fixed or rate-of-rise), the spacing of the detectors will be affected. The temperature rating of the detector must be 20° F below the expected maximum temperature at the ceiling.
3 Steps to Setting Detector Spacing
Once the type of heat detector has been selected, the spacing of the detector may begin. Through the years, I have used the following three steps: 1. Ceiling height; 2. Ceiling slope; and 3. Ceiling projections. This is not the order these items are found within NFPA 72, but I have found that taking into consideration the height of the ceiling first will simplify matters in the end. When looking at the ceiling height, slope should also be looked at. Not all ceilings are flat.
Table 188.8.131.52.1, Heat Detector Spacing Reduction Based on Ceiling Height, should be looked at first. If the ceiling height is 10 feet or less, then the manufacturer’s published spacing is to be used. For ceilings higher than 10 feet and up to 30 feet, the table is to be used in reducing the spacing. For ceilings higher than 30 feet, performance-based design would be required. For example, a ceiling height of 15 feet would require a reduction of 0.77.
Slope is to be factored into the layout next. The majority of times, you may have a flat ceiling. But this is not always the case and should not be assumed when laying out the detectors from a set of building drawings. If the information is not clear, then it needs to be obtained either from the framing plans for the ceiling or a site visit if the building is existing. As I have told many a designer, you need to “look up” when performing a site visit.
There are two typical styles of sloped ceilings, peaked and shed. A peaked ceiling comes to an apex in the center, while a shed ceiling has a high wall on one side and a slope to a lower wall on the opposing. The degree of the slope is the next variable to consider. If it is less than 30°, all of the detectors shall be spaced based on the height of the peak, or the high point of a shed ceiling. If the slope is 30° or greater, the spacing shall be based on the average height of the ceiling. To determine the average, take the sum of the height of the highest and lowest point and divide by half.
The configuration of the ceiling is to be looked at next. While a number of ceilings are smooth, as with slope it should not be assumed. When the ceiling is smooth, the limits of the spacing is per the listing, ceiling height and slope. Half of the distance shall be used at right angles to walls.
One may also apply the rule of .7 (point seven), in which the ceiling shall have a detector within a distance equal to .7 times the listed spacing. This is very useful when dealing with irregular areas.
When looking at beams or joists within a ceiling, any protrusions of 4 inches or less are classified as a smooth ceiling. If the protrusions are greater than 4 inches and spaced at 36 inches or less to center, then the heat detector is to be placed at 50% of the listed spacing at right angles to the joist. The detectors are to be mounted at the bottom of the joist.
If the spacing of the protrusions is greater than 36 inches to center, they are to be considered as beams. In the case of beams, the ratio of the beam depth to the ceiling height is to be taken into consideration. When the ratio of the beam depth to ceiling height is greater than .10 and the ratio of the beam spacing (center to center) is greater than .40, the heat detectors are to be mounted within each beam pocket. When the ratio for the beam depth to ceiling height is less than .10 or if the ratio for beam spacing is less than .40, the detectors are to be installed at the bottom of the beams.
When a beam projects more than 4 inches, the detector shall have a spacing of two-thirds the listed spacing at right angles to the beams. When the beams project more than 18 inches and are greater than 8 feet to center, each beam pocket shall be considered as a separate area. Finally, when the beam is less than 12 inches AND less than 8 feet on center, the detectors may be placed at the bottom of the beams.
The rules for the spacing of heat detectors can be confusing at first. However, when you map out the placement of the devices on a set of plans for the protected premise using the rules, the locations for the detectors will become second nature.
Shane Clary, Ph.D., has more than 37 years of security and fire alarm industry experience. He serves on a number of NFPA technical committees, and is Vice President of Codes and Standards Compliance for Pacheco, Calif.-headquartered Bay Alarm Co.