What makes a fire sprinkler special?

Special sprinklers provide unique solutions for conditions that are not adequately addressed by standard spray sprinklers, though they come with limitations on use that designers should familiarize themselves with

By Kyle J. Lehman June 6, 2022
Courtesy: SmithGroup


Learning Objectives

  • Understand the basics of NFPA 13: Standard for the Installation of Sprinkler Systems requirements for special sprinklers.
  • Learn about the various types of special sprinklers.
  • Review an example of specialty window fire sprinklers.


NFPA 13: Standard for the Installation of Sprinkler Systems, 2019 edition, Section defines a special sprinkler as one that has been “tested and listed” to protect a specific condition in accordance with the requirements of Section 15.2. Manny Silva, chief engineer at Tyco/Johnson Controls (previously Tyco Fire Protection Products), indicated during an interview, that this allows manufacturers to satisfy unmet needs that are not addressed by NFPA 13.

NFPA 13 does not identify specific pass/fail criteria for the testing and evaluation of special sprinklers; however, Section 15.2.1 lists several considerations, such as the test configuration and ignition location, which should be accounted by the listing agency when they develop the specific test procedures and pass/fail criteria to be used in evaluating the special sprinkler. The sprinkler listing then governs the design and installation of the special sprinklers.

Special sprinkler development, evaluation and listing process

As Kevin Maughan, fire suppression technology director at Victaulic (previously at Globe Fire Sprinkler Corp.), said in an interview, development of special sprinklers generally begins with a problem. Owner, insurer and contractor feedback often provide the initial spark, though sometimes the ideas come about by happenstance.

For example, Central Sprinkler Co., a sprinkler manufacturer before it was purchased by Tyco in the late 1990s, developed the attic sprinkler after a chance run-in with BF Goodrich, the original chlorinated polyvinyl chloride sprinkler pipe manufacturer, at a UL test facility. BF Goodrich was having difficulty passing an attic fire test with its CPVC pipe because the pipe was failing before standard spray sprinklers could control the fire.

Stephen Meyer, then the executive vice president of research and development at Central Sprinkler, was at UL for other testing, saw BF Goodrich’s struggles and led development of the first attic sprinklers. Because these sprinklers were optimized for attic applications, they successfully protected the CPVC pipe, even appearing to outperform standard sprinklers.

Before a special sprinkler can be installed in the U.S., it must generally be evaluated and listed by UL. The manufacturer describes the sprinkler and target conditions, but UL ultimately develops the evaluation standard against which the sprinkler will be measured. Because this is unique to each application, there may be an iterative standard development process between the manufacturer and UL before the standard is set and tests are conducted.

UL 199: Automatic Sprinklers for Fire-Protection Service is the evaluation standard for fire sprinklers; evaluation standards for special sprinklers are generally published as an outline for investigation such as UL 199G, which is the current standard for the evaluation of attic sprinklers. Some of these outlines for investigation, such as UL 199J, were developed for the evaluation of window sprinklers, are eventually incorporated into UL 199 as part of the base standard. Other manufacturers can develop sprinklers for listing under the outline for investigation standards, though they may be limited for a time by patent regulations.

FM Global, a commercial and industrial insurer that also tests sprinklers for compliance with its own standards, follows a similar approach when approving sprinklers. Most sprinklers are FM approved as either storage or nonstorage sprinklers, with a third category of special protection sprinklers. These special protection sprinklers may overlap with NFPA 13-compliant/UL-listed special sprinklers, but they also cover additional categories, such as corrosion-resistant sprinklers, that are covered under standard UL sprinkler listings.

In addition to the UL listing process, some manufacturers will obtain additional evaluation reports from third-party services, such as the International Code Council’s Evaluation Service (known as ICC-ES). ICC-ES reports typically contain a technical evaluation of the sprinkler and identify acceptable applications with respect to International Building Code requirements. ICC-ES reports generally contain the same requirements and limitations as the UL listing, but sometimes include additional requirements or more specific examples of design approaches. They are particularly helpful for obtaining approval from a reluctant authority having jurisdiction because the reports provide a widely accepted basis for approval under the alternative methods provisions of the IBC.

Special sprinklers and their application

There are several special sprinkler makes and models that are currently available from the major U.S. sprinkler manufacturers (see Table 1).

Table 1: This lists special sprinkler models that are currently available from the major U.S. sprinkler manufacturers, sorted by application. While accurate at the time this article was written, designers are encouraged to check with manufacturers for the latest product lineup because manufacturers are continuously expanding their product lines and listing criteria for their existing sprinklers. According to Victaulic, the concealed pendent and vertical sidewall window sprinklers have passed UL testing, though they are not commercially available at this time. Note that open spray nozzles, such as the Viking C-1 and Victaulic FL-SA/NZ, are not addressed in this table. Courtesy: SmithGroup

Attic sprinklers

Attic sprinklers have deflectors that are designed to accommodate typical pitched-roof configurations (see Figures 1 and 2). While typical sprinkler spray patterns create an umbrella-shaped distribution in a 360-degree radius around the sprinkler (or 180 degrees for sidewalls), attic sprinkler deflectors are shaped to provide a specific spray pattern that follows the slopes, hips and valleys found in typical pitched-roof attic construction, protecting the same space with fewer sprinklers.

When designing systems with listed attic sprinklers, it is important to pay close attention to the design parameters in the sprinkler listing. Every sprinkler has specific limitations as to the attic geometry it can protect. It is generally not possible to mix and match attic sprinklers from different manufacturers because the various models of attic sprinklers are designed, tested and listed to work together as a system.

Figure 1


Figures 1 and 2: Shown are examples of attic sprinklers from Victaulic. Figures 1 (left) and 2 show variations of the back-to-back sprinkler, meant to be installed at the attic ridgeline so it can throw water down both slopes of the roof, which demonstrate the need to select a sprinkler model that corresponds to the attic application. Courtesy: Victaulic

The design criteria in the attic sprinkler listings establish position and spacing limitations and generally also establish the minimum flow, pressure and number of sprinklers to be included in the required hydraulic calculations. It should also be noted that additional standard spray sprinklers — designed using NFPA 13 criteria — may be required in portions of the attic with geometry that cannot be protected within the boundaries of the attic sprinkler listing. Designers must be careful in such cases to design and calculate the system in such a way that addresses both the attic sprinkler and standard NFPA 13 design criteria. Additional compartmentation may be required in these cases.

Most manufacturers offer technical support when designing attic systems, particularly for edge case conditions that don’t fully conform to the product listing. Fire protection engineers and designers are encouraged to check for the latest design criteria before starting design. In addition, because the design criteria are specific to each product, designers who are required to develop nonproprietary specifications should seek early sole-source approval from the client for cases where only a single product will meet the design geometry and requirements.

Corridor sprinklers

Corridor sprinklers are essentially a specialized extended-coverage sprinkler, with a deflector (see Figure 3) and spray-pattern intended to direct most of the flow along one long axis–down the length of a corridor–with shorter throw distances along the perpendicular axis.

Figure 3: This shows two examples of corridor sprinklers — a concealed pendent version that is hidden from view by a flat cover plate and a second version that is installed fully exposed or in a semi-recessed position. Note the elongated, oblong sprinkler deflector in both models. In both cases, this deflector must be oriented with the long edge parallel to the length of the corridor. Courtesy: Johnson Controls Inc.

Because heat is channeled down the narrow corridor, distances as far as 28 feet between sprinklers can be accomplished without compromising the system performance. Corridor widths are limited to somewhere between 8 to 14 feet, depending on the sprinkler type and listing. Obstructions, such as soffits, require extra attention to the listing.

During installation and inspection, it is crucial to review the orientation of the deflector relative to the corridor. Unlike standard spray sprinklers with unidirectional deflectors and spray patterns, the “long throw” axis of the deflector must be oriented down the length of the corridor. In addition, there are specific restrictions on the installation of other sprinklers in the vicinity of the corridor sprinklers to prevent cold soldering; this must be accounted for in design and verified during installation.

Note that Viking’s corridor sprinkler is listed as an extended-coverage sprinkler, while Reliable and Johnson Controls corridor sprinklers are listed as special sprinklers. In practice, their application is similar, though they fall under different provisions of NFPA 13.

Window sprinklers

Architects often try to visually connect spaces within their buildings by introducing interior windows, even in locations that require fire-rated walls. In many applications, this requires the installation of fire-resistive glazing that has been tested to the same test standard, ASTM E119: Standard Test Methods for Fire Tests of Building Construction and Materials, as solid walls. While fire-resistive glazing is a very appealing option from a design standpoint, it is very expensive, costing several hundred dollars per square foot just for the material. Window sprinklers can be a more cost-effective alternative.

Note that this article focuses on window sprinklers, which include a heat-activated element and does not address deluge spray nozzles such as the Viking C-1 or Victaulic FL-SA/NZ. Some window sprinklers may be used as open sprinklers in an exterior deluge system, though others are limited to use with interior partitions and for protection of exterior walls against a fire originating within the building. If protection against exterior fire exposure is necessary, as is often required for fire separation distances of less than 10 feet, rated wall construction or spray nozzles/sprinklers in a deluge configuration must be used instead.

Designed to spray directly and primarily against the glazing, window sprinklers work by cooling the glazing to keep its temperature, and corresponding expansion, below the failure point of the glazing. Gasketed window frames, a key requirement of the assembly, allow the glazing to expand without failing as the glazing temperature increases.

Sprinkler designers should coordinate closely with architects early in the design process for window sprinkler applications. The architectural design must account for sprinkler listing requirements, such as glazing type, maximum height and frame type.

Glazing is generally limited to 13 feet in height and must be nonoperable. Heat-strengthened, tempered or ceramic glass is generally required and typically must be a minimum of ¼ inch thick. Horizontal mullions are prohibited since they may block water distribution across the glazing. In addition, some method of keeping combustibles away from the glazing is required.

Per typical listings, a 36-inch-high pony wall or knee wall is recommended, though alternatives like guardrails may be approved by the AHJ. Fire-rated glazing — the ceramic glass noted above — can often be used in conjunction with the window sprinklers when a knee wall or other physical barrier is not feasible.

Sometimes the listing specifies a particular make and model of glass to be used in this application, such as the TGP Firelite Plus WS ceramic glass identified in the Tyco/Johnson Controls window sprinkler listing. Though more expensive than nonrated glazing, the ceramic glazing used in this case is still less expensive than the ASTM E119-tested glazing that would be required without the window sprinklers.

It may be tempting to use operational controls, such as making housekeeping responsible for maintaining materials away from the glazing, as a method of compliance, but in practice these are nearly impossible to enforce and should not be used in lieu of a physical barrier.

Listed window sprinklers typically require spacing of 6 to 12 feet between sprinklers. Window sprinklers must generally be located between 6 to 12 inches away from the glass and it is critical that no window treatments, such as rolling shades or curtains, be located between the glass and the sprinkler, though they may be allowed on the room side of the sprinkler (see Figure 4).

Figure 4: This shows a section view of a typical window sprinkler application, with dimensional restrictions and requirement for a pony wall/knee wall. Courtesy: Johnson Controls Inc.

One might recognize some similarities between the installation requirements for special-application window sprinklers and the closely spaced, standard spray sprinklers that, per IBC-2018 Section 404.6, Exception 1, may be installed at nonrated glazing around atriums in lieu of one-hour atrium walls.

However, the prescriptive requirements of the IBC only permit use of standard spray sprinklers and glazing in the atrium separation application; the prescriptive provisions do not permit sprinklers and glazing to serve as a general substitute for fire-rated walls.

In contrast, the special-application window sprinklers discussed here are intended for use in a broader range of applications and, with approval from the AHJ, may be substituted for fire partitions, fire barriers and rated exterior walls up to a two-hour rating. Approval must generally be obtained through the alternative methods provisions of IBC Section 104.11 or the local equivalent. Keep this in mind the next time an architect asks if they can use a “water curtain” in lieu of rated glazing; it may be possible but comes with constraints based on the sprinkler listing and approval is not guaranteed.

Interstitial space sprinklers

Large combustible concealed spaces, which are particularly prevalent in recent construction practices using engineered wood products, often require sprinkler protection in the concealed space. This can present a challenge for traditional, standard-spray sprinklers due to the constrained dimensions and presence of obstructions that prevent development of standard sprinkler spray patterns. Interstitial sprinkler deflectors are designed with flatter spray patterns than standard spray sprinklers, allowing them to work better in the confined dimensions of the concealed space.

In addition — and perhaps a more important factor from an economic standpoint — CPVC sprinkler pipe can only be installed in combustible concealed spaces that require sprinkler protection if it is protected with sprinklers that are specifically listed for that application. Much as attic sprinklers were originally developed to enable installation of CPVC sprinkler pipe in combustible attics, the primary economic advantage of interstitial sprinklers is enabling the use of CPVC sprinkler pipe in combustible concealed spaces where it would otherwise be prohibited.

Note that different CPVC pipe manufacturers may have slightly different requirements, even for products based on the same source material. For example, both Tyco/Johnson Controls and Viking CPVC pipe products allow installation in combustible concealed spaces with special-application sprinklers, while manufacturer instructions for IPEX CPVC pipe prohibit installation in combustible concealed spaces altogether for NFPA 13-compliant systems. All three are manufactured from Lubrizol Blazemaster CPVC material but have different installation requirements and limitations.

CPVC sprinkler pipe generally may be used without additional sprinkler protection in concealed combustible spaces that do not require sprinklers in accordance with NFPA 13, 13R or 13D. These spaces are typically limited in size or filled with noncombustible insulation. Again, the manufacturer requirements should be referenced and compared against the NFPA requirements.

Designers who are required to specify multiple sources or who are tasked with reviewing product data submittals should review the manufacturer instructions for each proposed product against the unique conditions of the project to verify proper application. As with the other special sprinklers discussed, compliance with the product listing is critical for a code-compliant design and installation.

Author Bio: Kyle J. Lehman is a principal, fire and life safety engineering, at SmithGroup.