NFPA 13’s latest version adapts rules for dry and preaction systems, supplemental sprinklers, and more
NFPA 13: Standard for the Installation of Sprinkler Systems is the critical standard for commercial fire sprinkler system design and installation, adopted into law by reference in government codes in the US and internationally. Fire protection is an evolving field, so NFPA 13 committee members update this standard every three years based on data gathered in the field and the laboratory. As of this writing, the NFPA 13 2025 edition is the newest version.
The latest standard brings many changes for sprinkler system designers and sprinkler fitters—and we can’t cover all of them in detail. However, this article takes a look at some significant topics that have big impacts:
- Changes to the rules for dry and preaction systems, including new technologies
- The introduction of “supplemental sprinklers” and new design criteria for their use
Also, stay tuned for part two of this series, where we’ll cover:
- New protection options for sloped ceilings in storage occupancies and other settings
- Updated criteria for high ceilings in non-storage areas
QRFS aims to be your fire sprinkler resource. Check out our selection of commercial and residential fire sprinklers, escutcheons, cover plates, wrenches, pipe and fittings, and more parts for your system.
NFPA 13 2025 updates for dry and preaction sprinkler systems
Some of the most important changes to NFPA 13 focus on dry and preaction systems. Among them are:
- A rewrite of the section on dry sprinkler system requirements
- The introduction of two new anti-corrosion technologies for dry and preaction systems
- Updates to some C-factors (pipe friction measures) in hydraulic calculations
Let’s dive in:
1. Dry system requirements rewrite—the new version of section 8.2
Section 8.2 of NFPA 13 covers dry sprinkler systems, and it got a significant rewrite in the 2025 edition. The information provided is largely the same, but it’s just presented in a more logical order. There are 11 subsections, and the two with the biggest changes are 8.2.1 and 8.2.4.
Section 8.2.1 is about gridded pipe systems (defined in section 3.3.224.5), a design where parallel cross-main pipes feed branch lines from both ends, allowing multiple paths for water to flow to an open sprinkler. The only purpose of this new section is to forbid gridded pipe in dry systems.
Water reaches sprinklers slower in dry systems than in wet systems because a drop in air pressure within the pipe network from an open sprinkler must trip the dry pipe valve to open and flood the system. (This delay must be no more than 60 seconds, though usually fewer.) Experience has shown that gridded systems can cause significant delays in water delivery in dry systems, which is why the NFPA committee members added the prohibition.
Section 8.2.4, “Dry Pipe System Water Delivery,” combines two sections from the 2022 edition. The time it takes between a dry valve opening and water delivery is an essential concept in the design of dry systems. The 2025 rewrite consolidates all the information about this topic under one heading, including maximum water delivery times based on hazard level, pipe subdivisions using check valves, manifold test connection rules, and more.
2. Vapor corrosion inhibitors and vacuum systems used in dry and preaction systems
Corrosion can be a significant problem for dry and preaction systems. As NFPA explains, a combination of oxygen and leftover or condensed water in an otherwise dry pipe network drives the corrosion of iron, steel, copper, and brass pipes. When dry or preaction systems are filled with regular air and even a little bit of moisture, these pipes can degrade fairly soon.
Removing oxygen (one of the three sides of the corrosion triangle) can stop corrosion in pipes, and filling them with inert nitrogen gas instead of air is a common way to accomplish the goal.
Vapor corrosion inhibitors and vacuum systems are two newer solutions to the corrosion problem, and NFPA 13 2025 now gives rules on their use.
Vapor corrosion inhibitors, also called vapor phase corrosion inhibitors (VPCIs), are additives in air-filled dry and preaction systems. The vapor emitted by these additives coats the inner surfaces of pipes and can protect them from corrosive chemical reactions. The 2025 edition of NFPA 13 now includes this definition:
3.3.244 Vapor Corrosion Inhibitor (VCI) A chemical compound (substance) that emits rust-inhibiting vapor to protect ferrous and nonferrous metals against corrosion in air-filled dry pipe or preaction sprinkler systems.
NFPA 13 2025 provides specific rules for VCI installation in section 8.2.11, under the broader “Dry Systems” section. Systems with VCI need to include a listed assembly that is permanently installed “in accordance with the manufacturer’s instructions.” These assemblies must also be maintained following Chapter 32 of NFPA 13 and the manufacturer’s instructions and have “a means of verifying vapor concentration.” Interestingly, there aren’t references to VCIs in section 8.3 covering preaction systems, though preaction systems are mentioned in the standard’s VCI definition.
Vacuum systems take a very different approach than other anti-corrosion measures. Rather than replacing oxygen-filled air with an inert gas like nitrogen or blocking corrosion with vapor, this new technique sucks oxygen and moisture from the pipe with negative pressure. The latest version of NFPA 13 now has these definitions:
3.3.224.10 Vacuum Dry System.
A sprinkler system employing automatic sprinklers that are attached to a piping system containing air under negative gauge pressure, the release of which (as from the opening of a sprinkler) permits the air pressure detection to open the water flow valve, and the water then flows into the piping system and out the opened sprinklers.
3.3.224.11 Vacuum Preaction System.
A sprinkler system employing automatic sprinklers that are attached to a piping system containing air under negative gauge pressure, with a supplemental detection system installed in the same areas as the sprinklers.
Vacuum systems work and are broken out into definitions for both dry and preaction applications. So, they’re now covered under their own section, 8.11 Vacuum Systems, in NFPA 13 2025 instead of being referenced under the system type sections.
The rules here are relatively brief, stating that the vacuum equipment must be listed, the sprinklers in these systems “shall be listed for use under vacuum conditions,” and that dry and preaction systems must otherwise comply with all previous system-specific rules. Otherwise, the installation rules rely heavily on adherence to the equipment manufacturer’s instructions.
3. Updated C-factor values in NFPA 13 2025
The friction coefficient of pipes, also known as the C-factor, measures the roughness of pipes’ interiors based on their material. This number is vital for hydraulic calculations during sprinkler system design because rougher pipes eat away at the pressure and flow that drive water out of a sprinkler head—and water pressure and flow are crucial for suppressing or controlling fires effectively.
In its C-factor tables, NFPA 13 penalizes steel pipe used in dry systems because of the increased risk of corrosion. A corroded pipe is rougher than fresh, smooth pipe.
The standard assigns galvanized and black steel pipes a C-factor of 100 when used in dry systems instead of their usual 120. In the 2022 edition, the NFPA 13 committee acknowledged that having nitrogen instead of air in the pipes reduces corrosion, so they allowed designers to use the standard 120 C-factor in calculations for nitrogen systems with these materials. The 2025 update now extends this bonus to dry pipes that are protected from corrosion by VCI or vacuum systems (28.3.4.8.1).
Supplemental sprinklers—a new name for an old concept
Past editions of NFPA 13 have called for placing extra sprinklers underneath obstructions. Where large obstacles such as beams, ducts, or catwalks block the standard spacing or spray of sprinklers, designers and installers can run pipe below the obstacle to place a sprinkler, filling a coverage gap. The 2025 edition of NFPA 13 now gives these heads a name: supplemental sprinklers.
Sections of NFPA 13 2025 have been rewritten to use this new term, which has an official definition: “A sprinkler that is installed below an obstruction” (3.3.223.3.6). Beyond the vocabulary change, the NFPA 13 committee provided new guidance and requirements surrounding supplemental sprinklers. These include rules on what kinds of sprinklers can be “supplemental” and instructions on their spacing. There are also new procedures for hydraulic calculations when using these sprinklers.
Types of sprinklers used as supplemental sprinklers
There are three major points to understand about choosing supplemental sprinklers. First, the new rules state that supplemental sprinklers should be quick-response or otherwise have fast-response elements. (For example, early-suppression fast-response (ESFR) sprinklers fit the latter bill in storage environments.)
Second, supplemental sprinklers should generally have the same K-factor, orientation, and coverage type as ceiling sprinklers (9.5.5.3.3.2). There are exceptions to this rule that can apply when supplemental sprinklers need to be included in hydraulic calculations—specifically that their “design approach” can change “to be based on the hazard located directly below the obstruction.” (9.5.5.3.3.3, 19.5).
Third, supplemental sprinklers need water shields or must be “otherwise shielded” (9.5.5.3.3.4) when they are:
- Under non-flat obstructions
- Under non-solid obstructions
- Beyond the outer edges of the obstructions
- Under open grating
Like in-rack sprinklers that are common in storage environments, the placement of supplemental sprinklers can expose them to overspray from sprinklers above them. Water shields block this overspray from cooling a head’s heat-sensitive element, which can slow or prevent activation during a fire.
Spacing of supplemental sprinklers
For spacing supplemental sprinklers in non-storage environments—meaning excluding ESFR and CMSA models—the major takeaway is that supplemental sprinklers can be spaced using unobstructed construction requirements for the appropriate hazard level of what they’re protecting (9.5.5.3.4.2), no matter what is being done at the ceiling level.
For example, if someone is planning to use supplemental sprinklers in an Ordinary Hazard Group 1 (OH-1) 1 facility where the ceiling is obstructed construction, “where beams, trusses, or other members impede heat flow or water distribution,” they can space the supplemental sprinklers for OH-1 unobstructed construction (no beams, trusses, etc.). (For more info on sprinkler spacing, check out QRFS’s previous multi-part series on the topic.)
However, things are less simple for ESFR and CMSA sprinklers used in more challenging storage environments. In these situations, supplemental sprinklers have stricter spacing rules, especially where the sprinklers are placed relative to non-flat or non-solid obstructions. Why?
First, as Roland Asp at the National Fire Sprinkler Association (NFSA) explains, obstructions can present a challenge for heat collection. Hot air must accumulate around sprinklers to activate them, and some obstructions (like open grates, for example) allow heat to dissipate or collect where it’s less than ideal. In addition, ESFR and CMSA sprinklers are designed to deliver a lot more water over a hazard than other sprinkler types; thus, any obstructions can have a bigger negative impact on effective discharge in these storage scenarios.
To account for these factors, supplemental sprinklers need less space between them. The new section 14.2.10.3.5 has various spacing rulesfor these sprinklers. These rules vary based on sprinklers being below non-flat or non-solid obstructions or flat and solid obstructions of different sizes, as well as when a “flat, solid continuous horizontal barrier having the same footprint of the obstruction” is below obstructions of different sizes.
The video below shows ESFR sprinklers going off in a challenging storage fire—it’s vital that enough of these sprinklers can put out a lot of water without it being blocked:
Fire sprinkler fundamentals remain the same, but NFPA 13 updates enhance protection
The principles behind the design of fire sprinkler systems are time-tested and pretty consistent. However, NFPA 13 still needs periodic updates. New challenges and problems emerge in the real world, and new technology and data from testing laboratories create solutions. The updates to NFPA 13 2025 provide valuable guidance on new technology and clarify approaches to protecting challenging environments.
Stay tuned for part 2 of this series, where we detail additional changes:
- New protection options for sloped ceilings in storage occupancies and other settings
- Updated criteria for high ceilings in non-storage areas
If you need equipment for your fire sprinkler system, check out QRFS’s selection of commercial and residential fire sprinklers, escutcheons, cover plates, wrenches, pipe and fittings, and more parts for your system. If you have questions or need help placing and order, contact us at 888-361-6662 or support@qrfs.com.
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