When installing a residential sprinkler system in accordance with NFPA 13D, Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes, the purpose is limited to detecting and controlling residential fires so that the room of origin does not flashover and occupants have time to escape.
NFPA 13D was developed to provide an economical option to install life-safety systems in homes, but when it comes to installing sprinkler systems in residential occupancies, NFPA 13D is not the only choice. The higher levels of protection provided by NFPA 13, Standard for the Installation of Sprinkler Systems, and NFPA 13R, Standard for the Installation of Sprinkler Systems in Low-Rise Residential Occupancies, may be preferred.
1. Scope. Both NFPA 13D and NFPA 13R have limited scopes. NFPA 13R limits installations to residential structures of four stories or less and no more than 60 feet in height, measured from the grade plane to the top of the building. For NFPA 13D, the scope is limited to one- and two-family dwellings. Notably, there is no size restriction on the dwelling, so a massive mansion is treated no differently than a cozy cottage when applying the requirements of NFPA 13D.
NFPA 13 has the broadest scope. Any building can be sprinklered in accordance with NFPA 13, and the additional protection to the structure may be valuable to an owner. When considering a cost-benefit analysis, the additional sprinklers necessary to provide protection against injury, life loss, and property damage in an NFPA 13 system may be equitable when compared to the more lenient requirements of NFPA 13R/13D that focus more on life safety.
NFPA standards provide minimum requirements that provide a reasonable degree of protection. It is the role of the engineer to determine if any additional requirements are necessary to provide satisfactory performance on a specific project.
2. Water supplies. Once the installation standard is selected, it is important to highlight the differences in the installation requirements among the three standards. For the water supply, NFPA 13 and NFPA 13R require similar sources. Acceptable sources of water for the sprinkler system include:
• Waterworks systems;
• Fire pump in accordance with NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection;
• Water storage tank in accordance with NFPA 22, Standard for Water Tanks for Private Fire Protection; and
• Other sources acceptable to the Authority Having Jurisdiction (AHJ).
To provide a cost-effective system under NFPA 13D, the following sources can be used:
• Waterworks system;
• Elevated tank;
• Pressure tank in accordance with American Society of Mechanical Engineers (ASME) standards;
• Pumps with sufficient pressure and capacity.
NFPA 13D systems drop the references to NFPA 20 and NFPA 22 and do not require listed fire pumps or tanks to supply these residential systems. This makes NFPA 13D much more economical.
3. Listing requirements for system components. NFPA 13 and NFPA 13R will again fall along the same lines for listing requirements of system components. NFPA 13 requires all materials and devices essential to system performance to be listed (unless otherwise specified), and NFPA 13R requires all materials and devices to be listed as specified in the standard.
Essentially, if the product affects the ability of the sprinkler system to operate correctly in the event of a fire, it must be listed. The one exception to this is metallic pipe. Metallic pipe and fittings only need to meet the ASTM/ASME standards listed in Chapter 7 and Chapter 5 in the 2019 editions of NFPA 13 and NFPA 13R, respectively. Nonmetallic pipe must always be listed.
For NFPA 13D, the converse is true. Pretty much every component is not required to be listed except for sprinklers and nonmetallic pipe. Since sprinkler systems in single-family homes are often installed by plumbers, in addition to CPVC that is recognized for use in NFPA 13 and NFPA 13R, PEX tubing is permitted to be used in NFPA 13D systems when listed for fire protection use.
4. Underground pipe. The pipe connecting the sprinkler system to the point of connection of a public water supply is required to follow the requirements of Chapter 6 in NFPA 24, Standard for the Installation of Private Fire Service Mains, 2019 edition, which are extracted into NFPA 13. In the 2019 editions of NFPA 13R and NFPA 13D, only the plumbing codes need to be followed for the installation of underground piping.
One clarification is that when the size of the incoming service is 4 inches or greater in an NFPA 13R system, the installation should follow NFPA 24. For the 2022 edition of NFPA 13R, which is currently available, the committee moved the suggestion to the body of the standard and now requires NFPA 13R systems to have underground piping installed in accordance with NFPA 24 when the pipe size is 4 inches or greater.
5. Use of sprinklers. NFPA 13 requires sprinklers installed in light hazard occupancies to be listed as quick-response sprinklers and permits listed residential sprinklers to be installed in dwelling units and their adjoining corridors.
NFPA 13R and NFPA 13D take a listed residential-sprinkler-first approach. Residential sprinklers are required to be used in NFPA 13R systems. Up to four quick-response sprinklers can be installed in dwelling units that meet the definition of a compartment. Mechanical closets are permitted to use quick-response sprinklers due to the higher temperature rating typically required and not available in residential sprinklers.
NFPA 13D also requires listed residential sprinklers with the same exception for spaces requiring sprinkler types that are not manufactured in the residential style, including unheated areas using dry-type sprinklers, mechanical closets and saunas/steam rooms (if you are so lucky to have one in your home).
6. Water flow alarms. With life safety being the primary focus of residential systems, occupant notification becomes paramount to begin the egress process. Water flow alarms are required on all sprinkler systems, with two exceptions. For an NFPA 13 system, when the total number of sprinklers installed on the system is less than 20, no local alarm is required.
The second is when smoke alarms are installed in accordance with NFPA 72, National Fire Alarm and Signaling Code, NFPA 13D permits the installation of the local alarm to be omitted. With virtually every single-family home required to have smoke alarms, the water flow alarm is rarely installed on an NFPA 13D system. While the smoke alarms will activate before a sprinkler, it may be beneficial to specify a water flow alarm anyway to alert the owner or tenant of any potential leaks.
7. Hydraulic calculations. When calculating residential sprinkler systems, each standard requires a slightly different design area and discharge density. For NFPA 13 sprinkler systems using listed residential sprinklers, the four most demanding adjacent sprinklers, without respect to walls and compartments, are selected as the remote area and must discharge at a minimum of 0.1 gallons/minute (gpm)/square foot or the listing of the sprinkler, whichever is greater.
When quick-response sprinklers are used in residential occupancies in an NFPA 13 system, design options are limited to the density/area method or room design method. In the 2022 edition of NFPA 13, the committee clarified that the room design method is an option for listed residential sprinklers.
NFPA 13R systems also require a four-sprinkler calculation, but with one caveat — only up to four sprinklers within a compartment are required to be calculated, and a reduced density of 0.05 gpm/square foot is permitted to be used or the listing of the sprinkler, whichever is greater. So, if a room meeting the definition of a compartment only has three sprinklers installed, only three need to be calculated. An extra sprinkler in an adjacent space does not have to be picked up.
NFPA 13D also permits the lower density of 0.05 gpm/square foot or the listing of the sprinkler, whichever is greater, but with even fewer sprinklers flowing. In NFPA 13D, the discharge and hydraulics calculations chapter describes five ceiling scenarios in section 10.2.1. Ceilings meeting those specifications require up to two sprinklers within a compartment to be calculated.
For any ceiling arrangement not described in section 10.2.1, the design criteria must be in accordance with the sprinkler’s listing or the AHJ must be consulted when no data is available.
The requirements of residential sprinkler systems are similar across the three installation standards, with a few variations to provide cost-saving options for homeowners. In the end, a sprinkler system is a life-safety system with the intent of providing a reasonable degree of protection for life (NFPA 13/13R/13D) and property (NFPA 13/13R) from fire.
Where additional protections are necessary or desired, it is up to the engineer to specify. Many clients might want the additional property protection an NFPA 13 sprinkler system provides.