Residential sprinklers — Part II

By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu

Last month we noted the 2012 edition of the International Residential Code will continue to contain a requirement for sprinklers for all new one- and two-family dwellings and with that all major U.S. model building and fire codes now are of one voice in their mandate for sprinklers for all new residential occupancies including one- and two-family dwellings.

The standard used for the design and installation of these sprinklers systems is NFPA 13D, Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes. The current edition of this standard is the 2010 edition. In this article I would like to address some of the considerations for NFPA 13D systems designs. First, however, a little history.

In 1973 the National Commission on Fire Prevention and Control published “America Burning.” The report documented the fire problem in the United States and in particular the significant numbers of injuries and deaths which were occurring in our nation’s dwellings. Included in the report’s many recommendations was a recommendation to establish the U.S. Fire Administration and a recommendation that the proposed USFA “… support the development of the necessary technology for improved automatic fire extinguishing systems that would find ready acceptance in all kinds of dwelling units.” This technology would, of course, be the residential sprinkler system. The focus on residential fire deaths provided in this report led to two significant developments in the industry. The first was the development of a sprinkler standard for use in homes, NFPA 13D, the first edition published in 1975. The system contemplated by this standard would have the primary objective of life safety. It was also intended to provide for a relatively low cost fire sprinkler system. The second significant development was the development of fast response sprinkler technology and the residential sprinkler, a sprinkler that could prevent flashover in a compartment and meet the life safety objective of NFPA 13D.

Here are a few of items I feel are important to consider when designing 13D systems. Because the primary goal of a 13D system is protection of life rather than property, NFPA 13D allows the omission of sprinklers from areas where, statistically, the potential for life loss due to fire originating in the areas is low. These areas include bathrooms with an area of less than 55 square feet. Also, closets that are less that 24 square feet with a minimum dimension of 3 feet and are lined with non-combustible or limited combustible materials (the term limited combustible is most often intended to describe gypsum board). Per NFPA 13D paragraph 8.6.5 sprinklers are not required “in attics, penthouse equipment rooms, elevator machine rooms, concealed spaces dedicated exclusively to and containing only dwelling unit ventilation equipment, floor/ceiling spaces, elevator shafts, crawl spaces, and other concealed spaces that are not used or intended for living purposes and do not contain fuel-fired equipment.” Sprinklers may also be omitted from garages, open attached porches (in Hawaii we call these lanais), and carports. One interesting note here is there have been some recent notable life loss incidents in sprinklered homes in which a fire originating in a porch area and then spread into the home. We may see some changes in future editions of the standard resulting from these incidents.

The allowance to omit sprinklers from certain areas is intended to optimize the life safety benefits of the sprinkler system with the cost. Though full coverage is not required by NFPA 13D, the document does recommend full coverage noting that the additional protection serves to enhance the life safety benefit and add a measure of property protection. I definitely agree that if at all possible provide a full coverage system.

In addition to sizing piping by hydraulic calculation NFPA 13D now permits an alternative prescriptive pipe sizing method. This method starts with a static pressure and uses a series of tables to determine pressure losses to the most remote sprinkler. Regardless of the method chosen the system must meet requirements for both a one sprinkler and two sprinkler flow. For residential sprinklers that have no specific flow and pressure listings the minimum flow from a single sprinkler must be 18 gpm and the minimum flow from two or more operating sprinklers is 13 gpm each. For residential sprinklers that are listed with specific discharge criteria then the one- and two-sprinkler flows must be in accordance with the listing. For these specially listed residential sprinklers there is also the additional requirement that the discharge density shall not be less than 0.05 gpm/sf. The minimum operating pressure for residential sprinklers must be the higher of 7 psi or the listed operating pressure of the sprinkler at the listed flow. It is noted that the maximum two sprinkler design applies to rooms with “flat, smooth, horizontal ceilings.” The standard does not provide requirements in the case where ceilings are not “flat, smooth, and horizontal” but instead, suggests in the Annex that a design including three or more sprinklers may be needed. Also, in addition to the sprinkler flow, if the water service supplies more than one dwelling unit, the flow calculation must include an additional 5 gpm unless the system is provided with an interlock preventing domestic flow upon operation of the sprinkler system.

When the domestic water service is used for the sprinkler supply one must ensure that the service pipe diameter and water meter are capable of supporting the required sprinkler flow. Given that the minimum two sprinkler flow is 26 gpm it is questionable that a 5/8-inch water meter will be adequate. Where the two sprinkler calculation results in flows 30 gpm or greater a 3/4-inch or 1-inch meter may be necessary. If costs of larger sized meters from the water purveyor become an issue one may wish to consider having a pump and tank for the water supply. A minimum 10-minute supply would be required. If the dwelling unit is only one story in height with an area less than 2,000 square feet the stored water supply need only contain a seven-minute supply.

Unlike systems designed to NFPA 13 or 13R, NFPA 13D systems do not require a fire department connection. NFPA 13D are required to have a local water flow alarm unless the dwelling is provided with smoke alarms conforming to NFPA 72 The National Fire Alarm and Signaling Code. With respect to sprinkler control valves, if the 13D system has a control valve separate from the domestic service shut off then that valve must be supervised by either locking the valve in the open position or by electrical supervision with alarm at a constantly attended location, or by central, proprietary or remote station service.

Since the 2007 edition of NFPA 13D, the standard has recognized the use of dry pipe and preaction systems in addition to wet pipe sprinkler systems. One must exercise caution in using either of these systems because the standard requires sprinklers used on dry pipe or double interlock preaction systems be specifically listed for use on these systems while also requiring the use of listed residential sprinkler areas in living areas. The problem is I believe there are currently no listed residential sprinklers that are also listed for use in dry pipe (or double interlock preaction) systems.

In the 2010 edition of NFPA 13D it is clarified that a townhouse may be sprinklered using NFPA 13D. The standard defines a townhouse as:

“A one-family dwelling constructed in attached groups of three or more units in which each unit extends from the foundation to the roof and has open space on at least two sides.”

This should eliminate some confusion as some authorities having jurisdiction were requiring NFPA 13 or 13R systems because the townhouse building contained more than two dwelling units.

Also new to NFPA 13D is a requirement for qualified designers and installers. Early in the history of the 13D document some tried to make a standard simple enough so a system could be installed by a do-it-yourselfer. However, it soon became clear that qualified design and installation was a must. With the expected dramatic increase in 13D systems brought about by recent code developments it is crucial that the sprinkler industry develop a nationwide pool of qualified installers. The standard however, does not define what training or qualifications are required. It is hoped that the fire sprinkler trade associations will take the lead in developing the programs and criteria in this area.

Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer and past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc. He can be reached via email at SDannaway@ssdafire.com.