By Dave Purkiss

The purpose of NSF/ANSI Standard 50 (NSF 50) is to establish minimum requirements for materials, design, construction and performance of equipment commonly included in the water circulation systems of residential and public swimming pools, spas or hot tubs. NSF 50 was first published in 1977 and incorporates requirements for products previously covered by several individual NSF standards for pool and spa equipment.

The standard is a dynamic document and continues to be reviewed and revised on an annual basis by the NSF Joint Committee on Swimming Pools and Spas. The Joint Committee consists of regulatory officials from various U.S. states and industry representatives and product users.

The scope of NSF 50 currently includes criteria for the following materials, components and devices: materials; pipe and fittings; suction fittings; filters; pumps; multiport valves; surface skimmers; mechanical chemical feeders; flow-through chemical feeders; process equipment including: ozonators; UV treatment systems; electrolytic chlorinators and brominators, in-line and brine tank; and copper/silver ion generators.

Regulatory enforcement
Currently, 28 U.S. states have swimming pool and/or spa regulatory codes that specifically require compliance or third party certification of circulation system components to NSF Standard 50. Many counties and cities in states across the U.S. also have pool and spa codes that require compliance with NSF 50.

Overall, regulatory oversight in the U.S. is strongest for commercial swimming pools rather than residential. Regulatory acceptance/oversight usually occurs at two levels:

  • Before and after construction
  • On-going annual inspections

Most jurisdictions require permits for the construction of residential and commercial swimming pools/spas, which typically require a plan review to the applicable code. Residential and commercial pools may receive an inspection after construction, but on-going annual inspections are typically not required for residential pools.

Most jurisdictions do perform annual inspections of commercial pool and spa facilities. However, few jurisdictions are able to perform more than one or two inspections per year of regulated facilities.

Hydromassage bathtubs and therapeutic spas intended for medical care facilities are typically covered separately under plumbing codes, which typically receive inspections once after construction. Portable spas are typically not covered under any type of regulation.

Specific requirements

Material requirements
NSF 50 requires materials to be corrosion resistant and to meet minimum criteria established to ensure that the materials do not contribute harmful contaminants to the water.

Materials that have more than 650 square centimeters of water contact area are required to meet leaching test requirements designed to identify harmful contaminants that may migrate out of the material and into the water. Material samples are exposed to a defined extraction water for three 24-hour periods. The extractant water from the final 24-hour exposure period is analyzed for contaminants, which must be below acceptable levels based on U.S. EPA drinking water standards.

Materials are exempt from this requirement if the materials comply with the requirements of U.S. Code of Federal Regulations 21 CFR 170-199. Alternately, materials are also acceptable if they comply with the American National Standard for drinking water materials: NSF/ANSI Standard 61: Drinking Water System Components—Health Effects. NSF 61 has defined exposure protocols for various types of materials and components, which are more aggressive than the NSF 50 exposure protocol. Allowable levels of contaminants under NSF 61 are based on U.S. EPA and Health Canada levels for drinking water. NSF 61 also contains toxicology evaluation criteria for unregulated contaminants.

Chemicals that are used as an integral part of a system covered by NSF 50 are also considered acceptable if they meet the requirements of the American National Standard for drinking water treatment chemicals: NSF/ANSI Standard 60: Drinking Water Treatment Chemicals—Health Effects.

Circulation system components
NSF 50 has specific design, flow, durability and safety requirements for the integral components of the circulation system including pumps, skimmers, pipe and fittings, suction fittings and valves.

Plastic pipe and fittings are required to conform to the American National Standard NSF/ANSI Standard 14 that establishes material, performance and durability requirements for plastic piping components. The safety and applicability of suction fittings is addressed via ASME A112.19.8M.

Centrifugal pumps are required to withstand a hydrostatic pressure of 150 percent of the maximum working pressure. There are design criteria for strainers, drain plugs and shaft seals. Each pump must be sold with operating instructions that must include a manufacturer’s pump performance curve. NSF 50 includes a test method to verify these pump performance curves.

Multiport valves are required to meet a burst pressure of four times the maximum working pressure and shall not leak, rupture or burst when subjected to 1.5 times the maximum working pressure for five minutes. The ability of a multiport valve to seal off ports not in use during the filter and backwash cycles is judged by a differential pressure/leakage test. Valves are tested to verify the pressure loss claims of the manufacturer and waste ports are tested for leakage.

Recessed automatic surface skimmers are required to meet dimensional requirements of NSF 50 for the housing, weirs and strainer basket. The structural integrity of a skimmer housing is evaluated by subjecting the housing to a vacuum of 85kPa for five minutes. Weirs are required to automatically adjust to changes in the water level when operating at the maximum design flow rate. Skimmers for commercial pools are required to have an equalizer line that prevents air from becoming entrained in the suction line. Leakage of water through the equalizer is not allowed to exceed 10 percent of the total flow through the skimmer under normal operating conditions.

Electrical requirements
All electrical components of equipment are required to meet the requirements of the National Electric Code and referenced standards.

Filtration systems
NSF 50 establishes criteria for cartridge filters, diatomaceous earth (DE) filters and sand type filters. In addition to the material requirements mentioned previously, NSF 50 addresses structural durability and filtration performance.

Filters must meet structural durability requirements that include:

  • Design burst pressure equal to four times the maximum working pressure
  • Hydrostatic pressure equal to 1.5 times the maximum working pressure for five minutes
  • 20,000 low-pressure (207 kPa) cycles followed by two times the maximum working pressure for one minute
  • Vacuum filters shall be designed to withstand 1.5 times the vacuum pressure developed by the weight of the water in the tank.
  • Vacuum filters shall sustain a five-minute vacuum of 85 kPa without leak, rupture or collapse.

Filtration performance is mainly measured by turbidity reduction requirements where filters must reduce influent turbidity of 45 ±10 NTU by 70 percent. The startup effluent of a DE filter system is not to exceed 10 NTU during the first 60s of flow. Pressure head loss from the filter inlet to the filter outlet is not to exceed the maximum head loss specified by the manufacturer.

There is also a verification test of manufacturers’ cleaning procedures. Cartridge, sand type and DE filters must meet defined filtration rates based on filter design and the intended application (i.e., residential/commercial and pool/spa).

NSF 50 also establishes criteria for filter media in sand type filters. The standard currently has specifications for filter sand in terms of content, particle size and uniformity coefficient. NSF 50 allows the use of alternative medias in filters where the media is specified on the data plate of the filter. The filter must meet all applicable requirements of NSF 50 when tested with the alternate media.

Disinfection systems
NSF Standard 50 contains criteria for a wide range of equipment that is designed to disinfect pools and spas. These include chemical feeders, (mechanical and flow through), as well as electrolytic chlorinators and brominators, ozonation and UV systems and copper/silver ion generators.

Chemical feeders are required to sustain a hydrostatic pressure of 1.5 times the maximum working pressure for five minutes without distortion or leakage. Chemical feeders are subjected to a chemical resistance test to ensure the materials are resistant to degradation. Mechanical feeders are also subjected to an erosion test using dry chemicals or a DE suspension for slurry feeders. Mechanical feeders are subjected to a 3,000-hour life test and must meet uniformity of output requirements ±10 percent of feeder setting from 25 percent to 100 percent of rated capacity.

Flow through feeders are not subjected to an extended life test, but they must meet uniformity of output requirements. The nature of the chemical type, size and configuration greatly affects the performance of flow through feeders. Using the wrong the type of chemical in a flow through chemical feeder can result in over or under dosing of disinfectant, or cause serious hazards such as fire or explosions. There are many incidents reported each year where calcium hypochlorite and trichloroisocyanuric acid are inadvertently mixed in flow through feeders, resulting in explosions. For this reason flow through feeders are evaluated for use with specific chemicals (chemical type, configuration, trade designation). NSF 50 requires manufacturer’s instructions and caution statements on the unit to advise the user of the chemicals recommended by the manufacturer.

Process equipment such as ultraviolet light (UV), ozone, electrolytic chlorinators/brominators and ion generators are required to meet 3,000-hour life tests. Ion generators and chlorinators/brominators are required to meet uniformity of output requirements, as well as chemical resistance requirements.

NSF Standard 50 contains a disinfection efficacy test procedure for process equipment that is intended for supplementary disinfection of water such as UV, ozone and ion generators. The procedure requires a three-log reduction in challenge organisms.

NSF 50 also requires UV systems, ozone and ion generators to be utilized with residual levels of chemical disinfectants.

  • Ion generators shall be used in conjunction with not less than 0.4 ppm free chlorine or 0.8 ppm bromine.
  • UV systems are required to be used with not less than one ppm free chlorine or two ppm bromine.
  • Ozone systems are also required to be used with residual disinfectants and ensure the ozone concentration in the finished product water does not exceed 0.1 ppm.

About the author
Dave Purkiss is General Manager of the Water Distribution Systems certification program at NSF, with oversight of certifications to NSF/ANSI Standards 50, 60, and 61. He has been with NSF for 19 years, serving in various roles including Managing Director of WRc-NSF Ltd from 1999 to 2003. Purkiss can be reached at 1-800-NSF-MARK or at



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