Water softeners are common household appliances found in regions of the country where hard water (or water that contains a lot of dissolved calcium or magnesium) is prevalent. Hard water can cause scales to form on the inside of pipes, water heaters, and other appliances and equipment. Scales do not conduct heat well and can reduce the flow of water through pipes. Hard water also reduce soap's ability to lather and reacts with the soap to form a sticky coating on skin.

Hard water can be managed by softening or filtering the water. Water softeners are often used due to the high costs associated with filtration. The calcium and magnesium ions in the water are replaced with univalent hydrogen, sodium, or potassium ions (i.e., cation exchange). To achieve the ion replacement, the water in the house runs through a bed of small plastic beads or through a chemical matrix called zeolite. The zeolite is covered with hydrogen, sodium, or potassium ions. As the water flows past the hydrogen, sodium, or potassium ions, they swap places with the calcium and magnesium ions. Eventually, the zeolite contains nothing but calcium and magnesium, and at this point it stops softening the water. It is then time to regenerate the zeolite. Regeneration involves soaking the zeolite in a stream of concentrated brine, usually of sodium chloride or potassium chloride, or acid solution. The strong brine displaces all of the calcium and magnesium that has built up in the zeolite and replaces it with hydrogen, sodium, or potassium. The remaining brine plus all of the calcium and magnesium are flushed out through a drain pipe.

While the volume of water consumed by these fixtures during the regeneration phase has decreased significantly in recent years, water softeners still generate and discharge a significant volume of wastewater. To minimize the water consumption of these fixtures and reduce the amount of salt discharged into septic and sewer systems, the NSF/ANSI 44–Residential Water Softener Testing Standard and the Water Quality Association’s WQA S-100 Residential Water Softener Testing Standard include a voluntary set of requirements for efficiency-rated (ER) residential cation exchange water softeners. (These two standards are essentially identical. Therefore, a residential cation exchange water softener can be certified to either standard.)

All residential cation exchange water softeners sold in the United States must be certified to the general requirements of NSF/ANSI 44 (or WQA S-100). The voluntary efficiency requirements found in Section 7 of NSF/ANSI 44 are for manufacturers looking to differentiate and market their products as water- and sodium-efficient. Under Section 7, an ER system must meet the following criteria:

• Be a demand-initiated regeneration (DIR) system. In other words, it must use a flow meter or water hardness sensor to initiate regeneration. Devices that use time clock-initiated regeneration (fixed time schedule) do not qualify for the efficiency rating. Softeners that use time clock-initiated regeneration automatically regenerate on a fixed time schedule set by the user (typically every four days to weekly). This can result in unnecessary regeneration during times of reduced use and waste large volumes of water.
• Have a rated salt efficiency of not less than 3,350 grains of total hardness exchange per pound of salt, based on NaCl equivalency (477 grams of total hardness exchange per kilogram of salt).
• Not generate more than 5.0 gallons of water per 1,000 grains of hardness removed during the service cycle (18.9 liters per 64.8 grams of total hardness removed).

Most manufacturers indicate on the product packaging and literature that their product meets the NSF/ANSI 44 Residential Cation Exchange Water Softeners Efficiency Rating. In addition, the following organizations that independently certify water softeners to the NSF/ANSI standards maintain a listing of products that meet the voluntary efficiency rating:

• Water Quality Association (WQA) Gold Seal Certification Listings 
• NSF International (NSF) Product and Service Listings
• Underwriters Laboratories, Inc. (UL) Online Certification Directory