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The 3M Aqua-Pure Whole House Scale Inhibition Inline Replacement Water Filter AP431 is designed for use with 3M Aqua-Pure Whole House Scale Inhibition Inline Replacement Water System AP430SS. The system is designed to help reduce scale buildup and corrosion - thereby helping to extend the life on hot water-using appliances such as water heaters (standard (gas or electric) and tank-less), boilers, clothes washers and dishwashing machines, and plumbing fixtures like faucets, pipes, and shower heads. Scale is calcium and magnesium deposits that often form when hard water is heated. Water hardness is responsible for most scale formation in pipes, water heaters, finishes and flat surfaces, and other water using appliances. Over time scale buildup can significantly shorten the life of any water-using appliances. Also scale buildup inside pipes causes loss of water pressure and reduced flow as the interior gets smaller and smaller over time. The AP431 replacement water filter contains polyphosphates that bind dissolved calcium and magnesium hardness minerals, helping to prevent them from precipitating out (or becoming a solid) and building up on metallic surfaces. Polyphosphates also help prevent corrosion by forming a thin protective layer on the metal surface. Cartridge change-out is easy for professionals as well as end users. Change disposable filter cartridge every 6 months or sooner if you observe noticeable reduction in water flow rate.