Filter for use with: Whole House Filter System with Timer (AO-WH-PREL). Use only certified, genuine A. O. Smith filters for continued contaminant removal and system performance. Longer lasting appliances - whole house filter system designed to reduce sediment from every tap. IAPMO tested and certified to NSF/ANSI standard 42 for materials. 40 micron filtration - reduces sediment visible to the naked eye. Replacements made easy - no plumber necessary. Replace filter every 6 months. Large capacity (4.5") filter is perfect for household sizes 4 or more people. Includes one replacement filter.