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Some plumbers successfully size residential supply pipes based on long experience or rules of thumb. But in unusual circumstances — when a house has low pressure, long pipe runs, an extensive irrigation system, or a large whirlpool tub — rules of thumb may result in undersized pipes that perform poorly or don’t meet code (see Figure 1, next page). To accurately size residential water supply pipes, you need to have information on six important variables: • code requirements • available minimum static pressure at the water meter or pressure tank • the pressure-reducing effect of any water meter, backflow preventer, water softener, and/or whole-house filter • length of pipe to the most remote fixture in the building • height of the building • water