Many builders are aware they are paying too much for their water meter connections, and for materials, including supply pipe. But how many know it's because the most commonly-used pipe sizing formula is almost 90 years old?

To bring plumbing pipe sizing into the twenty-first century, the International Association of Plumbing and Mechanical Officials (IAPMO) has developed the Water Demand Calculator, which automatically computes the peak water demand flow rate (in gpm) for the plumbing system. IAPMO has essentially done the math to help home builders build better plumbing systems for less money.

The problem is that pipe sizing has traditionally been done with a fixture-unit method, which uses Hunter’s Curve, a formula plumbers have used to calculate peak water usage in single-family homes since the 1930s. This formula does not account for today’s high-efficiency fixtures and appliances, says Dan Cole, Senior Director of Technical Services for the International Association of Plumbing and Mechanical Officials (IAPMO). Hunter’s Curve grossly overestimates water demand, so plumbers routinely end up oversizing pipes. Oversized piping can create hot water delivery delays to fixtures in the home. This not only wastes energy but also increases the dangers from water aging and stagnation in residential water pipes. (For more on the link between pipe sizing and hot water delivery delays, see “Efficient Hot-Water Piping” by Gary Klein.)

The IAPMO WDC method is based on up-to-date probabilities and improved statistical modeling to derive a truer water-demand estimate. And a more accurate estimate of water demand allows for smaller-diameter piping to deliver the right amount of water to a home's fixtures.

The WDC calculation is based, in part, on the friction losses and water velocity limitations for a given piping configuration. So, for example, the WDC might estimate a peak demand of 20-gpm for that piping, which would require a minimum diameter pipe of 1-inch. By comparison, the old fixture-unit method would derive a peak demand of 30 gpm for the same configuration, requiring a 1 ¼-inch diameter pipe.

According to IAPMO, the Water Demand Calculator provides as much as a 65% reduction in water demand estimates for single-family homes and multi-family units. “Our research shows that home builders will realize a 1-to-3 pipe-size service line diameter reduction, depending on the size of the residential building," said IAPMO’s Dan Cole. “This can save hundreds and even thousands from lower meter connection fees and pipe costs.”

The IAMPO Water Demand Calculator can be dowloaded as an Excel file, or is available as a free app for Android and for iPhone.