CHALLENGE:
Accurate water supply pipe sizing is critical for sustainable building design. For decades, plumbing designers relied on “Hunter’s Curve” to assess the expected maximum flow rate for which they need to design a building’s plumbing system. Hunter’s Curve is widely embedded in codes and standards. However, this approach has significant limitations: it assumes all fixtures operate simultaneously, leading to inaccurate pipe specifications. Additionally, the design curve was developed when fixtures consumed far more water than today's water-efficient models, making its calculations increasingly irrelevant for modern buildings.
While various alternative methods have been attempted over the years, none achieved the consistency or standardization necessary for widespread adoption—leading to unnecessary costs and missed opportunities for water conservation.
SOLUTION:
Developed by experts at the IAPMO, the Water Demand Calculator® accurately predicts peak water demand in single-family homes and apartment buildings—enabling proper sizing that saves consumers on both water and water heating-related utility bills for the life of the plumbing system.
After research and development, the Water Demand Calculator was incorporated into the 2018 edition of the Uniform Plumbing Code, which is an American National Standard (ANS), by IAPMO, using its ANSI-accredited procedures. The methodology has gained significant traction: multiple jurisdictions now recognize the Water Demand Calculator with some—such as the City of Vancouver, Canada and Castle Rock, Colorado—formally requiring its use.
The Water Demand Calculator can be obtained at: https://iapmo.org/we-stand/water-demand-calculator.
Building on this momentum, work is currently underway to establish IAPMO Z1403, a binational ANSI/CAN standard for the Water Demand Calculator, which will further harmonize pipe sizing practices across North America.
IMPACT:
The innovative approach offers a data-driven alternative to conventional pipe sizing and delivers multiple benefits across the building industry. Right-sizing pipes eliminates unnecessary material expenses, supports water conservation, and improves overall water quality.
It also reduces water heating demands, lowering energy consumption while minimizing the carbon footprint of plumbing infrastructure.
Ultimately, it accommodates modern water-efficient fixtures while providing a framework that can adapt to future plumbing technologies–ensuring its relevance as plumbing technologies continue to evolve.
Sources:
Guidelines:
Case studies will be added to the Standards Boost Business webpage at ANSI’s discretion.
