Extending the Water Demand Calculator to Commercial and Institutional Buildings

2021 ASPE Tech Symposium attendees discuss the future of pipe sizing, high-efficiency fixtures, and the Water Demand Calculator with four experts.

by Toritseju Omaghomi, Natascha Milesi Ferretti, PE, Gary Klein, and Steven Buchberger, PE

In 1992, Congress passed legislation called the Energy Policy Act (EPAct92), which set national water-efficiency standards for toilets, faucets, and showerheads, introducing the era of “U.S. Legal” or “low-flow” plumbing fixtures. Since then, other agencies and organizations (e.g., U.S. Green Building Council [USGBC], U.S. Environmental Protection Agency [EPA]) have encouraged water use that is more stringent than EPAct92, leading to the emergence of high-efficiency fixtures.  Widespread adoption of water-conserving fixtures over the past several decades has profoundly impacted indoor water use, which has prompted the development of new methods for estimating peak indoor water demand.

The Water Demand Calculator (WDC) computes peak indoor water demands in residential buildings that are fitted with U.S. Legal plumbing fixtures as mandated by EPAct92. The current version of the WDC is a Microsoft Excel spreadsheet available online from the International Association of Plumbing and Mechanical Officials (IAPMO) and included in Appendix M of the 2021 Uniform Plumbing Code (UPC) and in Appendix C of the 2020 Water Efficiency and Sanitation Standard (WE-Stand). The WDC is the first significant codified modification to estimating peak indoor water demand since the development of Hunter’s curve 80 years ago. Comparisons of predicted and measured peak water demands at new residential buildings in North America and Australia show very good agreement between theoretical estimates from the WDC and actual readings at water meters.

2021 ASPE Tech Symposium Panel Session

At the 2021 ASPE Technical Symposium in San Diego, four experts participated in a panel session to discuss the next steps in the evolution of the WDC to estimate peak water demands. The panel session, entitled “Extending the Water Demand Calculator to Commercial and Institutional Buildings,” was presented to a standing-room crowd of more than 75 plumbing industry professionals.

Toritseju Omaghomi opened the session with a succinct history of Hunter’s curve, including key assumptions, inherent limitations, and typical applications. Some highlights are summarized in Table 1. Natascha Milesi Ferretti followed with a detailed overview of new experimental research efforts at the National Institute of Standards and Technology (NIST) to update estimates of pressure losses at pipe fittings, especially for reduced pipe sizes, appurtenances, and materials that are increasingly common in the current era of decreasing indoor water demand.

Table 1.  Key Assumptions and Limitations of Hunter’s Curve
1.     Congested service is assumed (users que at fixtures, e.g., halftime at a sports arena).

2      Only three fixture groups are used to develop the design curve: bathtubs, flush valve toilets, and tank toilets.

3      The sample size used to estimate probability of fixture use is small.

4      Flow rates are based on plumbing fixtures from the 1930s.

Gary Klein provided a presentation that highlighted how use of the WDC is allowed in various building codes (International Plumbing Code [IPC], UPC, WE-Stand) and demonstrated the dramatic effect of using the WDC on the required pipe size. The final panelist, Steven Buchberger, reviewed the theoretical basis of the WDC and described the University of Cincinnati’s current research efforts, funded by NIST, to gather and analyze field data needed to extend the WDC beyond residential users to the commercial/institutional sector.

Answers to Plumbing Engineers’ Questions

The four presentations were followed by a lively Q&A session. All questions from the audience were recorded and grouped into one of the three categories below.

Questions Related to the WDC

Does other data show measured water use to validate the WDC?

Yes, extensive measurements of water use at several new large residential buildings in the U.S. and Australia show good agreement with predictions from the WDC.

Are there any issues (e.g., low pressures) in operational systems designed with the WDC?

No, there are no issues of which the panelists are aware.

Is there a version of the WDC for nonresidential buildings?

A version of the WDC for commercial buildings is in development now with funding from NIST and assistance from IAPMO. A beta version is expected by early 2024. A progress update on the WDC will be presented at the next WDC Summit in November 2022.

Will fixture p-values for residences change due to COVID work-from-home mandates?

The work-from-home mandates affected the total volume of residential water use, but this is not expected to have a significant impact on the peak household demand; hence, no changes to the fixture p-values are expected.

Is there a simplified chart for the WDC?

Additional information about the WDC can be found at iapmo.org/water-demand-calculator.

When sizing water meters, how is outdoor water use (e.g., irrigation) accounted for?

It depends on the timing of the outdoor water use. If outdoor use occurs during the peak indoor period, meter sizing should be based on the sum of the outdoor demand and the WDC indoor prediction. If outdoor use occurs late at night, meter sizing should be based on the larger of the two (outdoor use or WDC prediction). The upcoming fourth edition of AWWA M22: Sizing Water Service Lines and Meters has more detail and examples about this scenario.

Is the end goal to change water supply fixture units (WSFU)?

The WDC does not use fixture units to compute peak water demand. The goal is to switch from WSFU to actual flow rates and representative probabilities of fixture use. This will keep plumbing designs current as fixture flow rates change and more information on simultaneous fixture use becomes available in more occupancies.

Questions Related to General Research

Most of the data for the WDC came from homes in California. Did you consider the effects of colder climates where people tend to use the restroom more frequently?

Climate was not considered. The p-values for residential restroom fixtures are assumed to be valid in warm and cold climates.

Are you testing instantaneous water heaters?

No, instantaneous water heaters are not being tested.

Are you measuring temperatures at other points besides at the tank and discharge?

No, research to develop and test the WDC does not involve measurements of water temperature.

Decreased flow also affects sewers, or how water is needed to push waste. Do you consider waste when testing for reduced flow?

Reductions in household water demand are an important consideration for wastewater flow; however, wastewater flow is not part of the WDC. In addition, the WDC does not establish the flow rates; it allows for right-sizing of premise plumbing based on improved estimates of peak water demand.

Will you study what new standards and new materials affect gravity flow?

No, this is beyond the scope of work for the WDC.

Any thoughts on using new probabilities to update the minimum number of fixtures in buildings?

There is indeed a link between total fixture count and the probability of individual fixture use. Hence, as more data becomes available and p-values are refined, it may be possible to provide some guidance on the minimum number of fixtures needed in certain types of buildings.

Would building owners have to pay for the sensors and labor, if they are interested in instrumenting their facility with sensors to measure flow?

That depends on circumstances. If the building were included in a sponsored (funded) research project to monitor fixture use, there would likely be no charge to the building owner to have water use sensors installed.

Would it help if we are building a school and could work with getting permission for instrumenting them at construction?

Yes, installing water use sensors during the construction phase would likely save time and money.

Questions Related to Plumbing Codes

Is there a formal plan to incorporate the WDC into the IPC?

There is not currently a plan to do this.

How do we get states to adopt the WDC, especially states using the IPC?

The first known single-family and multifamily applications of the WDC were in Texas and New York respectively. It has also been used on projects in California, Oregon, Washington, and Canada. The WDC has been adopted as part of the state plumbing codes in Hawaii, Nevada, New Mexico, North Dakota, and Oregon, and in Seattle King County, Washington, all of whom base their plumbing code on the UPC. In California, Foster City and San Jose have adopted the WDC for use in their jurisdictions, and a petition to consider the adoption of the WDC into the California Plumbing Code will be heard during the 2022 Intervening Code Cycle. The State of Wisconsin and City of Fort Collins, Colorado have also adopted the WDC into their codes.

Current IPC rules for sizing water distribution systems make it straightforward for a plumbing engineer to seek permission to use an alternative method to design the system. Section 604.1 of the IPC states: “The design of the water distribution system shall conform to accepted engineering practice. Methods utilized to determine pipe sizes shall be approved. Accepted engineering practice is that which conforms to accepted principles, tests, or standards of nationally recognized technical or scientific authorities.”

Like other such requests, this will require documentation and approval by the code official. Most jurisdictions have the authority to adopt the use of a nationally recognized standard into their code. The WDC has been part of WE-Stand since 2017. Extensive documentation was prepared to support the petition in California that clearly demonstrates that the WDC provides a generous margin of safety when compared to measured peak flow rates. Inviting code officials, plumbers, water service providers, architects, and plumbing engineers to lunch-and-learn seminars is a great way to get started sharing the information to properly apply the WDC to single-family and multifamily projects. Work with those who show interest to help them through the learning curve. Get a few projects built. Measure results and share them with those who participated in the lunch-and-learn. Repeat these efforts until the WDC becomes standard practice.

Plumbing Engineers’ Perspectives

A survey with 11 questions prepared by the University of Cincinnati was distributed to all members of the audience. The purpose of the survey was to learn first-hand from plumbing engineers about key design challenges facing the profession and methods used to estimate peak indoor water demands. Nearly 75 percent of the attendees responded to the survey questions, and a summary of the survey responses is given below. A copy of the two-page survey can be found here.

Summary of 55 Survey Responses

  • 22 percent of the respondents were in the plumbing industry prior to EPAct92.
  • 98 percent percent of the respondents identify as design engineers, while 9 percent and 2 percent are also plumbing contractors and building engineers. Others were retirees, vendors, equipment manufacturing engineers, and plumbing instructors, designers, or installers.
  • The percentage of respondents who install premise-plumbing systems as part of their work is listed in Table 2.
Table 2. Percentage of Survey Respondents Who Install Premise-Plumbing Systems as Part of Their Work
Building Type U.S. Legal High-Efficiency
Residential 53% 53%
Government/institutional 75% 71%
Commercial/office 93% 91%
Healthcare/school 78% 76%
  • 27 percent and 38 percent of the respondents have changed their sizing approach for U.S. Legal and high-efficiency fixtures respectively. Some use the WDC, and others adjust fixture units or use engineering judgment to estimate the design flow. Some respondents have not adjusted their sizing methods due to policy requirements.
  • 50 percent of respondents identified poor fixture performance and pressure drop as major problems related to U.S. Legal and high-efficiency fixtures. Other problems mentioned were:
    • Long hot water delivery times
    • Issues with tankless water heaters
    • Longer duration of fixture use (e., multiple flushes, longer showers, hand washes)
    • Oversized pipes
    • Pressure boosters required to meet fixture pressure requirements
  • Respondents indicated that all parts of the water supply system (service entrance, hot water supply, cold water supply, top floor fixtures, wastewater drains) have been negatively affected by U.S. Legal or high-efficiency fixtures for the following reasons: oversized pipe, increased water residence time, lowered residual chlorine, Legionella issues, low pressure, and poor performance of fixtures.
  • The following percentage of respondents identified these fixtures to experience the most-plumbing-related issues. Other fixtures with plumbing-related issues included janitor sink, emergency shower/eyewash, urinal, and water heater.
    • Shower: 44 percent
    • Toilet: 69 percent
    • Lavatory faucet: 22 percent
    • Kitchen faucet: 15 percent
    • Water fountain: 5 percent
  • Respondents identified the following as specific design considerations that need to be updated to address issues related to U.S. Legal and high-efficiency fixture use:
      • Study water use in buildings for longer periods.
      • Update data and design criteria in plumbing codes.
      • Update pipe sizing requirements for all building types (including hot water systems).
      • Update pressure requirements for fixtures.
      • Study contaminants, pathogens, and biofilm formation in supply pipes.
      • Study flush valves and faucets manufactured with sensors for frequency of use.
  • 69 percent of respondents would like to see improved estimates of pressure losses in fixtures. Also, a high percentage of respondents mentioned the need for detailed estimates on pressure loss in modern piping and fittings made from copper (58 percent), CPVC (73 percent), and PEX (69 percent).

Plumbing Engineers’ Questions to Be Considered

  • Should age (senior living), culture, and economical status be monitored for water use? Note: Some people shower twice a day, others once a week.
  • Are pressure requirements for fixtures based on old data?
  • Do low-flow and high-efficiency fixtures work well with lower pressures?
  • How are you going to incorporate mechanical and lab equipment use into the WDC?
  • Is there a seasonal change to indoor water use due to the use of humidifiers?
  • How does peak water use vary due to regional, climatic, and seasonal changes?
  • How will healthcare water use be broken out?
  • How are flows for sterilization, lab equipment, makeup water for the chilled water system, RO/DI, water softening systems, and humidifiers handled?

Plumbing Engineers’ Suggestions

  • Demographics in a residential building should be considered.
  • Measure cooling towers, swimming pools, and swimming pool coolers separately. Monitor groups of fixtures on all occupied floors of an office and not the individual fixtures.
  • Fixtures do not use water—people and processes do. The number of fixtures depends on the building use category.
  • The LEED water savings spreadsheet has daily use for commercial fixtures.
  • Try contacting the Huntington Library and Gardens in Pasadena, California. They already have sensors on all of their facility fixtures; this could be a source of useful data.

Issues Related to Wastewater Drain Lines

Many issues were raised related to wastewater drain lines and frequent drainage blockages as a result of low-flow/high-efficiency fixtures. Below is a summary of comments.

  • How does reduced return flow affect scouring requirements in the drainage system?
  • Update wastewater/drain line design (slope).
  • The current slope in the drain line is not effective with low-flow conditions.
  • Maintenance increases due to frequent dry p-traps, odors, cleaning, and blockages.
  • Study the effects on downstream sanitary piping design standards, size, slope, minimum scouring, and viscosity changes for dirty water.

About the Authors

Toritseju Omaghomi graduated from the Environmental Engineering program at the University of Cincinnati where she developed IAPMO’s Water Demand Calculator as part of her Doctoral Dissertation. Natascha Milesi Ferretti, PE, is a Mechanical Engineer at the U.S. National Institute of Standards and Technology. Gary Klein is President of Gary Klein & Associates Inc. Steven G. Buchberger, PE, is a professor of Civil and Environmental Engineering at the University of Cincinnati.

The opinions expressed in this article are those of the authors and not the American Society of Plumbing Engineers.

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