The Future of Domestic Water Sizing

MEA’s Plumbing Coordinator, Patrick Ryan, recently attended a seminar on domestic water pipe sizing for modern high efficiency fixtures. The below is the summary of Patrick’s experience during the Hunter’s Curve Summit.

I recently attended “The Water Demand Calculator: Updating the Hunter’s Curve Summit”, a symposium on domestic water pipe sizing for modern high efficiency fixtures, co-hosted by the American Society of Plumbing Engineers (ASPE) and the International Association of Mechanical and Plumbing Officials (IAPMO). The summit was 8 hours long with around 180 attendees. The speakers discussed a range of topics from the history of Roy Hunter and the development of Hunter’s curve to the development of the Water Demand Calculator (WDC) to methods of domestic water piping sizing in Germany. The Summit was concluded with a panel discussion where they would answer questions from the attendees.  At the conclusion, if one thing was clear it was that Hunter’s curve is outdated and a new method of calculating flow rate in domestic water systems is required.

Hunter’s Curve

The current standard for sizing plumbing systems in the U.S. is utilizing Hunter’s curve developed by Roy Hunter in 1940. Hunter’s curve is a probability curve, based primarily on water closets, which converts a fixture unit value into a flow rate. Each plumbing fixture is assigned a fixture unit (FU) value (i.e., Flush Tank Water Closet = 2.2 FU, Lavatory = 0.7 FU, etc.) if you want to size a pipe you must sum all fixture unit values that the pipe serves and then use Hunter’s curve to convert that value to GPM. This has been the standard for 80 years and is used in all current U.S. plumbing codes but there are issues this approach.

When Hunter’s Curve was created the average toilet flushed approximately 7 gallons per flush. Modern water closets range anywhere from 1.6 gallons per flush all the way down to 0.8 gallons per flush for ultra-high efficiency models. There has been a large focus in the last several decades to drastically improve efficiencies for all plumbing fixtures. As such, the use of Hunter’s curve has led to over sizing of domestic water piping.  Oversized piping has a host of issues with it. Water will sit in piping longer when it is oversized, and it increases the probability for stagnation in the water lines. With increasing concerns over Legionella, reducing stagnation in domestic water systems should be a primary concern. Over sizing also reduces energy efficiency of hot water piping through increased heat losses. This means hot water return systems will use more energy to maintain the temperature for the extra volume of water stored in the piping as well.

Hunter’s curve is a clean and simple method for sizing domestic water systems that was invaluable in a time when computers were not accessible. Because of that simplicity, Hunter’s curve has become a one size fits all solution for all building types. As the saying goes it is a jack of all trades and a master of none. This approach is used to size single family residences, multifamily buildings, high rises, commercial buildings, hospitals, restaurants, and stadiums but the use of these buildings can differ greatly from one another, so the result is inevitably going to be an inefficient design. Computers are now ubiquitous and the ability for designers to complete complex calculations is greatly simplified. With the drastic changes in plumbing fixture efficiency and computing technology since Hunter’s curve was developed, it only seems fitting to develop a new method of sizing domestic water systems utilizing modern technology.

The Water Demand Calculator

IAPMO, ASPE, and the Water Quality Research Foundation (WQRF) organized a task force in 2011 to create an improved method of sizing domestic water systems with modern low flow fixtures.  The task force acquired water survey data of over 1000 single family homes from Aquacraft Inc. The result is the Peak Water Demand Study and the Water Demand Calculator.  The WDC is a free excel calculator that uses 3 different algorithms depending on the number of fixtures and a dimensionless number called Hunter’s Number. I won’t get into the specifics of their methodology in this article but if you wish to learn more about these algorithms and the development of the WDC, I encourage you to read the Peak Water Demand Study . The main result of the WDC is reduced pipe sizes but that brings many benefits with it:

  1. Reduced meter charges – the savings of reduced meter charges can be drastic. For instance, in New Jersey going from a 2” meter to a 1 ½” meter can save potentially up to $130/Month. The Savings provided by each jurisdiction will be different, but the savings will be there, and it isn’t just a one-time savings it.
  2. Less water storage in the pipes – this means less stagnation which will help against the ever-growing concerns of Legionella. It also means hot water is delivered to the fixtures faster which means less wait time and water waste. Reducing pipe sizes also makes achieving the hot water storage requirements easier for programs like Zero Energy Ready Homes (ZERH), LEED, etc.
  3. Reduced Energy losses – The hot water return system is tasked with maintaining the temperature and circulating the hot water in the system. Smaller piping will reduce the heat losses of the HW and HWR systems leading to smaller pump and valves as well as reducing water heater operation.
  4. Reduced raw material usage – this is good for the environment and for the owner through reduced construction costs. IAPMO conducted a Study that compares a single-family residence, a 6-unit building, and a 45-unit building each sized using the International Plumbing Code (IPC), the Uniform Plumbing Code (UPC), and the WDC. A pricing exercise was completed for each version and then compared. That study is worth a read if you are interested in the potential construction costs savings involved.

It is also important to note that the Water Demand Calculator is already accepted by the Uniform Plumbing Code and National Standard Plumbing Code as alternative sizing methods.  They can be found in appendix M of the 2021 Uniform Plumbing Code and Appendix G.8 of the 2021 National Standard Plumbing Code. I am interested to see if the WDC will be included when New Jersey adopts the 2021 version of the National Standard Plumbing Code.

Nothing is perfect and there are certainly areas I would like to see improvement for the WDC. First is that almost half of the data for the WDC came from California which often has water shortages. I have concerns over whether that has an influence on the water usage and whether these results represent the average home in the U.S. I would like to see more random data locations instead of it being concentrated in the west coast and California. Of course, this is easier said than done and the water demand calculator is still a marked improvement over Hunter’s curve. The Peak Water Demand Study also only utilized single family homes and as such the water demand calculator is currently only applicable to single family and multifamily buildings. During the conference they mentioned that the next step in the development of the WDC was to expand into commercial buildings. This requires data and time so it may be some time before we see expanded building categories for the WDC. The last hurdle is that the IPC currently doesn’t expressly recognize the WDC as a method for domestic water sizing. Although section E101.1.2 of the 2018 IPC does state “other sizing or design methods conforming to good engineering practice standards are acceptable alternatives.”  So, the WDC can be used under the IPC but isn’t officially recognized from the largest accepted plumbing code in the U.S.

While Hunter’s Curve has served the plumbing industry well for over 80 years, an updated method for sizing domestic water systems is well overdue. IAPMO, ASPE, and WQRF started off on the right foot with the Water Demand Calculator. There is still room for improvement, but the future appears bright for the Water Demand Calculator. I look forward to being able to utilize this tool in the not-too-distant future.

References, International Association of Plumbing and Mechanical Officials (IAPMO),

Buchberger, Steven, et al. “Peak Water Demand Study Executive Summary – IAPMO.”, International Association of Plumbing and Mechanical Officials (IAPMO), Jan. 2017,

“Water Demand Calculator Study.”, International Association of Plumbing and Mechanical Officials (IAPMO),