Over the course of my 17-year career, I’ve mentored more than a dozen junior engineers. I’ve made sure each one gains a foundation of technical engineering concepts and how to work with other disciplines to find the best design solution, gain a total project perspective, manage conflicts and know what questions to ask.

While these are all marks of great engineers, there was another that I made sure my mentees understood: A great engineer knows when to admit something is a bad idea, even if it means starting over and finding another solution.

Today, our industry is challenged with the uncomfortable reality that many plumbing codes still require or strongly recommend us to size domestic water systems with outdated methods, greatly oversizing piping and carrying public health risks. I contend that continued use of these methods is a bad idea, and we should look for another solution.

Oversizing: The Problem and a Solution

Many engineering systems can be oversized with no negative impact on performance or risk to the public. However, domestic water is not one of them. One of the biggest risks of oversizing domestic water piping is the higher likelihood of waterborne pathogen growth. While there are others, water age and stagnation are two of the biggest factors in Legionella and other pathogen growth, and oversized piping is a root cause of both. 

In addition, oversized piping can make it difficult to maintain disinfectant residuals in all areas of the water system. Beyond the risks to public health, oversized water piping adds unnecessary project costs in materials and labor, which is challenging in an industry where we’re all pressured to design under tightening budgets.

The International Plumbing Code and Uniform Plumbing Code, as well as many other state codes, contain appendices that discuss recommended rules for sizing domestic water systems. The most common of these methods involves Hunter’s Curve, which establishes a relationship between the types and quantity of plumbing fixtures and water system demand. 

Hunter’s Curve was developed in 1940 based on limited empirical data. While it has been updated several times since, it has not kept up with modern plumbing technology and user behaviors. The letter of most codes says that this and other rules are only recommended and not required, and an engineer can still use different methods if approved by the authority having jurisdiction. 

However, my experience has shown that many plan reviewers treat these recommendations as requirements and hold engineers to them to obtain a building permit. Additionally, many plan reviewers I’ve worked with have hesitated to allow sizing methods they are unfamiliar with or are not discussed in the appendices. I know from other colleagues that my experiences are not isolated — many have been forced to size domestic water systems with methods they would rather not use because of the risks involved.

The shortcomings of Hunter’s Curve and other sizing methods recommended by the plumbing codes were blatantly illustrated by the arrival of the International Association of Plumbing and Mechanical Officials (IAPMO) Water Demand Calculator (WDC). The WDC was developed based on modern plumbing fixtures, user behavior and an abundance of empirical water usage data. While the calculator is still only applicable to single and multifamily buildings, I saw its impact first-hand when I met with one of the calculator’s developers. 

We compared a 192-unit senior living building I had designed with the Hunter’s Curve method against the same building with the WDC. I nearly fell off my chair when I saw the disparity — the peak flow rate calculated by the WDC was less than a third of that using Hunter’s Curve, and most of the load was the extra scope I added for the commercial kitchen. The building water service would have been adequate at half the size, and most of the distribution piping could have been similarly reduced. 

While this exercise was only limited to a multifamily building, IAPMO is working on a similar calculator for higher-risk populations in health care and other markets and expects to find similar results. However, from that limited exercise alone, the message was clear: By continuing to use Hunter’s Curve and other traditional methods in the plumbing codes, we are putting the public at risk and doing our building owners a disservice.

Plumbing technology and engineering methods have always evolved faster than codes can respond. This has been especially evident in the last 40 years with the rise of water-efficient fixtures. The Energy Policy Act of 1992 went a step further in mandating these fixtures along with other water efficiency standards. Still, there has since been little response from the plumbing codes in updating sizing methods to account for this. 

Due to this and some other reasons, I’ve often questioned why plumbing codes speak at all about water sizing. Perhaps codes should give requirements for minimum pipe sizes on given fixtures so contractors working on remodels or small systems could have some guidance. Beyond that, it should be left up to the engineer to determine the appropriate sizing method. 

The engineer is ultimately the one who best understands the building, its systems, fixtures and occupants, so, logically, the engineer knows best how to size the domestic water systems. I believe our industry needs to seriously consider decodifying domestic water sizing and leave it to the design professionals who put their stamps on the drawings.

Precedent

Some engineers and code officials may balk at the idea of decodifying water sizing. Somewhat unfortunately, this is understandable since water sizing methods have been in the plumbing codes for so long that few remember when they weren’t. However, looking at other engineering disciplines shows that there is plenty of precedent for this. 

The most obvious to those practicing plumbing engineering is medical gas. While medical gas is, in many cases, a life support system, the codes are largely silent on how to size it. While NFPA 99 establishes minimum pipe size requirements for medical gas systems, it only says that the losses must not exceed 10% of the system operating pressure (Section 5.1.10.11.1). 

Beyond that, it is up to the engineer to determine the demand of the systems based on patient types, connected equipment, simultaneous use, future and surge loads, and other factors that influence losses and sizing. Plumbing codes are similarly silent about sizing on several other systems that plumbing engineers design: laboratory gases and vacuum, purified and process water, compressed air, corrosive wastes and siphonic roof drainage. 

Mechanical codes also have this precedent, where HVAC system requirements tend to be performance-based and leave it to the engineer to determine how to achieve them. (The 2021 International Mechanical Code only says that “hydronic [pipe] must be sized for the demand of the system,” which is otherwise determined by the engineer (Section 1201.2)). 

The few specific sizing requirements for mechanical systems usually come out of the energy codes, such as limiting the fan power needed to produce a given airflow. Altogether, putting the sizing of domestic water systems in the hands of the engineer is not a far-fetched idea.

A Call to Action

Given the public health risks of oversizing domestic water systems, the industry needs to take action on the code side. At a minimum, the codes could keep the recommended sizing rules they have if they also adopt language that they are guidelines only and not enforceable. Engineers would be free to use other sizing methods that are appropriate for their design conditions. Code officials would also be free to require submission of these calculations to ensure a proper line of thinking in them. 

A better course of action would be to adopt performance-based requirements used by medical gas and mechanical codes and leave it to the engineer to meet them. What those requirements should be would no doubt stoke a lot of much-needed debate in the industry, but I believe they should focus on water age, average temperatures, flow rates and velocities, and fixture consumption. 

Those who practice plumbing engineering and participate in code development can be great voices for change in this endeavor, along with professional societies such as the American Society of Plumbing Engineers and the American Society of Heating, Refrigerating and Air-Conditioning Engineers. As design professionals, we owe it to ourselves and those we serve. Continuing to do it the way we’ve always done is a bad idea; we need to find another solution.

Aaron Bock, PE, has been designing plumbing systems for the health-care industry across the United States for more than 17 years. He is a past president and current vice president-legislative for the Wisconsin chapter of the American Society of Plumbing Engineers.