Plumbing engineers are facing many new challenges in their day-to-day work. Domestic hot water and return system design has become more than just energy efficiency, first cost or occupant comfort. Engineers must also be aware of the foreseeable risks — both from pathogens and design temperatures — and the degree to which designers took reasonable steps to mitigate them.

The rising incidence of Legionnaires’ disease, combined with heightened regulatory guidance, has changed the approach and legal landscape surrounding domestic hot water supply and return system design.

Public health data demonstrates why Legionella has become a major risk factor. Reported cases of Legionnaires’ disease in the United States have increased steadily since the early 2000s, and “Legionella have emerged as the predominant causes of hospitalizations and deaths from waterborne- and drinking water-associated disease,” according to the Centers for Disease Control and Prevention (https://bit.ly/4spb8Hy). 

Between 2015 and 2020 alone, drinking-water-associated outbreaks resulted in thousands of illnesses, hundreds of hospitalizations and dozens of fatalities. What’s important to understand is that many cases are still unreported, meaning the actual exposure risk is higher than the known statistics indicate.

From a liability perspective, this matters because knowledge of risk establishes duty. Failing to act on that duty can contribute to a negligence finding. 

When authoritative bodies such as the American Society of Plumbing Engineers (ASPE), the American Society of Heating, Refrigerating and Air-Conditioning Engineers, the CDC, OSHA and the World Health Organization all publish guidance stating that elevated hot water temperatures may be necessary to control Legionella growth, that guidance becomes discoverable evidence of industry awareness. Consequently, the expectation is that plumbing engineers are aware of these guidelines and follow them.  

Following code is no longer good enough, because in litigation the question is no longer whether Legionella was known to be a risk, but whether the design appropriately responded to that knowledge.

As a result, many designers are now distributing hot water at higher temperatures — often 130 F to 140 F — to maintain return temperatures above 120 F throughout the system. While these temperatures may align with pathogen control guidance, they simultaneously increase the foreseeable risk of scald injury. This creates a dual exposure: failure to address Legionella risk on one hand, and failure to protect occupants from thermal injury on the other.

The plumbing industry has been aware of scalding and thermal shock injuries for a very long time. These incidents are well understood, well documented and highly predictable. The relationship between water temperature, exposure time and burn severity has been established for decades. 

Vulnerable populations, including children, older adults and individuals with physical or cognitive impairments, are known to be at elevated risk. From a risk management standpoint, these injuries are mysterious; they are foreseeable outcomes when elevated temperatures are delivered without appropriate safeguards.

The role of ASSE standards and devices

As plumbing engineers, it seems we are caught between a rock and a hard place. Fortunately, this is where American Society of Sanitary Engineering (ASSE) standards and devices come to play. 

ASSE-listed devices do more than provide technical functionality — they establish a recognized standard of care. And for those unaware, standard of care typically is the ordinary, reasonable skill and diligence exercised by a competent, licensed professional in the same discipline, locality and time under similar circumstances. 

Basically, it is going to be the benchmark for a court of law to determine if you did your job adequately or not.

If you ever get to a point where litigation becomes reality, you will likely encounter questions from internal or external counsel (lawyers) similar to the following:

• Was an ASSE-listed device required for this application?

• Was the correct ASSE standard selected?

• Was the device installed, sized and applied within its 

certified operating range?

• Was downstream mixing or point-of-use protection

provided where required?

Let’s talk about the ASSE listed devices mentioned earlier. As plumbing engineers, we must be able to understand and utilize the many different ASSE devices correctly, since not using them correctly can harm people.

ASSE 1016 valves, for example, are specifically intended for individual shower and tub/shower applications. Within this single ASSE listing, there can be three different types of valves.  

1. Pressure balancing only valves, which may comply with ASSE 1016, are effectively “blind” to inlet temperatures. They are designed only to balance the pressure of cold and hot water coming into the valve. If the temperature of the cold or hot water were to change, the valve makes no adjustment. Because of this, pressure balancing valves are increasingly difficult to utilize in systems with elevated distribution temperatures due to the risk of scalding.  

2. Thermostatic valves listed with ASSE 1016 will adjust for fluctuating temperatures but cannot adapt to a pressure change (think of flushing a toilet and getting a sudden surge of hot water in an adjacent shower). These valves have always been open to exposing users to thermal shock injuries. 

3. Combination pressure balancing and thermostatic ASSE 1016 valves are the only devices that provide both scald and thermal shock protection. It is no surprise that these valves are more expensive than the other two options, but considering the state of the industry, they are a very viable option.

Similarly, ASSE 1017 valves are often misunderstood from a liability standpoint. These devices are designed to control system distribution temperature, not to protect end users. 

ASSE 1017 valves are for controlling distribution temperatures only. Devices to protect against scalding and thermal shock may still be required at the point of use. Additionally, ASSE 1017 devices have a relatively large allowable temperature fluctuation, which increases as flow through the valve increases. 

For these reasons, using only an ASSE 1017 listed valve can be framed as a misapplication of a device outside its intended purpose, which weakens any legal defense that “a listed valve was provided.”

Other ASSE standards — such as 1062, 1069, 1070 and 1071 — address specific applications, each with explicit limitations. What engineers must be critically aware of is that each valve has requirements for inlet pressures and temperatures, and outlet flow rates. Using these devices outside their published values invalidates their listing — and the fun part is each manufacturer might have slightly different values.  

You’ll need to review the product data for the ASSE valve you are specifying to ensure it meets the system and downstream fixture requirements. From a legal perspective, this is critical: if a device is used outside its listing, it ceases to represent compliance with the standard of care, regardless of its presence in the system.  

If you need help understanding the ASSE listings, ASSE offers a free white paper titled, “Guidelines for Temperature Control Devices in Hot Water Systems” (https://bit.ly/47LDaoo).

Equally important is documentation. Design decisions related to elevated temperatures, valve selection and system configuration should be clearly documented in the basis-of-design narrative. When guidelines are followed, the rationale should be stated. When tradeoffs are made, such as higher temperatures requiring additional scald protection, those decisions should be explicit. Silence or ambiguity in the design record often becomes a liability issue after the fact.

The good news is that the modern legal expectation is not perfection, but reasonableness. Designers are expected to recognize known risks, apply industry standards appropriately and design systems that balance competing hazards in a defensible way. Legionella control and scald prevention are no longer separate conversations; they are inseparable components of a single risk-management problem.

As domestic hot water temperatures continue to rise, the most defensible designs will be those that treat ASSE standards not as optional accessories, but as essential risk control devices. The question is no longer whether elevated temperatures are justified, but whether the system includes the safeguards necessary to ensure that controlling one hazard does not create another.

James Dipping, PE, CPD, GPD, ARCSA AP, FASPE, is a nationally recognized plumbing engineer and thought leader with over 28 years of experience. As Senior Principal, Plumbing Engineering Discipline Lead - North America at Stantec, he has advanced plumbing design standards across Stantec’s Buildings practice. An ASPE Fellow, James has played a pivotal role in shaping industry best practices, particularly in water safety and sustainability. He co-authored ASPE’s “Engineering Methodologies to Reduce the Risk of Legionella,” is the chair of ASPE Working Group 99 Thermal Disinfection Design Committee and is the vice-chair of the Plumbing Engineering Professional Engineer Exam Development Committee for the National Council of Examiners for Engineering and Surveying.