Best Practices for Legionella Mitigation – Part One

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Provided by Plumbing & Mechanical Engineer

Best Practices for Plumbing Design

There are several best practices for plumbing engineers to help prevent Legionella growth when designing new plumbing systems.

“Engineers should install ports throughout the water supply infrastructure to enable sampling and testing of the water to help better monitor the temperature and disinfectant levels,” notes Sigler. “Additionally, plans should incorporate double elbow fittings — such as Viega’s double drop elbow — to maintain proper water flow from either side of the plumbing loop. Furthermore, at a minimum, pipes should be installed with proper insulation in accordance with the 2021 IECC Section C404.4 for non-residential buildings and Section R403.5.3 for residential buildings.”

Sigler also points out that engineers should eliminate low-flow areas and minimize the distances between plumbing fixtures and hot water sources.

“In the International Plumbing Code (IPC), the maximum length for hot or tempered water piping must not exceed 50 feet from the source to the fixture,” he explains. “This helps reduce the possibility of bacteria growth while limiting the amount of energy needed to maintain water temperature.”

Another consideration to keep in mind, Sigler adds, is the distance between a building and water treatment plant. “The further a building is from the plant, there may be a need for a supplemental disinfectant system to ensure there is sufficient disinfectant residual throughout the water supply system.”

Fathers sees hot water recirculation as a critical weapon in battling Legionella.

“One thing to always look for and to eliminate are plumbing dead legs where warm, stagnant water can fester, serving as an ideal breeding grounds for legionella growth,” he says. “Also, it’s important to properly size all equipment in a safe and efficient plumbing design. Call it system optimization. This may entail a thorough inspection of a facility’s entire plumbing system — from the backflow device at entry to the water heater to distribution piping, and even sanitization of small aerators at all points of use — they often harbor bacteria.”

According to Mullen, the most common design decisions being made relating to Legionella prevention at present come in the form of automatic faucets and valves, with advanced scheduling tools that assist in flushing aged water.

“These components are typically associated with building maintenance systems and allow centralized control by building operators, whether onset or remote,” he explains. “Simple, yet often overlooked design enhancements include laminar flow aerators to help ease common areas of stagnation or considering tankless hot water storage with plate and frame heat exchangers in lieu of tank type.”

Mullen also notes that unfortunately, cost plays a large role in design decisions.

“Often, without the knowledge of these horror stories of legionella outbreaks, the wrong choices get made,” he says. “What is observed in the industry is that value engineering exercises early in the bidding process tend to scrap these new aged components from the design, which could lead to challenges in producing documentation if an outbreak were to occur. The barrier of entry on these components are slowly starting to come down, and this will, in turn, advance the popularity of installation leading to more data collection and interpretation.”

Additionally, tools such as IAPMO’s Water Demand Calculator are helping plumbing engineers design smarter, safer and more sustainable systems with smaller pipe diameters.

“For years, we’ve used pipe size in ways that do not represent the way we live today,” Mullen says. “With the increased popularity of digital meter faucets, water-conserving fixtures and intuitive building designs, we simply don't need the demand we once thought we did in various environments. The Water Demand Calculator includes a modern approach to designing water systems, based upon an updated probability of use. This engineering tool is the most interesting advancement of plumbing design we've seen in decades. By engineering building systems with Legionella mitigation at the tip of the spear, professional engineers and certified plumbing designers can provide cost-effective solutions to their clients that do not require the additional cost of a broadband connection or dedicated power.”

Currently, the Water Demand Calculator is used for residential plumbing design; however, an updated version for commercial plumbing is currently in the works. Mullen is optimistic about getting the commercial version to the masses but notes it will be a laborious task and that more participation from industry professionals will be required. He encourages anyone interested in joining IAPMO's innovation task group to check out and consider attending the 2022 Water Demand Summit in November, to learn more.

Lansing also recommends limiting the diameter of water supply piping and the length of uncirculated branch piping to fixtures.

“For decades now, we’ve been installing low-flow fixtures with ‘high-flow’ piping,” he says. “When I say ‘high-flow’ piping, what I'm really referring to is oversized piping, which results in greater water stagnation and reduced water quality, and is generally understood to be a contributing factor in the rise of Legionnaires’ disease cases. To be honest, the industry was slow to pull together resources to develop an updated pipe sizing method. I don’t see that as the industry’s fault either. It has been a struggle to find the resources, both financial and human, to focus on advancing the efforts of Dr. Hunter’s work from 1940.

“When Hunter’s Curve went into widespread use, it was almost as the perception was that we had solved water pipe sizing forever and didn’t need to revisit this for at least 80 years,” he continues. “That’s obviously not the case, and there’s been discussion on revising Hunter’s Curve dating back decades. UPC Appendix M method, also known as the Water Demand Calculator, has done just that and it’s being used throughout the country now, with some states even including it within local plumbing codes. The next effort currently underway is extending the functionality beyond residential applications into commercial applications.”

Pierre believes the most common consideration for designing new plumbing systems is not necessarily specific products or components, but rather the compatibility of the components and materials with the facility’s water management plan.

“We should consider how the materials and components we select will impact water safety at the facility,” he explains. “Working with the water management team is essential to understanding these elements. For example: Are components and materials able to withstand high temperatures or chemical disinfection, if the plan includes these as corrective actions? If our water management plan includes sampling or monitoring of the hot water return, or storage tanks — has the design incorporated sampling ports or temperature gauges?”

What’s next?

So what’s next in terms of Legionella prevention and mitigation? Mullen notes that engineers should keep an eye out on the advancement in critical system automation and the use of artificial intelligence designed for the built environment.

“There’s a massive opportunity for data collection from these emerging technologies, and partnerships will be crucial to ensure that the information does not go to waste. The intellectual property extracted from these tools will undoubtedly be something we as an industry have a need for, considering the currency field data has in research and development. The more intelligent our buildings get, the more intuitive plumbing components and equipment will have to become to function in these ecosystem systems. If we don't continuously stay ahead of these innovations as engineers and installers, the adoption of these systems will see an unnecessary stifling effect. The industry is managing very well with the growing pains related to digitalization, and that makes me very hopeful.”

Fathers agrees that engineers will see greater value through evolving IoT technologies — especially in terms of how data can be interpreted and used.

“Some very surprising things are beginning to emerge as a result of the rich resources we’re finding with all this new data — call it ‘building profiling,’ he says. “Engineers, system designers, building owners and data analysts alike are finding it possible to learn so much more about the buildings in which we live and work — data that now allows us to better-design systems, fine-tune system performance, streamline or tailor maintenance schedules or more accurately size system components — even down to details like better sizing of mixing valves, precisely matched to their intended use.

“There’s always been a predisposition to oversize systems hot water systems, such as valves, pumps, etc., to ensure adequate hot water at all times,” Fathers adds. “But now, if data confirms that an average 300-bed hospital consumes a certain amount of domestic water, it allows facility designers to design building systems with far greater accuracy and efficiency.”

Sigler points to the use of treated wastewater inside of a building becoming more of a common practice. “It is important to ensure that proper backflow protection is provided between potable and non-potable water sources as cross connections can introduce Legionella bacteria into the potable water supply system,” he explains. “Additionally, algae and bacteria growth patterns should be monitored at treatment facilities nationally.”

Lansing suggests engineers should produce more case studies going forward.

“If you’ve designed and implemented a system that addressed the issue of Legionella growth well, document what you did and get it out there as a case study,” he says. “It doesn’t have to be perfect. We need as much information as possible assessing the effectiveness of these strategies so we advance together as an industry. We need to be implementing measures, studying the effectiveness and then circling back and modifying the approaches accordingly. That's the only way to move forward without making arbitrary decisions that could impact other aspects of public health, like thermal injury, which may not appropriately address the risk of Legionnaire's disease. There are a lot of different standards and a lot of different risks, and it’s really important that we are taking all of them into context as we move forward with standard strategies.”

Pierre notes that engineers should look for the UPC and UMC to incorporate water management into its building and plumbing codes. “This will place a larger role on designers and engineers to consider Legionella and water management in the design phases. Additionally, the increased awareness could possibly lead to more legislation. At least six states have introduced legislation addressing the risk of Legionella in our building water systems, including the state where it all began — Pennsylvania.”


Nicole Krawcke, is Chief Editor of Plumbing & Mechanical and PM Engineer magazines. She joined BNP Media in 2014 and served as Web Editor and Business Management Editor of The ACHR NEWS. Prior to joining BNP, she worked at, a hyper-local news platform focused on community journalism. She has 10 plus years of writing and editing experience and holds a bachelor’s degree in Journalism from Michigan State University.


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Originally published in August 2022