Designing a hot water system that meets a building’s hot water requirements while ensuring best-in-class performance and safeguarding occupant health and safety is a complex challenge. Engineers must address a variety of threats, including extreme water temperatures and waterborne pathogens such as Legionella, all while maintaining organizational and code compliance.

Though there isn’t a single way to achieve a healthy hot water system, engineers can position themselves for optimal outcomes by developing a Water Safety Management Plan (WMP) focusing on core design and operational principles. 

A WMP is an important document often co-authored by an industry specialist and each organization that informs both plumbing system designers and those responsible for onsite operations — emphasizing four key pillars: infection control, scald protection, regulatory compliance and sustainability.

Let’s zoom in on infection control to learn how it supports an overarching plan that ensures a healthy hot water system’s safety, reliability and efficiency.

Infection control

Few would argue that waterborne pathogens, most notably Legionella, require a thoughtful risk management strategy. However, due to the variability and complexity of water systems and water quality throughout America, combined with the multiple environmental conditions in which Legionella bacteria can thrive, there is no single, feasible approach that is accepted industrywide. 

As a result, specialists in Legionella risk management opt for the term “barrier” when employing strategies to reduce the risk of Legionella growth and proliferation. This approach focuses on the various processes, tools and procedures that can act as barriers and control the spread of infection.

The first and most essential barrier is water temperature. This means water temperatures in all parts of the hot water system infrastructure must be maintained above the Legionella growth range high limit of 122 F/50 C, as well as periodically raised to 140 F/60 C or above, which kills the bacteria within 32 minutes. 

The water’s age must also be considered. Are there regions of the system (such as a pipe run from the recirculation loop to a low-usage fixture) where the water ceases to move? If so, water temperature is in danger of dropping into the optimal Legionella growth range of 95 F/35 C to 115 F/46 C, putting users at risk. 

Piping technologies, fittings and system design recommendations are available to minimize these distances. Periodic fixture flushing to evacuate aging water is also a positive infection control protocol. 

• Digital water temperature control technology. Today’s technologies have come a long way in maintaining proper infection control barriers. For example, digital recirculation valves have made system water temperature management much more consistent. These mixing valves, whether installed at the point of source (water heater) or zoned throughout the building, can deliver hot water within 2 degrees of the set point. 

Models operating with a 1-degree inlet-to-outlet differential are particularly effective. This tight tolerance allows low-temperature-loss systems to maintain the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)-guided minimum system temperatures without setting the mixing valve at an elevated, potentially harmful outlet temperature. 

Positioning the recirculating hot water temperature control close to the point of use will simplify access, but plumbing designs that specify point-of-use thermostatic mixing valves (TMVs) run into several challenges. TMVs often include narrow pathways and rubber seals that provide environments conducive to the accumulation and growth of harmful waterborne pathogens. 

Also, if a periodically scheduled or system remediation thermal disinfection regime is the preference, point-of-use TMVs, by design, will attempt to restrict disinfection temperature water from flushing through the fixture unless a manual override and post disinfection reset is performed. This adds complexity to the already significant maintenance load inherent in placing point-of-use TMVs in multiple locations. 

Operating circulation pumps continuously, which means excluding them from any energy codes or regulations that might require the pumps to be turned off to save energy, will reduce the waterborne pathogen incubation risk potential that is elevated in stagnant, warm water. 

• Mineral deposition (scale) and infection control. The infection control discussion would not be complete without addressing biofilm, an organic slime that can accumulate on pipework walls within a hot water system. Unremedied, biofilm can provide nutrient-rich living quarters where waterborne pathogens can incubate. 

A 2024 study conducted by the Montana State University Biofilm Engineering Department concluded that “scale in pipework provides a scaffolding upon which biofilm can accumulate.” The American Society of Plumbing Engineers (ASPE) published a document supporting these findings. 

In its 2022 publication titled “Engineering Methodologies to Reduce the Risk of Legionella in Premise Plumbing Systems,” ASPE states: “Studies have shown that scale buildup inside premise plumbing systems acts as a nutrient source for biofilm and Legionella bacteria and can lead to inefficiencies in water heaters … This scale development reduces flow and creates an environment for biofilm growth [, and, thus,] bacterial development.”

Historically, combatting this scale was challenging and required significant maintenance and overhead costs. Today, however, the development of nanobubble technology has effectively automated the process. A static, side-stream application transforms the noncondensable gases naturally present in water systems into abundant nanoscopic bubbles. 

These nanobubbles flow throughout the hot water system, naturally removing existing scale and any biofilm attached to it and inhibiting the formation of future mineral deposits without adding anything to the water. Particulates simply flow downstream for easy removal, leading to cleaner, healthier and more efficient water systems. 

This technology has proven effective in new plumbing systems or retrofitting into existing pipework. Keeping pipe and components free from mineral deposition is a hot water system best practice.

• Optimizing infection control strategy. The message is clear: A detailed water safety management plan for implementing and maintaining multiple barriers against waterborne pathogen incubation is critical for optimal hot water system infection control. The first and most essential barrier is appropriate water temperature management, which is monitored and documented. 

Continuously recirculating hot water, programmable automatic flushing of pipe runs ahead of fixtures to reduce the water age and applying one or more biofilm or bacterial inhibition/removal technologies will optimize facilities’ infection control strategy.

Compliance

A healthy hot water system must comply with a standard of care, which is defined as acknowledged applicable laws, standards and guidelines. The OSHA Technical Manual; Joint Commission Environment of Care; ASHRAE Guideline 12-2020; ASHRAE Standard 188-2021, Legionellosis: Risk Management for Building Water Systems; and VA Directive 1061 are standards of care that include critical recommendations for establishing and monitoring hot water system temperature-control limits. 

Following a standard of care provides safe and comfortable user outcomes that protect against diseases caused by waterborne pathogens such as Legionella.

The previously discussed WMP is vital to ensuring a compliant, healthy hot water system, as it defines the system’s operational principles, water heating and temperature control methodologies, and user safety protocols. ASHRAE 188-2021 and its accompanying Guideline 12-2020 for guiding system water temperature metrics are well-accepted base documents from which a WMP can be developed. 

Within healthcare, WMPs took on renewed visibility in 2017 when the U.S. Centers for Medicare & Medicaid Services (CMS) issued a directive requiring healthcare facilities to develop and adhere to policies and procedures that inhibit microbial growth in building water systems to reduce the risk of growth and spread of Legionella and other opportunistic pathogens in building water systems.

Hospitals receiving reimbursement from CMS (the agency that pays for Medicaid and Medicare) must maintain CMS certification. This can be done either by a state inspection, or the hospital can opt to pay to be certified by the Joint Commission (a CMS certification subcontractor). If a hospital loses CMS certification, it no longer qualifies to receive reimbursement for Medicaid or Medicare patients. 

The CMS directive (which used ASHRAE 188 as a guideline) marked the most significant change in how facilities were required to manage and prevent the spread of Legionella since the bacteria was identified following the 1976 Legionnaires convention. 

• ASHRAE Standard 188-2015, Legionellosis: Risk Management for Building Water Systems 

“6.1.3 CONTROL LIMITS: Establish limits within which a chemical or physical parameter must be monitored and maintained. 

“6.1.4 MONITORING: Establish a system for monitoring the parameters associated with the control limits established in 6.1.3. 

“6.1.7 DOCUMENTATION AND RECORDKEEPING: Establish documentation and maintain records.” 

• ASHRAE Guideline 12-2000 

“4.1.6 RECOMMENDED TREATMENT: Hot water stored above [140 F (60 C)], minimum recirculated return [124 F (51 C)].”

Finally, an important element for the health of a hot water system is ensuring all products are certified and comply with the relevant standards for hot water systems by the International Association of Plumbing & Mechanical Officials (IAPMO) and the American Society of Sanitary Engineering (ASSE). 

These IAPMO-ASSE water heating standards include ASSE 1082, Performance Requirements for Water Heaters with Integral Temperature Control Devices for Hot Water Distribution Systems, and ASSE 1084, Performance Requirements for Water Heaters with Temperature Limiting Capacity. 

The notable water temperature control standards include ASSE 1070-2020/ASME A112.1070-2020/CSA B125.70:20, Performance Requirements for Water Temperature Limiting Devices; ASSE 1016/ASME A112.1016/CSA B125.16, Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations; and ASSE/ANSI 1017-2023, Performance Requirements for Temperature Actuated Mixing Valves for Hot Water Distribution Systems.

For digital mixing valves, the 2022 standard ASSE/IAPMO IGC 384-2022, Digital Mixing Valves for Recirculating Hot Water Systems, elevates requirements to meet digital technology’s higher level of performance.

A holistic system approach

Every hot water system will shoulder its own unique operational dynamic according to variable and fixed factors. These factors include a building’s purpose, hours of operation, peak occupancy periods, fixture location/usage diversity, geography, seasonal water temperature change, system maintenance frequency, water chemistry, code compliance, and more. 

However, because no two hot water systems are alike, the focus of a healthy hot water plan should follow core design and operational principles. Infection control plays a pivotal role in this framework, working alongside scald protection, regulatory compliance and sustainability to ensure the system is safe, efficient and tailored to the precise requirements and conditions of the building it serves. 

Brian Armstrong is part of the fifth-generation family ownership of Armstrong International, a globally recognized thermal utility manager with manufacturing and sales locations worldwide. As director of hot water, he brings extensive expertise in domestic and industrial hot water systems. Brian has a proven track record of driving innovation in smart technology solutions, optimizing hot water delivery systems and advancing energy-efficient practices in institutional and industrial markets.