Designing a hot water system that fulfills a building’s hot water requirements while also safeguarding the health of its occupants is no small feat. One of the most significant threats engineers must consider is Legionella bacteria, which thrive in warm water environments and can pose serious health risks if not managed properly.
Legionella bacteria, which is responsible for Legionnaires’ disease, can proliferate in various parts of a building’s hot water system, especially in areas where temperatures fall between 95 degrees F. (35 degrees C.) and 115 degrees F. (46 degrees C.). This range provides optimal conditions for bacterial growth, making it crucial to monitor and manage water temperature carefully. Without the right strategies, stagnation in pipes and fixtures can create a potential breeding ground for Legionella.
However, because of 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. These barriers, which focus on the various processes, tools and procedures that control the incubation of bacteria, should be part of a comprehensive plan to reduce the risk of Legionella growth. Let’s take a closer look at these barriers and how to properly implement them in a holistic Legionella risk reduction plan.
The Role of Temperature Control
The cornerstone of Legionella prevention is maintaining water temperatures in all parts of the hot water system infrastructure above the Legionella growth range high limit of 122 degrees F./50 degrees C. Therefore, engineers should set the water heater to maintain a consistent temperature of 140 degrees F./60 degrees C. or higher, ensuring that water enters the system at this temperature each time there is a fixture draw-off. A Digital Recirculating Valve (DRV) can then reduce the temperature to a safe and consistent range of 122-125 degrees F./50-52 degrees C. at the point of use, meeting hot water safety standards while preventing Legionella growth throughout the system.
Precise temperature control technologies, such as digital mixing valves, are particularly effective in achieving this goal. These valves allow engineers to set specific temperature ranges, minimizing fluctuations and ensuring that water temperatures remain consistently safe. By integrating such technology, either at the water heater or zoned throughout the building, systems can maintain optimal conditions within a narrow margin of the set point while reducing the risk of scalding.
While point-of-use thermostatic mixing valves (TMVs) provide temperature control at individual fixtures, they may also present challenges. TMVs, due to their narrow pathways and rubber seals, can sometimes create environments where bacteria can thrive if not properly maintained. Therefore, a balanced approach that integrates these technologies with broader system controls is necessary for reducing the risk of Legionella.
Water Age Management
Another critical aspect of Legionella control is addressing water stagnation. Stagnant water can cool down into the temperature range that supports Legionella growth, particularly in pipe sections leading to low-usage fixtures. Engineers should identify and address these areas by minimizing pipe lengths and ensuring consistent water movement. This can be achieved through system designs that reduce the distance between fixtures and promote continuous flow.
Additionally, implementing periodic flushing of fixtures can help refresh stagnant water by bringing in fresh, heated water to maintain safe temperatures. Automated or scheduled flushing systems are a practical way to manage this aspect of Legionella control, ensuring that water circulation is consistent even in low-usage areas.
Continuous Water Circulation for Safety
Maintaining continuous circulation of hot water is another vital measure in preventing Legionella. By ensuring that water remains in motion, facilities can avoid stagnant conditions where temperatures might fall into the bacterial growth range. Energy regulations or codes that mandate pump shut offs should be carefully evaluated, as exemptions may be necessary to prioritize safety over energy savings when it comes to managing the risk of waterborne pathogens.
Combating Biofilm and Scale Accumulation
Biofilm, a slimy substance that can accumulate on pipe walls, poses a significant risk as it provides an environment where Legionella bacteria can thrive. The American Society of Plumbing Engineers published a document titled Engineering Methodologies to Reduce the Risk of Legionella in Premise Plumbing Systems, which showed that scale buildup in pipes acts as a scaffold for biofilm development. This highlights the need for strategies that prevent scale accumulation.
Modern nanobubble technology has emerged as an effective solution for managing scale and biofilm. This technology uses microscopic bubbles created from naturally present gases in water, which flow through the system to remove existing scale and prevent future deposits. By keeping pipes clean and free of buildup, nanobubble technology enhances the overall safety and efficiency of hot water systems, reducing the risk of Legionella.
Developing an Effective Water Safety Management Plan (WMP)
Finally, to systematically manage Legionella risks, facilities should develop a Water Safety Management Plan (WMP). Often created with the guidance of industry experts, a WMP serves as a blueprint for maintaining the hot water system. It details the operational practices, temperature control measures, and occupant safety protocols necessary to minimize the risk of Legionella.
Standards such as ASHRAE 188-2021 and Guideline 12-2020 offer valuable frameworks for developing these management plans. By incorporating these guidelines, facilities can ensure that their hot water systems operate within safe parameters, while adhering to industry best practices for infection prevention.
Conclusion
An effective Legionella prevention strategy is not built on a single measure but rather a combination of barriers. The primary defense is maintaining water temperatures above the growth range of the bacteria through automated flushing systems, continuous circulation, and advanced technologies like nanobubbles, all of which play a vital role in creating a robust prevention plan. The implementation of a WMP that includes these multiple strategies ensures that facilities can maintain consistent control over water quality and minimize the risk of Legionella outbreaks, creating a safe, efficient environment for the occupants of a building.
