Legionnaires’ disease is arguably the hottest topic in the plumbing industry today.

First identified after an outbreak in 1976, this potentially fatal form of pneumonia is contracted by aspirating Legionella bacteria and presents with coughing, shortness of breath, fever and pain.

Since its initial discovery, the bacteria has been found to be pervasive in large-scale water systems, leading to offices, hospitals, hotels and other sizeable buildings to double down on their sanitization, identification and prevention efforts.

Legionella bacteria can be found in many domestic hot and cold water systems in trace amounts, but does not present a health concern until it colonizes, and the bacteria are atomized and aspirated. In warm, stagnant water, the bacteria can grow and multiply to high concentrations, which is when it becomes dangerous.

Organizations differ on the specific temperature at which the bacteria will grow, but the most common range stated for Legionella bacteria survival is 68°-122°F (20°-50°C). The bacteria is dormant below 68° (20° C) and does not survive above 140° (60° C). 

The key to preventing Legionnaires’ disease is to make sure that building owners and managers follow a water management program. Unfortunately, there is no one guideline to follow on how to reduce the risk of Legionella growth and spread. Go to any ASPE meeting and ask the members what the best way is to prevent Legionella and you are likely to get as many different answers as there are attendees.

Organizations that mandate domestic hot water systems design guidelines and recommendations are similarly scattered and contradictory in their requirements, torn between the need for energy conservation and Legionella prevention.

ASHRAE, which touts energy conservation practices, suggests that temperature maintenance systems should be automatically switched off “during extended periods when hot water is not required,” whereas OSHA states that in the interest of Legionella prevention, DHWS should be excluded from energy conservation measures and run continuously.

The Uniform Plumbing Code and International Plumbing Code set minimum guidelines for plumbing system and component safety, but do not provide concrete “how to” documents and checklists for Legionella prevention.

The Center for Disease Control simply suggests periodically culturing potable water samples to test for bacteria growth.

Which codes and guidelines dictate your DHWS design typically falls to your local jurisdictions and personal or corporate preferences. However, it still leaves the question of just how can plumbing engineers best design their systems to prevent, or at the very least discourage, Legionella bacteria from growing while still adhering to the necessary requirements so as not to find themselves on an inspector’s infraction list?

As stated earlier, many design engineers and building operators will have a different answer for preventing Legionella growth and treating it. Copper silver ionization, chlorine dioxide gas injection, hyperchlorination, heat and flush; the methods for controlling Legionella growth are numerous and constantly growing as more research is conducted. Whichever method you prefer, it is important to design the system to accommodate chemicals and/or hot water flushes.

Although important, system flushes are only one way you can minimize Legionella bacteria in your system. For thwarting growth before it begins, implement the following practices and components into your next DHWS design.

Avoid dead legs in piping designs: Legionella bacteria’s ideal environment to cultivate and multiply is in stagnate water ranging from 77° to 108° (27-42.2° C). By minimizing the distance between supply lines and fixtures in your system design, you reduce areas of stagnation in the hot-water piping, which could accumulate scale and biofilm buildup. 

Maintain water-temperature control: Hot-water tanks should store water above 140° (60° C) and use a mixing valve to recirculate it to the fixtures at 120° (48.9° C) to comply with anti-scalding regulations. The hot water in the storage tank kills the Legionella bacteria, and the water in the supply lines is outside the preferred temperature range for growth.

Keep pumps running continuously: Another method for avoiding stagnant water and an outbreak of Legionella is to always keep your recirculation pumps running, even during periods of low demand. The constantly circulating water will help prevent the bacteria from grouping together and adhering to pipe walls.

Perform routine maintenance of equipment: Legionella can form and hide in biofilm on the walls of hot-water tanks. To prevent an outbreak, routinely clean the tanks, including a periodical drain to remove scale and sediment. This will prevent biofilm, organic debris and corrosion from contaminating your water system.

There are a number of factors to consider when designing a domestic hot-water system and how to best avoid Legionella growth in your design. The most important step you can take is to do your research so that you are aware of all the most current practices and products available.