Preventing Legionnaires’ Disease with DHWS Design

Legionnaires’ disease is arguably the hottest topic in the plumbing industry today. pme0619 TechTopic Photo2 - Preventing Legionnaires' Disease with DHWS Design

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 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 are dormant below 68° (20° C) and does not survive above 140° (60° C).

The key to preventing disease 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?

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