How hotels can mitigate risk of Legionnaires' disease

As an engineer, I appreciate the obligation we share to specify building systems that enhance the health and welfare of people in the structures we design. But I’m not just an engineer, as over the past decade I’ve been actively involved with mitigating the risks of spreading legionella, a genus of pathogenic bacteria that can cause Legionnaires' disease, through tankless water heater design. Last spring there was an outbreak of Legionnaires’ disease in the Bronx area of New York City involving dozens of people being diagnosed with the disease—and most sadly, several fatalities. It was this horrible outbreak that inspired me to share this guidance.

This information is not intended to scare you about legionella—rather, I want to provide some background about the bacteria and how it can multiply, together with tips for how higher risk market segments can recommend systems to help mitigate its risk.

Size and Scope of the Problem

There are around 6,000 reported cases of Legionnaires' disease each year in the United States, according to the Occupational Safety and Health Administration. Scientists believe the actual number of cases is much higher, given the difficulty in distinguishing Legionnaires' disease from other types of pneumonia. The most recent U.S. population-based study estimated that 8,000 to 18,000 people are hospitalized each year with Legionnaires' disease.

Yet recent data on legionella outbreaks have suffered under the COVID pandemic. Fewer people are getting tested for legionella because most testing is focused on COVID. In this compromised testing environment, it appears to take a multiperson outbreak in a specific area before legionella is considered a suspect.

While testing numbers are down, the good news is that there has been an advance in legionella detection. Clinicians now use a urinary antigen test, which detects L. pneumophila serogroup 1, one of many organisms that cause the disease, and the one responsible for more than 70 percent of the reported cases. This testing approach is now available and widely used throughout the U.S.

Legionella and its Potential Growth

The legionella organism lives and proliferates in a temperature between 68 degrees and 122 degrees F. In temperatures below 68 degrees it doesn’t die; it simply goes dormant and is less likely to replicate. Additionally, the organism can survive at 122 degrees for eight hours and doesn’t instantly die until temperatures reach 176 degrees. Clearly this organism is tough and can thrive in many common plumbing systems, including stratified hot water storage tanks.

Plenty has been written about the most common breeding pools for legionella: cooling towers; spas/hot tubs; fountains and similar water features, but there’s been an absence of information available about legionella and domestic use water systems, so we’ll focus on this specific area. Facility owners, managers and engineers are responsible to create healthy built environments.

Legionella and Domestic Use Water Systems

There are three primary conditions required for legionella growth: 

  • low water temperature (that is, somewhere between 68 degrees and 122 degrees F)
  • stagnant water
  • presence of biofilm

Clearly these conditions exist in many modern plumbing systems, so let’s examine each of these risks.

Water Temperature

The water in a typical hot water storage tank is stratified by temperature—some of which is inhospitable to legionella, but there are frequently pockets of temperatures that are more friendly to the organism. 

Depending on the facility or application, the required hot water temperatures will differ. A sink in a hotel room might be limited to 120 degrees, while the hotel laundry facility and restaurants might need 130 degrees or higher. Engineers often solve this issue with mixing valves, but mixing valves are mechanical devices, and are therefore prone to problems that can lead to water being too hot or too cold. Additionally, mixing valves mess with the system equilibrium and may create risk pockets of stagnate water due to oversized piping, dead piping or poorly balanced systems. It’s like the human circulatory system—if you have a clogged artery, it creates problems for the entire body, not just for a particular area.

Water Quality and Movement

Water quality in a storage tank is rarely uniform either, with sediment of various types gathering at the bottom of the tank. This sediment is often both a food and insulator for organisms. So, all it takes is for a disruptive event to the tank—for example, water hammer or fire hydrant use nearby—for the tank to stir and create a situation more favorable for legionella.

To meet the requirements of building owners, many hot water systems are oversized to address peak-demand situations when by chance everyone demands hot water at the same time. Some even add a backup hot water storage tank for insurance. And it’s this stored (stagnant) water that invites legionella. Aside from legionella, oversized hot water systems also create efficiency and maintenance problems.


Biofilm is simply a polymeric solution that is excreted from certain organisms. It forms a protective and adherent substrate, is a great insulating factor and is also a food source, making it a popular breeding ground for bacteria. 

As an insulator, biofilm can exacerbate the stratification in a hot water storage tank, making some areas even more attractive for legionella. Biofilms can even grow in high velocity situations where lots of water moves across a surface. Legionella does not produce biofilm directly, but it does feed on it.

How Facility Owners and Managers Can Reduce Legionella Risk

To prevent Legionnaires’ disease, it’s imperative to reduce the risk of legionella growth and transmission in water systems. We can achieve this by making sure building water systems are clean and well-maintained while monitoring water regularly for legionella bacteria.

Modern technology also plays a key role in the mitigation of legionella growth, particularly when it comes to the selection of water heaters. A decade or two ago, commercially sized tankless heaters were not available or were not up to the task. But times have changed and there are great commercial tankless options available for plumbing engineers now. 

While tankless technology does not eliminate all legionella risk, it does significantly mitigate it by creating a less hospitable environment for legionella, which is achieved in multiple ways:

  • With no stored water, we eliminate the bacteria, sediment and bio-film risks found in stagnant water. Reducing or eliminating these impurities is key to success.
  • Tankless units maintain a consistent output water temperature without any additional controls in place, eliminating mechanical points of failure found in mixing valves. This consistent water temperature eliminates the stratification of water, with its resulting pockets of attractive temperatures for legionella. Creating temperatures that are inhospitable to legionella is key to removing its risk.
  • Finally, tankless units use advanced flow control, which keeps the water moving rather than storing it in stagnant tanks. Legionella cannot thrive in moving water.

Putting Tankless Plans Into Action

Hotels, healthcare facilities, multifamily housing units and senior living centers are particularly attuned to mitigating legionella risks, and these industries have exponentially increased their use of tankless water heaters. We urge facility owners, managers, and their planning engineers to reach out to water heater manufacturers to learn the latest technologies available to mitigate legionella risk.

Sri Deivasigamani is the founder and CEO of Intellihot.