Jody Samuell, Project Development Manager, Viessmann Manufacturing, U.S

Minimizing the Risk of Legionella Infection in Building-Wide DHW Systems

Viessmann White Papers: Technical updates and thought leadership from Viessmann US.

Since its discovery in the late 1970’s, Legionnaires’ Disease has been a constant challenge to battle in hotels, dormitories, onboard cruise ships, and other large-scale shared living spaces. Thanks to decades of dedicated research, scientists and engineers have discovered not only the cause and treatment of this dangerous illness, but also prevention measures to help aid in the deterrence of transmission. This white paper outlines these methods, while exploring how the disease was discovered, and challenges presented in inhibiting growth and avoiding infection.

Discovery of Legionnaires’ Disease

Legionnaires’ Disease was first identified in 1976 by the Centers for Disease Control and Prevention after they, along with other federal, state, and local authorities launched one of the largest disease-related investigations in US history. An outbreak of severe pneumonia was discovered throughout the participants of an American Legion  convention (referred to as Legionnaires) in Philadelphia, Pennsylvania. Within a week of the convention’s end, during  which all participants had stayed in the same hotel, approximately 2,000 of the Legionnaires had contracted an acute  pneumonia-like symptoms; more than 200 of the attendees had been hospitalized; 34 had died. The investigation lead  to the discovery of a new bacterium, named  Legionella pneumophila, that had been spread throughout the hotel’s air  conditioning system ⁴.

Legionella bacteria occurs naturally in rivers, lakes, soil, wells, and can be found in stable water piping and storage systems.  The bacteria is able to survive drinkingwater treatment processes, and thrives in biofilm that develops naturally inside these systems. Warm, stagnant water + air + nourishment from corrosion and biofilm/scale = ideal growth conditions for the bacteria. Because the bacteria feed and reproduce in these biofilms found within piping systems, disinfection of a water storage system alone is not enough to kill it. These biofilms can remain stagnant for long periods of time, allowing for bacteria to flourish, even during and after sanitation practices. When  the biofilms then rupture due to turbulence, mechanical impact, or other  shocks, the large amounts of bacteria that were able to develop within the film are released into circulation, increasing the levels of system contamination. As the bacteria can be “hidden” in the biofilm for unknown periods of time, even system testing of contamination levels can be inconclusive.

Pictured in figure 1: Legionella biofilm crosssection

Ideal growth temperature for the bacteria is between 95º and 115ºF (35-46ºC). Below 68ºF (20ºC), the bacteria can survive, but remains dormant. At higher temperatures, above 112ºF  (50ºC), Legionella can survive, but stop reproduction. At 131º F (55ºC) the bacteria dies within 5-6 hours; within 32 minutes at 140ºF (60ºC), within 2 minutes at 151ºF (66ºC), and instantly at around 160ºF (70ºC). Therefore, it is suggested to maintain thermal disinfection temperatures between 158º and 176ºF (70º-80ºC) in  DHW systems ¹.

Pictured in figure 2:  thermal range of legionella

The bacteria thrives in potable water systems, including piping to showers and bathtubs, spas, swimming pools, fountains, and cooling towers before being spread through the air where it can infect humans through inhalation of infected water vapor. Many cases of Legionnaires’ Disease are concentrated in shared public spaces, like cruise ships, hotels, dormitories, health clubs and senior living centers. According to a study performed across 15 Philadelphia hospitals, sixty percent, or 9 out of the 15 facilities inspected, were contaminated with the bacteria. These contaminated water supplies were found to be significantly attributed to the design of the hot water delivery system. This study suggests that contamination of the  Legionella pneumophila  bacteria can be predicted based on the design on the water distribution system ².

People can contract Legionnaires’ Disease from inhaling mist containing the bacteria. Once in the lungs, the bacteria can thrive due to the warm, damp conditions, where it can multiply and cause symptoms including cough, shortness of breath, fever, muscle aches, and headaches. It is not spread from person-to person, and symptoms can take up to 10 days to appear once exposed. Older people, current and former smokers, and those with underlying illnesses or weakened immune systems are at the highest risk of contraction. Once contracted, the illness is treated with antibiotics; most healthy people will recover, though hospitalization may be necessary. Approximately 10% of individuals diagnosed with Legionnaires’ Disease will die from the infection.

Cases of Legionnaires’ Disease have been on the rise. According to the National Notifiable Diseases Surveillance System, between 2000 and 2017, cases of the infection have increased 550% to approximately 2.25 individuals in 100,000 contracting the illness ⁴. This increase can be attributed in part to several things, including the increased interest in conservation efforts, which lead to higher usage of low-flow plumbing fixtures and lower storage temperatures. An aging population is also attributed to increasing rates of infection, as individuals over age 50 are more likely to contract the illness ⁴.

Pictured in figure 3:  rate of reported cases Legionnaires’ Disease

Methods of control and prevention

To help stave off the infection, the HVAC industry has begun taking action with a number of new standards and guidelines aimed at preventing Legionella bacteria and infection. The American Society of Heating, Refrigerating and Air-Conditioning Engineers Standard 188 establishes a minimum of Legionellosis risk management requirements. The American Society of Plumbing Engineers, and The American Society of Inspectors of Plumbing and Sanitary Engineers have also developed a number of certifications and qualifications directly associated with Legionella prevention training ¹.

Parts of the methods of prevention and control include focal and systemic disinfection. Systemic disinfection can be performed in a number of ways. Copper-silver ionization and chlorination are among the most popular, but both have drawbacks. For effective ionization, the water’s pH must maintain levels below 8.5 in order to prevent interference of the disinfection practices with the copper-silver ions. For this method to be effective, a constant dosing of the water system with copper-silver ions must be present. Systems are available which can automate the testing and dosage, but can present a cost barrier in some circumstances.

Chlorine can also be affected by the water’s pH, and is also limited in its efficacy because it takes prolonged time to significantly reduce the growth of the bacteria. For efficacy in preventing legionella, however, chlorine levels must be higher than acceptable for drinking. Periodic system flushing with higher chlorine levels can aid in disinfection practices, but system shutdown is required before restarting the water system to flush the chlorine out of the potable water. This type of full-system flush can be complicated by “dead legs,” or areas of the water system that are not directly flushable. These areas of the system may sit for long periods of time, enabling bacterial development and growth. These areas of the system must be manually flushed by opening fixtures in those areas in order to complete a full system flush.

Focused disinfection methods currently in use across the industry include point-of-use filtration methods, ultraviolet radiation (either point-of-entry, or full-system disinfection, or point-of-use single appliance treatment options are available), and increasing the storage temperature of domestic hot water systems to above the disinfection temperature of 158º F. According to the review article Controlling Legionella in Hospital Drinking Water: An Evidence-Based Review of Disinfection Methods, point-of use, especially ultraviolet radiation, is the most effective method of preventing Legionellosis infection. For this type of disinfection to be effective, water must be filtered to remove all particles which could block bacteria from the light, and lights must be regularly monitored and replaced periodically. Despite the number of methods reviewed, it is always critical that “...water temperatures...must be rigorously maintained and monitored...” ⁷.

Protecting Domestic Hot Water Systems from Legionella

Due to their nature, the DHW systems that supply hotels, dormitories, and other larger-scale multi-resident facilities are more prone to growth and transmission of Legionella than smaller scale hot water systems. Their larger storage tanks and longer stretches of piping allow for more room for bacteria to grow, and create larger overall supply systems that must be treated for disinfection. Because Legionella grows and thrives in warm, stagnant water, the DHW storage tank is a place where the bacteria can thrive. With an on-demand DHW system, on the other hand, this risk can be partially mitigated. An on-demand DHW system, also referred to as an “instantaneous water heater” heats water as it is used, meaning no hot water is stagnated inside the hot water heater.

Certain on-demand  DHW systems like the  Viessmann Vitotrans 300, shown above, offer a sanitation function which, when enabled, will heat water up  as hot as 190ºF (88ºC), well above the temperature at which the bacteria is  instantly killed once per day. As this water is heated and moved throughout  the distribution system, the temperature of the super-heated water does  drop, but is able to maintain sanitation temperatures throughout the entire  building, killing off any bacteria that may have found a home inside the water  delivery systems throughout the entirety of the building. Once the sanitation  function’s daily cycle is complete, the system’s temperature will return to  the set point temperature programmed into the DHW system. This type of  sanitation functionality is not a complete prevention method, but rather a  part of a facility-wide Legionella prevention strategy.

The supply temperature (and especially the extra-heated sanitation function) water is far too hot to be used safely for  showers, hand washing, etc.. To compensate for this, electronic mixing valves are used to reduce the temperature  of the DHW water before it reaches use-points. These mixing valves provide safe water temperatures at the usage point,  which is required at points of use with DHW systems that supply temperatures above 120ºF. Both point-of-use and  point-of-source mixing valves are available, each with their own unique purpose and benefits. Point-of-use mixing  valves are used to moderate temperatures at a single fixture, ie: at a single sink or shower. These systems offer a  high level of protection against scalding, as they are designed to automatically regulate water temperature no matter  the storage temperature by combining auto-varying levels of hot and cold water as it is being drawn from the tap,  shower, etc. Point-of-source valves can be attached to the DHW system itself, mixing hot water from the heating unit  with cold water, mixing them as the water is pumped through the building. These types of valves can also be used for  recirculation systems in order to help maintain water supply temperatures at safe levels. Point-of-source valves also  allow for DHW to be stored at a higher temperature.

Cited Sources

  1. Legionella, Caleffi S.p.A, 2014:
  2. Determinants of Legionella pneumophila Contamination of Water Distribution Systems: 15-Hospital Prospective  Study, Vickers,Yu, Hanna, Murace, Diven, Carmen & Taylor; Infection Control, 1987
  3. Legionella (Legionnaires’ Disease and Pontiac Fever): Monitoring Your Building Water, Centers for Disease Control  and Prevention:
  4. CDC Discoveries; Legionnaires’ Disease, Centers for Disease Control and Prevention:
  5. In Philadelphia 30 Years Ago, an Eruption of Illness and Fear, Lawrence Altman; The New York Times, 2006:
  6. Thermostatic Mixing Valves, One Community:
  7. Controlling Legionella in Hospital Drinking Water: An Evidence-Based Review of Disinfection Methods, Lin, Stout &  Yu; Infection Control and Hospital Epidemiology, 2011
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