In Hot Water

By Barbara Horwitz-Bennett, Contributing Editor January 1, 2006

Industry experts discuss strategies for reducing Legionella outbreaks while, at the same time, maintaining water temperature at levels safe enough to prevent scalding.

CSE: How common is it for facility owners and designers to proactively address the issues of Legionella and scalding?

FITZGERALD: Unfortunately, not as common as it should be. I attribute this to the lack of understanding by infection control practitioners and facilities management about sources of bacteria and effective treatment protocols.

SHEERIN: But in general, media coverage, facility maintenance education and sometimes local and state regulatory enforcement have created a greater awareness of the Legionella hazard. And most health-care facilities we work with proactively monitor and maintain their domestic hot water systems.

HOOVER: While that’s generally true, one obstacle I foresee to a more proactive approach is the rising cost of natural gas. People will lower water heater thermostats to conserve energy without considering the potential for in- creased Legionella.

CSE: What types of facilities are most vulnerable to these problems?

KNIGHT: Hospitals, nursing homes, assisted-living centers and similar facilities whose occupants are often at greater risk due to their compromised immune systems. Other locations of special concern include critical-care units and those dedicated to organ transplants and cardio care, where Infection control is so essential.

HOOVER: Really, any older facility with systems that are outdated or facilities that have gone through a series of “quick-fix” alterations.

CSE: How does one strike a balance between water temperatures hot enough to kill Legionella but not so hot as to scald?

HOOVER: The American Society of Plumbing Engineers published a paper in 1988 recommending that stored water temperatures be maintained at 140°F to prevent Legionella. This is a safe temperature for hot water, when used as intended and in accordance with the warnings and instructions supplied with hot water products. Another option is to install a tempering valve at every fixture in order to reduce the risk of scalding. With this solution, the water can be stored at 140°F until it is tempered to the right level and delivered through the fixture to avoid scalding.

FITZGERALD: While maintaining hot water outlet temperatures of 140°F will slow down the growth of the bacteria, it won’t effect a total kill. As an alternative, I suggest installing semi-instantaneous or instantaneous water heaters to reduce the formation of Legionella colonies, and the installation of thermostatic mixing valves at point of use for scalding protection.

KNIGHT: Another problem with storing water at 140°F is distribution, at least in the United States. Outside the U.S., particularly in western Europe, 140°F is the standard recirculation temperature, because more advanced systems offer superior point-of-use thermal protection.

But in the U.S., lower temperature recirculation is a result of long-standing trends and budget-driven selection of inexpensive pressure-balance mixing valves for point-of-use showering and bathing. Except for a basic mechanical maximum temperature limit stop on the shower valve handle, there will always be a concern if facilities start to elevate recirculating hot water temperatures.

CSE: So what can one do?

KNIGHT: One way to achieve a balance would be to move away from pressure-balance shower valves for a better level of thermal protection at the points of use—at least in critical-care installations.

CSE: At what points in a system is Legionella most likely to appear and multiply?

SHEERIN: Any place where particulate can concentrate, whether a storage-type water heater, dead leg, shower head or faucet aerator fitting, is of concern. But it’s municipal water quality that determines the potential hazard.

KNIGHT: Another particular concern is the infrequently used point-of-use fixture, which can create amplification pockets for Legionella to multiply, serving to retard a remediation protocol.

FITZGERALD: Really, any copper system contains layers of bio-film that provide an environment for development of Legionella. Also, the misting effect of showerheads and leaking faucet aerators allows aerosolization of water droplets containing Legionella into the lungs. Cooling towers and evaporative condensers are other sources, but their greatest risk is to workers who clean and maintain the equipment. Cooling towers that have high levels of drift—a water vapor—can create a hazard to the public depending on their location and prevailing winds. Other potential sources of Legionella are hot tubs, spas, sprinkler systems and vegetable misters.

CSE: What are some of the typical flaws in current plumbing designs?

KNIGHT: With regard to flaws in plumbing design, some thermostatic mixing valves (TMV) are simply not designed for the jobs for which they are being specified. With some brands, one can only truly control system “temperature creep” by accessorizing the TMV with supplemental controls, such as placing an aqua-stat on the pump. This option, however, is in direct conflict with OSHA’s recommendation for Legionella risk reduction, as it allows complete plumbing systems to stagnate during periods of no fixture demand.

CSE: What technologies are available to deal with Legionella and scalding? What are their advantages and disadvantages?

KNIGHT: Legionella control systems are classed as “biocide treatments” or “thermal treatments.” There are pros and cons to both, but both clearly benefit from the ability to deliver the treatment to all points within the plumbing system.

A new technology gaining traction integrates electronics for water temperature and flow control at the point of use. Electronic point-of-use controls can be programmed using hand-held devices such as PDAs to evacuate infrequently used fixtures, allowing the Legionella abatement treatment to pass through what would otherwise be a “traffic jam.”

If thermal treatment is the protocol of choice, then electronic mixing valves (EMVs) for central recirculation system control offer a proactive solution. Even though they tend to be more expensive, EMVs can be tied into the resident BAS. Water temperatures can be pre-set and consistently monitored to remain at a level that reduces the Legionella risk.

In addition, EMVs with BAS offer remote set-point adjustment features that enable users to comply with OSHA and other recommendations that urge periodic system pasteurization or “thermal disinfection.” Ultimately, each situation should be evaluated in the context of the environment. Rarely is there a single solution that will eliminate the problem. Biocide treatments, for example, only treat waterborne pathogens, and chemical treatments can damage system components and require repeat treatments.

SHEERIN: Other Legionella and domestic water disinfection technologies include UV lights, ozone and gas chlorination. Certainly, the point-of-use and instantaneous-type water heater designs offer improved protection. No water storage means fewer opportunities for contaminants to collect and amplify. Non-scalding water temperature is achieved through any of a myriad of thermostatic or pressure-balance valves on the market. There are also faucets that incorporate sensor actuation and thermostatic mixing, which simplify a system and make it safe.

As far as evaluating these different options, UV light systems are highly efficacious, but performance declines with increased lamp life and decreased cleanliness. Also, UV offers no downstream residual protection: The system only kills bacteria where the water passes by the lamp.

Chlorination systems, on the other hand, provide residual kill and a variety of delivery methods, from batch to liquid injection to gas chlorinators. But at improper levels, chlorine can corrode copper piping systems.

FITZGERALD: Another methodology that has proven effective in controlling Legionella colonies in hot water distribution systems and cooling towers is copper/silver ionization. The process uses positively charged ions of copper and silver, which are attracted to the negative ionic structure of bacteria. Copper and silver ions penetrate the cell structure of the organism, limiting its ability to absorb nutrients and causing it to die by lysis. Ions are generated by passing low-voltage current through copper/silver electrodes contained in a flow cell installed in the hot water distribution system. Flow cells need to be installed on the discharge side of recirculation pumps so those ions are carried to distal sites and provide residual ions for continuous treatment.

Advantages of this approach include the fact that it can be readily retrofitted into existing hot water systems and has a proven track record in several hundred installations. On the other hand, equipment is relatively expensive. In large facilities with multiple hot water distribution systems, multiple ionization systems would be required. Furthermore, electrodes scale up in hard water and require frequent cleaning. Finally, accurate testing for silver ion levels require atomic adsorption, which normally has to be done at an outside lab.

As Michael intimated, the use of chlorine dioxide, a neutral compound of chlorine in the +IV oxidation state, is another effective strategy. It disinfects by oxidation; however, it does not chlorinate. Chlorine dioxide can be generated on-site using several methods, but one of the most popular is an electro-chemical method that directly converts sodium chlorite to chlorine dioxide.

Unlike chlorine, chlorine dioxide will not combine with other organics to form trihalomethanes, which are suspected carcinogens; it will not cause corrosion in copper pipes; and it maintains an effective residual in systems. Disadvantages include that it is sometimes difficult to monitor and maintain proper levels to effect a kill and yet remain at levels mandated by the EPA. Also, the number of installations and length of time installed are limited in comparison to copper/silver.


Robert W. Fitzgerald , Technical Sales, Neal & Assocs., Dallas, (An Aerco Distributor)

Dr. William Hoover , Chief Engineer, A.O. Smith Corporate, Technology Center, Milwaukee, with the assistance of Brad Plank , P.E., Product Safety Manager, A.O. Smith, Dallas

Paul L. Knight , Director of Sales and Marketing Armstrong, Hot Water Group, Three Rivers, Mich.

Michael P. Sheerin , P.E., LEED AP , Principal and Director, Healthcare Engineering, TLC Engineering for Architecture, Orlando, Fla.

The ABCs of Legionella

Legionella is ubiquitous in groundwater sources. The bacteria require an ecosystem to grow and multiply. Bio-film, which consists of scale (CaCO 3 ) microorganisms and other organic compounds, form the perfect growing medium on the surface of pipes and heat transfer equipment. The irregular molecular shape of CaCO 3 facilitates the formation of layers of bio-film when heated, and provides a surface to which the bacteria attach themselves. In a large storage-type water heater with tube bundles, these scale deposits proliferate. As a result, colony counts are much higher than those found in distribution piping.

The ideal temperature range for Legionella growth is 68°F to 122°F. At 130°F, its metabolism slows down, but it takes a hot water temperature of 158°F to effectively kill the bacteria. Thermal eradication, commonly known as “heat and flush,” requires raising the water temperature to 158°F, opening all hot water outlets and allowing a flush period of 20 minutes. However, this method has proven ineffective due to the insulating properties of scale. The upper layer of scale protects lower layers. Also, increased velocities in the system cause erosion of outer layers and result in exposure of higher colony counts in a relatively short period of time.

Laying Down the Law on Legionella

Legionella is gaining recognition as a hazard to contend with, but so far, there hasn’t been much incorporated into codes and standards on the subject.

ASHRAE is currently preparing Guideline 29P, Risk Management of Public Health and Safety in Public Buildings, which should help, says Robert W. Fitzgerald, a Dallas area rep for AERCO Boilers, Northvale, N.J. But he doesn’t anticipate any code changes in the near future.

And while the issue has been discussed at length among many associations, Paul L. Knight, director of sales and marketing, Hot Water Group, Armstrong, Three Rivers, Mich., has found there to be some conflicting recommendations and significant misunderstandings.

“For instance,” says Knight, “many facilities management people interpret the 110°F requirement at a point of use as 110°F recirculation. All they know is that when the local health inspector stops in and puts his thermometer under a faucet, there is big trouble if that water temperature exceeds the 110°F threshold.

“These people need some level of collective direction and Legionella risk-reduction education,” he concludes.