The goal is to design systems that minimize conditions conducive to these and other bacteria, include preventive measures in operating and maintenance specifications, and document preventive recommendations rejected by the owner.

Water should remain in domestic water heaters for no longer than 24 hours.


Legionnaires’ disease, a deadly pneumonia, is caused by inhaling small water droplets contaminated with Legionella bacteria, or by accidentally choking the bacteria into the lungs while drinking. People can be infected with Legionella while washing their hands or face, brushing their teeth, showering, walking by a decorative fountain, bathing in a whirlpool spa, drinking from a water fountain, or inhaling mist emitted from a cooling tower. The risk is reduced by minimizing Legionella bacteria in water systems, preventing transmission of the bacteria from water to people, or increasing an individual’s immunity. Based on currently available knowledge and technology, minimizing Legionella bacteria in water systems is the primary means of prevention.

It is not surprising that plumbing systems are implicated in a significant number of Legionnaires’ outbreaks - they provide a good habitat for Legionella as well as many points of exposure. Designing, installing, operating, and maintaining plumbing systems to minimize Legionella bacteria is, therefore, key to reducing risk of disease. This gives plumbing engineers a greater role in Legionnaires’ prevention than doctors, infection control professionals, HVAC engineers, water treatment specialists, microbiologists or epidemiologists. Only facility managers have equal or greater responsibility.

Plumbing engineers must therefore understand and implement Legionella-preventive designs and specifications, not only for the moral obligation to protect health and life, but to reduce legal risk. If you design a plumbing system for a building that is implicated in a case of Legionnaires’ disease, you may be sued by the person who got sick, or by his or her family if the case is fatal.

Here is a brief list of recommendations designers can follow to help prevent Legionnaires’ disease (there are many more that cannot be covered here). Most of these recommendations have been raised in lawsuits related to Legionnaires’ disease:


1. Ensure that the proper type of backflow prevention devices are installed at appropriate locations. At a minimum, backflow preventers should be installed on domestic water lines feeding cooling towers, heating water systems, lawn irrigation systems, pools and spas, garden hoses, chemical dispensers (e.g., in janitor closets), fire sprinkler systems, x-ray film processors, and dialysis equipment. Although usually not required by code, it is also a good idea to install double check valves on lines serving potable but bacteria-prone devices such as piped coffee makers, ice machines, soda dispensers and water softeners.

2. Design the domestic water heating system so that water does not remain in tanks for longer than 24 hours. Redundant heaters should be rotated so that each is off for no longer than 24 hours.

3. Operate domestic water pumps every day. For example, if two pumps are installed on the domestic hot water return line, but only one is operating at a given time, they should be alternated so that neither is offline for more than 24 hours. Stagnant water in idle pumps and the piping isolated with them can provide a habitat for Legionella and other bacteria that can enter the system when the pump is turned on.

A series of domestic water heaters.

4. Run domestic hot-water recirculation pumps continuously. They should be excluded from energy conservation measures.

5. Keep redundant and backup lines valved open or flush them before use. Where there are redundant or backup water lines, both branches should ideally be kept open to prevent water stagnation. If, however, one segment is valved off, it should be thoroughly flushed periodically and before each time it is used. The water should be flushed to a drain so that none of the potentially contaminated water is distributed downstream to the building. This may require an additional valve and drain at the downstream end of each segment.

6. Allow for good hot water recirculation. Consider automatic air vent valves at trapped high points of the hot water circulation system, and specify balancing devices that allow accurate and relatively easy balancing of the system. Ensure that the flow rate through each riser is enough to balance the hot water recirculation system with the balancing valves specified.

7. Specify proper preoccupancy disinfection for renovations as well as new construction. Flush tanks and piping with chlorine according to ASPE’s recommendation of 50 ppm chlorine for 24 hours, rather than merely meeting the local code requirements. And, specify that the contractor is to obtain a written report certifying the chlorine concentrations reached and the amount of time they were held. A qualified water treatment company will likely charge significantly more for the chlorination than will a plumbing contractor, but it may be worth the extra cost.

The lead booster pump should be alternated to ensure that each pump is used daily.

8. In general, prefer the simple to the complex regarding the design of domestic water systems. The more piping, fixtures, and valves, the more surface area and crevices where biofilm can develop and stagnation can occur. Heat recovery systems, mixing valves, and other complexities in the system may do more harm than good with respect to bacteria control.

9. Specify water heaters that have the capability of heating the entire system to 160°F. Although the system will not be operated at that high temperature, it may be necessary for emergency disinfection. Inform the owner in writing that both scalding and the control of Legionella and other bacteria should be considered in setting hot water target temperatures. Temperatures above 122°F inhibit Legionella growth but do not guarantee against it.

As for the risk of scalding, skin damage in adult males occurs in about 5-10 minutes at 120°F, 15-30 seconds at 130°F, and 3-5 seconds at 140°F. Children and others with sensitive skin will be scalded quicker and at lower temperatures.

10. Specify preventive measures during major construction, especially during excavation, pile driving, or the opening and closing of water feeds. Before opening water feeds that have not been used for more than three days, flush them to waste for a long enough time to flush the pipe volume several times and until the water is clear. Isolate any occupied portions of the building from water pressure shock as much as possible.

In occupied buildings, consider continuous disinfection of the domestic water system with chlorine or chlorine dioxide during the entire construction period, in particular to minimize the risk of bacteria released due to the dislodging of biofilm. If a new building is finished and occupied in phases, ensure that water is flushed at least weekly in the unoccupied areas.

In general, plumbing engineers should design systems to minimize conditions conducive to Legionella and other bacteria, include Legionella-preventive measures in operating and maintenance specifications, and document Legionella-preventive recommendations that were rejected by the owner.

Both of these backflow preventer lines should be left open. If one is closed and used as a backup, it should be flushed to waste before each use.

Increasing Your Business

Legionella gives plumbing engineers opportunities as well as responsibilities. Plumbing engineers can increase their income by offering services related specifically to Legionella. The demand for Legionella assessments is on the rise and will likely increase further after the new ASHRAE Legionella standard is released.

Plumbing engineers trained in Legionella control can play an integral role in such services, either directly to owners or as a subcontractor to a Legionella specialist, water treatment company, or environmental consulting firm - in particular in inspecting domestic water systems and drawings for conditions that may be conducive to growth of Legionella and other waterborne bacteria.

Plumbing engineers who offer Legionella assessments directly to owners need to disclose the limitations of the assessment, specifically describing the services you are not providing as well as the ones you are. For example, the plumbing engineer should state in writing that the assessment pertains only to the plumbing system, advising the owner that cooling towers, decorative fountains, whirlpool spas and other devices are also potential sources of Legionella contamination.

In sum, plumbing engineers who are knowledgeable of Legionella can reduce their legal risk, expand their business, and even save lives.

Course Focuses on High Quality Legionella Assessments

On May 19-21, author Matt Freije will hold an intensive course, “Performing High Quality Legionella Assessments,” at the Carlsbad Inn Beach Resort, in Carlsbad, CA. The course provides comprehensive training for engineers, water treatment specialists, industrial hygienists, and environmental health specialists who want to perform high quality Legionella assessments in hospitals, hotels, or industrial facilities. Because of the facility walk-through, the class will be limited to approximately 15 students.

Attendees will learn important foundational facts about Legionella and other waterborne pathogens, specific and cost-effective ways to minimize bacteria in water systems, how to inspect plumbing systems and cooling towers, the best methods for choosing sample locations and collecting water samples, secrets to conducting efficient and thorough facility assessments, and simple formulas for estimating the time and expenses required to conduct assessments.

On days one and two, the classroom training will be held from 8:30 a.m. to 3:00 p.m. and breakfast and lunch will be provided. On the third day, students will be shuttled (free of charge) in the morning to a hospital in San Diego for a hands-on facility walk-through, and then returned to the Resort in the afternoon.

Rooms at the Carlsbad Inn Beach Resort are available for attendees at $159 per night. To reserve a room there, contact Caroleann Petz at 800-235-3939 (x451) or CPetz@gpresorts.com, and tell her you will be attending the HC Info seminar. Rooms are limited so book early. For information on the course, visit www.hcinfo.com or call 800-801-8050.