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Legionnaires' Disease

Section II:C-1. Domestic Hot-Water Systems

Electric Water HeaterLegionnaires' disease has been associated with domestic hot-water systems in a number of outbreaks. Large water heaters like those used in hospital or industrial settings frequently contain cool zones near the base where cold water enters and scale and sediment accumulate.

Control Strategy:

TopHow to maintain a hot-water system
Fig.1: Recirculating pump
Fig.1: Recirculating pump
Properly maintaining a hot-water system includes being aware of the following.

Conditions that promote growth:
  • Scale and sediment supply the environment needed for growth of Legionnaires' disease bacteria (LDB) and other microorganisms.

  • Dead legs and non-recirculated plumbing lines that allow hot water to stagnate also provide areas for growth of the organism.

  • Temperatures maintained below 60°C (140°F) encourage growth of LDB and other microorganisms.
  • Maintain domestic water heaters at 60°C (140°F) and water delivered at the faucet at a minimum of 50°C (122°F). Where these temperatures cannot be maintained, control LDB growth with a safe and effective alternative method. Also see What to consider in the system design.

  • Proper insulation of hot-water lines and heat tracing of lines can help maintain distribution and delivery temperatures at 50°C (122°F).

  • If potential for scalding exists, employ appropriate fail-safe scald-protection.
System Controls:
  • Run recirculation pumps continuously and exclude them from energy conservation measures.

  • Eliminate or minimize the use of rubber, plastic and silicone gaskets in the plumbing system. These materials may serve as growth substrates for LDB. Frequent flushing of these lines also reduces growth.

  • Identify and test the integrity of all backflow preventers (to ensure protection of domestic water from cross-contamination with process water) through a building code-approved method.
  • Hot-water tanks should be drained periodically to remove scale and sediment.

  • Periodically chlorinate the system at the tank to produce 10 ppm free residual chlorine and flush all taps until a distinct odor of chlorine is evident as a means of control. The tank should be thoroughly rinsed to remove excess chlorine before reuse.

    • In-line chlorinators can be installed in the hot water line; however, chlorine is quite corrosive and will shorten the service life of metal plumbing.

    • Control of the pH in the range of 6.8 - 7.0 is extremely important to ensure that there is adequate residual chlorine in the system.
  • Additional information (App II:A-1) on biocides is also available.
Alternative Controls:
Please note: ozone and ultraviolet treatments are short-lived in their effectiveness and do not provide residual disinfection, as does continuous chlorination.
  • Metal ions, such as copper or silver, in solution have a biocidal effect.

  • Ozonization injects ozone into the water, killing microorganisms.

  • Ultraviolet (UV) radiation also kills microorganisms.

    • Commercial, in-line UV systems are effective and can be installed on incoming water lines or on recirculating systems.

    • Scale buildup on the UV lamp surface can rapidly reduce light intensity and requires frequent maintenance to ensure effective operation.

TopWhat to consider in the system design
Fig.2: Avoid points of stagnation
Fig.2: Avoid points of stagnation
  • Water systems designed to recirculate water and minimize dead legs will reduce stagnation.

  • Pressure-independent, thermostatic mixing valves at delivery points can reduce delivery temperatures.

  • Point-of-use water heaters can eliminate stagnation of hot water in infrequently used lines.

  • Eliminate dead legs when possible, or install heat tracing to maintain 50°C (122°F) in the lines.

TopHow often to test hot-water systems for LDB following identification of contamination
Outbreak Protocol:

Test the domestic hot- or warm-water system for LDB on the following schedule to ensure that recontamination has not occurred:
  1. Weekly for the first month after resumption of operation.

  2. Every two weeks for the next two months.

  3. Monthly for the next three months.

TopHow to collect hot-water samples
Fig.3: Sampling from an electric water heater
Fig.3: Sampling from an electric water heater
Collect water samples to determine potential contamination as outlined above.
Please Note: It is important not to flush a hot-water outlet before taking a sample because the end section of the water system may be a source of contamination.

Water Sampling Protocol:
Sampling information specific to hot-water systems is provided below. For more information about water sampling, please refer to Section II:E. Water Sampling Guidelines.
  • Sample the incoming water supply if the plumbing provides access.

  • Sample hot and tepid water tanks and reservoirs.

  • Sample faucets and showerheads throughout the facility, for example, those nearest, intermediate, and most distant from water heaters, storage tanks, and connections with municipal water supplies.

  • If a biocide is used, follow the manufacturer's instructions for proper neutralization.
Interpretation Guidelines:
  • The OSHA suggested guideline for LDB in domestic hot- and tepid-water systems is less than 10 CFU per milliliter.

    • If LDB concentrations are below 10 CFU per milliliter and no LDB were detected in swab or other samples, no further monitoring for LDB is necessary. Continue the maintenance program as long as the system is in use.

    • If water concentrations exceed 10 CFU per milliliter or LDB were detected in other samples, take steps to identify the source of contamination or amplification and treat the system.
      See How to treat a contaminated water system.

      • Sample the water system monthly until the source of contamination is identified and adequately treated. Once LDB concentrations remain below 10 CFU for a tree-month period, sampling may be stopped.

TopHow to treat a contaminated hot-water system
Thermometer - For problems with accessibility in using figures and illustrations, please contact the Directorate of Technical Support and Emergency Management at (202) 693-2300.
Use the treatment procedure below to treat all hot-water systems that have either been tested and found to contain detectable levels of LDB or are assumed to be contaminated based on epidemiological evidence.
  • Disinfect the system using an effective chemical, thermal, or other treatment method. For example:

    • Pasteurize the hot water system by heating the water to at least 70°C (158°F) and maintain this temperature during the flushing period. While maintaining the temperature at 70°C (158°F), continuously flush each faucet on the system with hot water for 5-20 minutes.

    • Use an accepted chemical disinfectant such as chlorine or an acceptable biocide treatment to clean the system. Thoroughly flush the system after treatment to remove all traces of the corrosive and possibly toxic chemicals.

    • Follow any other technique that has demonstrated effectiveness and safety.
  • After treatment, sample the hot water from each storage tank. If LDB are detected, re-treat and re-sample the water system. If no measurable levels are found and all other potential sources have also been addressed, proceed with follow-up testing as outlined above.
For additional information about treatment technologies, see:

For problems with accessibility in using figures and illustrations, please contact the Directorate of Technical Support and Emergency Management at (202) 693-2300.
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