Anti scalding warning label on an A.O. Smith gas fired power vented water heaterLegionella bacteria hazards in water heaters

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Legionella sp. bacteria hazards in water heaters & hot water plumbing systems:

This article explains why there is a risk of harmful or even dangerous Legionella sp. bacteria formation in hot water heating systems and describes the minimum temperature required in these systems to avoid the Legionella hazard.

What hot water temperatures protect against Legionella & related Anti-Scald Regulations?

Green links show where you are. © Copyright 2017, All Rights Reserved.

Legionella Bacteria & Legionnaires' disease hazards in water heaters, hot water tanks & hot water cylinders or geyers

Watch out: attempts to avoid scalding burn hazards by lowering the maintenance temperature of the water heater itself convey a new hazard: at lower temperatures in water heaters there is a risk of formation of Legionella sp. bacteria. Regarding prevention of Legionella, cf Veterans Administration source in citations at the end of this article. Quoting:

Water temperatures exceeding 124 degrees Fahrenheit (°F) (51 degrees Celsius (°C)) are necessary to prevent the rapid growth of Legionella, the causative agent of Legionella pneumonia (traditionally known as Legionnaires’ disease) in hot water systems. Cold water systems (below approximately 68°F (20°C)) tend to be too cold to foster rapid growth of the organism.

For normal or otherwise healthy individuals, 120°F at the sink tap is adequate to minimize the risk of scalding. Some patients, due to illness, disabilities, advanced age or side effects of medication, may be less sensitive to temperature and thus be at increased risk for tissue damage caused by extended exposure to hot water.

Patient exposure to heated water occurs via a number of mechanisms (e.g., bathing, hydrotherapy, showering). For immersion bath wa ter, scalding is possible at temperatures exceeding normal body temperature; however, the risk is small below 105°F. At 117°F, scalding risk increases significantly. At 140°F, second degree burns may occur after only 3 seconds of exposure. Some patients are extremely sensitive to scalding.

It is not possible to maintain water temperatures at the outlet that prevent the growth of Legionella and simultaneously eliminate the possibility of scald injury in persons partially or fully insensitive to hot water temperature.

The requirement to keep water quite hot in the water heater or piping systems of buildings in turn leads to the need for ANTI SCALD VALVES / MIXING VALVES to provide scald protection for building occupants.

Reference Citations for Anti-Scald Temperature Limits, information & regulations

  1. "AS/NZS 3500.2:2003 A4 Plumbing and drainage - Sanitary plumbing and drainage." Website:
  2. American Society for Heating, Refriger ating and Air-conditioning Engineers (ASHRAE). Guideline 12-2000. Minimizing the Risk of Legionellosis Associated with Building Water Systems, 2000.
  3. Centers for Disease Control and Prevention (CDC). Guidelines for Environmental Infection Control in Health-care Facilities. Recommendations of CDC and the Healthcare Infection Control Practices Advisory Committ ee (HICPAC). Morbidity and Mortality Weekly Reports (MMWR) 52 (RR10):1-42; 2003.
  4. Cerovac S, and Roberts AHN. Burns Sustained by Hot Bath and Shower Water. Burns 26:251-259; 2000.
  5. Henriques, FC. Studies of Thermal Injury; Predictability and Significance of Thermally Induced Rate Processes Leading to Irreversible Epidermal Injury. Archives of Pathology 43:489-502, 1947. NOTE: Although published several decades ago, this reference is a landmark study on thermal injury.
  6. Moritz AR and Henriques FC. Studies of Thermal Injury; the Relative Importance of Time and Surface Temperature in the Causation of Cutaneous Burns. American Journal of Pathology 23:695-720, 1947. NOTE [from the US VA]: Although published several decades ago, this reference is a landmark study on thermal injury.
  7. Stephen FR, and Murray JP. Prevention of Hot Tap Water Burns – a Comparative Study of Three Types of Automatic Mixing Valves. Burns 19(1):56-62, 1993.
  8. Standards for temperature controls for hot water systems
    1. ASSE 1016 - Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations
    2. ASSE 1017 - Temperature Actuated Mixing Valves for Hot Water Distribution Systems.
    3. ASSE 1062 - Temperature Actuated Flow Reduction Devices
    4. ASSE 1066 - Individual Pressure Balancing Valves for Individual Fixture Fittings
    5. ASSE 1069 - Automatic Temperature Control Mixing Valves
    6. ASSE 1070 - Water Temperature Limiting Devices
  9. "Domestic Hot Water Temperature Limits for Legionella Prevention an Scald Control", VHA Directive 2009-009, (Feb 2009) [USA] Department of Veterans Affairs, Veterans Health Administration, Washington D.C. retrieved 3/21/2014, original source:
  10. Domestic Hot Water Scald Burn Lawsuits... D. Bynum et als., "Legislation to Prevent Scalding from Hot Water, A quick guide to English, Welsh and Scottish Regulations", retrieved 3/20/14, original source:
  11. "Hot Water System Information", Ron George Design & Consulting Services, retrieve 3/21/2014, original source:
  12. "How the dual issues of scalding an Legionnaire's Disease are being handled around the world", AntiScald, Inc., 70-40 137th St., Flushing NY 11367, USA, Tel: 718-268-7126, email: retrieved 3/20/14, original source: This company is a distributor of anti-scalding products.
  13. "Legionella and the prevention of legionellosis", World Health Organization WHO, retrieved 3/21/2014, original source:
  14. Safe Water Ltd., Safe Water Ltd · PO Box 100 · Picton · New Zealand
    Mobile 021 800 825 · Email Website: distributes anti-scald safety products in New Zealand
  15. "Recommended Code of Practice for Safe Water Temperatures", Thermostatic Mixing Valve Manufacturers Association (TMVA), TMVA, Westminster Tower, 3 Albert Embankment, London SE1 7SL Tel: 020 7793 3008 Fax: 020 7793 9730 e-mail:, retrieved 3/21/2014, original source:
  16. "Guidelines for Safe Recreational Water Environments, Vol. 2, Swimming Pools & Similar Environments", World Health Organization, WHO, retrieved 3/21/2014. Original source:

Legionella Contamination in Drinking Water Research


Continue reading at LEGIONNAIRES' DISEASE INFO from CDC or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see



Legionella BACTERIA & HVAC Equipment

Suggested citation for this web page

Legionella BACTERIA in WATER HEATERS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Or see


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Or see


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Or see


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman