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

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?

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

Here we discuss steps to prevent dangerous Legionella bacteria growth in water heaters. Krause (2019), in developing a tool for improved assessment of the probable Legionellosis risk based on objective data from building investigation, cites these key factors in the probability of growth of Legionella bacteria in a building water system:

Aside from or perhaps even independent from testing potable water for legionella bacteria, most experts focus on setting a sufficiently-high water heater temperature to kill bacteria and to prevent its growth in water heater tanks or hot water cylinders, calorifiers, or geysers.

Separately at Legionella BACTERIA & HVAC EQUIPMENT we discuss Legionella hazards and Legionellosis or Pontiac Fever traced to building air conditioning systems and cooling towers.

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 & TEMPERATURE CONTROL / MIXING VALVES to provide scald protection for building occupants.

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

Legionella Contamination in Drinking Water Research


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Continue reading at LEGIONNAIRES' DISEASE INFO from CDC or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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