legionella-water-heater-temperature

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Legionella water heater temperature is a question about whether the setting on a hot water system changes how much Legionella bacteria can grow in the water. Two sources address it. A 2015 laboratory study with recirculating water loops found that Legionella pneumophila “still persisted up to 58 °C.” A 2025 review of heat pump water heaters describes 60 °C as the level that periodically controls Legionella. Neither source measured a typical single-tank home heater, and neither links a specific home to an illness. This guide reports what each one says and where the evidence stops.

Quick answer

• In the 2015 study, L. pneumophila “still persisted up to 58 °C, with evidence that it was growing under the conditions of this study.”
• The same study notes that “Water heater set points do not directly translate into the temperature experienced at the tap, where it can quickly cool to room temperature.” The setting on the heater is not the temperature at the faucet.
• The 2025 review states that “While most HWHP systems provide water at 55 °C, hot water will periodically need to reach 60 °C to prevent Legionella proliferation.” HWHP means heat pump water heater.
• The review found “no research in the scope of this review has demonstrated a direct link between water from HWHP systems and any disease related to Legionella directly.”
• Changing a heater setting, or servicing a heater, is work for a licensed plumber. This guide gives no steps for it.

What the 2015 study tested

The 2015 study is by William J. Rhoads, Pan Ji, Amy Pruden and Marc A. Edwards of Virginia Tech, published in Microbiome. Its stated aim was to “conduct a controlled, replicated pilot-scale investigation” of “continuously recirculating water heaters to examine five water heater set points (39–58 °C) under three water use conditions.”

The design used two identical systems. Both were first run for five months at 39 °C to build a stable baseline. The experimental system was then raised to 42, 48, 51 and 58 °C over the course of the experiment, while the control system stayed at 39 °C for the whole 15-month run. Water passed through the recirculating loop back to the heater. Along the loop, six replicate taps were flushed on schedules that represented high, medium and low use. The influent water was first flushed through three granular activated carbon whole-house filters.

The authors measured L. pneumophila by quantitative PCR, which counts DNA copies rather than culturing live bacteria. They also tracked a host amoeba and total bacteria in the same samples. The study’s own introduction describes the organism plainly: “Legionella is notorious for growing in hot water systems.”

The result that matters most for a reader is the top set point. Legionella was not eliminated at 58 °C, and the authors report evidence of growth at that temperature. That finding comes from a recirculating loop in a laboratory system, so it describes that setup. It is not a reading from an ordinary home with a single storage tank.

What the 2025 review covers

The 2025 paper is by Jodi Brookes, Helena Senior, Rebecca J. Gosling, Duncan Smith and Margaret Wade, all at the UK Health and Safety Executive. It appeared in Microorganisms. It is a review of heat pump water heater systems, which the authors expect to grow quickly in Great Britain. The paper states that “It is anticipated that by 2028 there will be a significant increase in the use of HWHP systems in Great Britain (GB).”

The authors describe their method carefully. They write that “This review was approached systematically but was not a full systematic review.” It is a review of sources, not a new experiment. The authors state that “The review of forty sources demonstrated that Legionella are an expected risk of contamination in HWHP systems,” and the paper’s main conclusion is that “Legionella presence is an expected risk in HWHP systems.”

The review says the risk comes from older knowledge about hot water systems: “Legionella are a known risk in hot water systems that can be carried over to HWHP systems.” It also says that “there is minimal evidence to suggest that the current control measures are being appropriately applied to reduce the risk of exposure.” That line is about whether systems are managed well in practice, which is a different question from the temperature a single heater is set to.

The temperatures the sources describe

The two sources give temperatures that fit together. The 2015 study found growth risk at and below 58 °C in its loop. The 2025 review describes a common target for protection. It states that “Research has shown that almost 90% of Legionella bacteria will be inactivated when held at 60 °C.” It also says that “While most HWHP systems provide water at 55 °C, hot water will periodically need to reach 60 °C to prevent Legionella proliferation.”

The review describes what is commonly recommended, and the picture is not uniform. It states that “The most commonly stated method is to maintain the temperature consistently at ≥50 °C, with the option to boost the temperature periodically to 60 °C.” It also notes that “the recommended boosting temperature also varied between 60 °C and >70 °C.” Across the reviewed documents, it reports that the most common range “reported to control Legionella contamination was between 50 °C and 60 °C.”

The review also describes design standards from some countries. In one example it says, “Inside the tank, the water must be maintained at 60 °C,” and it adds that “The design of a system must maintain at least 60 °C if there is a storage tank.” These are standards the review summarizes. This guide did not open the standards themselves, so it does not state them as rules for any other system.

The review also cites one study on storage tanks. It reports that the study “showed that there was an increased prevalence (12%) of Legionella in homes that were supplied with storage tanks or recirculated hot water.” The 12 percent figure comes from that cited study, not from the review, and the cited study was not opened for this guide.

Who the sources say could be affected

The review states that Legionella “can potentially cause disease in the general public and those who are maintaining and replacing these systems.” It does not give a rate of illness for households, and it says that “no research in the scope of this review has demonstrated a direct link between water from HWHP systems and any disease related to Legionella directly.” The authors add that this link “would be difficult to achieve.”

The review also looked at people who service these systems. It notes that “In the sources reviewed, there was no reference to worker exposure risk during maintenance activities.” That is a gap in the sources, not a finding that workers are safe.

The review says that, under countrywide guidance, “the risk is considered lower in single- or multi-family homes that do not require a centralised system.” That is a statement about how guidance treats different housing. It is not a risk estimate for any one home.

If you have a weakened immune system or a lung condition, talk to a doctor about any question of exposure. This guide does not give medical advice.

Changing a water heater safely

Both sources discuss temperature targets, but neither is an instruction manual. Setting a water heater, replacing one, or servicing its controls is work for a licensed plumber or the equipment maker. The sources do not describe installation, repair or setting steps, and this guide gives none.

The sources also do not discuss scalding. A higher setting raises the temperature of the water at the tap, and the sources do not say what temperature is safe to touch or how to mix it at the faucet. If you are unsure about the setting in your home, ask a plumber to check it.

How we chose

This guide uses two sources that could be read in full and that address Legionella and hot water temperature directly. Both were opened on 2026-10-28. The 2015 study is a controlled laboratory study with recirculating loops. The 2025 paper is a review of heat pump water heater systems in Great Britain, and its authors describe it as not a full systematic review. Other candidates in the research folder were not used. One returned no usable body text on water hardness, and a biofilm candidate rested on a single source, which is not enough under this site’s rule of two.

Each direct quote in this guide was checked against the saved text of the source. Where a sentence is the guide’s own summary, it does not add a fact beyond what the source states.

What the sources do not say

• Neither source measured illness in homes. The 2015 study measured bacteria in a laboratory system, and the 2025 review found no direct link to disease in the scope it covered.
• Neither source tested an ordinary single-tank home heater with a changed setting. The 2015 study used recirculating loops, and the review covers heat pump systems.
• Neither source gives a home cleaning or flushing protocol for taps, showers or a water heater. The review describes thermal disinfection, meaning periodic boosts in systems, but gives no steps for a home.
• Neither source discusses scald risk from higher settings, so this guide does not state a safe temperature for touching or bathing.
• Neither source gives a home-level answer to whether a particular setting is enough. The 2015 study reports growth at 58 °C in its system, and the review gives a general target.

What we could not verify

• The 12 percent storage-tank figure comes from a study the review cites. That study was not opened for this guide.
• The national design standards the review summarizes, including those it describes for France and Germany, were not opened for this guide.
• The 2015 study placed activated carbon filters ahead of its loops. The sources do not say how much this affects Legionella levels in a home without such filters.
• Whether the 2015 growth results at 58 °C would appear in a single-tank home heater, or in a heat pump system under real use.

FAQ

Does a higher water heater setting stop Legionella?

The sources do not say that a single setting makes a home safe. The 2015 study found L. pneumophila still present at 58 °C in its loop, with evidence of growth. The 2025 review states that about 90 percent of Legionella are inactivated at 60 °C and that systems periodically need to reach 60 °C. Those are two different measurements, and neither one is a home guarantee.

What temperature does the review call the target?

The review says the most commonly stated method is to keep water at or above 50 °C, with periodic boosts to 60 °C. It also notes that most heat pump systems deliver water at 55 °C. The review did not describe a single target that applies to every home.

Why is the temperature at the tap different from the setting?

The 2015 study states that “Water heater set points do not directly translate into the temperature experienced at the tap, where it can quickly cool to room temperature.” The sources do not measure a specific home’s tap temperature, so this is a general finding from the study’s loops.

Can I lower my heater to save energy?

The 2015 study begins from the point that “Lowering water heater temperature set points and using less drinking water are common approaches to conserving water and energy.” The study examines the Legionella effect of that approach. The sources do not give a safe setting for a home, and any change should be made by a licensed plumber.

Is Legionella a risk for healthy people?

The review says Legionella “can potentially cause disease in the general public.” It does not estimate the risk for healthy people in a home, and it states that no direct link has been shown for water from heat pump systems. Ask a doctor if you have symptoms or a weakened immune system.

Who should I ask about my water heater?

Ask a licensed plumber or the equipment maker. Both sources discuss temperature, but neither gives steps for setting, repairing or replacing a heater.

Talk to a doctor

If you have a weakened immune system, a lung condition, or symptoms such as fever or a cough that does not go away, talk to a doctor. Do not change a medicine or treatment based on this guide. A doctor can advise on exposure for your situation.

Bottom line

The two sources agree that Legionella can persist in hot water systems. The 2025 review describes 60 °C as the level for periodic control. The 2015 study shows growth risk at 58 °C in a recirculating laboratory loop, and the 2025 review finds no direct link to disease for heat pump water heater water. Neither source is a home guide, and any change to a heater belongs with a licensed plumber.

Sources

• Rhoads WJ, Ji P, Pruden A, Edwards MA. Water heater temperature set point and water use patterns influence Legionella pneumophila and associated microorganisms at the tap. Microbiome. 2015;3:67. PMC4666224. https://pmc.ncbi.nlm.nih.gov/articles/PMC4666224/ (opened 2026-10-28)
• Brookes J, Senior H, Gosling RJ, Smith D, Wade M. Legionella in Hot Water Heat Pump (HWHP) Systems. Microorganisms. 2025;13(5):1134. PMC12114087. https://pmc.ncbi.nlm.nih.gov/articles/PMC12114087/ (opened 2026-10-28)