Legionella in a Home Water System: What the EPA and DC Health Advise

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This post gathers legionella in home water system guidance from the EPA and DC Health. Legionella in a home water system is a question about growth and spread. Legionella is a bacterium found naturally in lakes, streams and soils, and it can also be found in water pipes and the devices that use water in a building. The EPA says it “can become a health concern when it grows and spreads in a building’s water system”. Legionella guidance for a house is mainly about keeping the plumbing from giving the bacteria room to grow, and about limiting the spray that water fixtures make.

Quick answer

• The EPA says “Legionella grows best at temperatures between 80-120⁰F”, so the home steps aim to keep water out of that range where possible.
• The EPA says keeping water in indoor plumbing above 124⁰F or below 80⁰F can help control the bacteria, and that a thermostat setting at or above 140⁰F can often keep hot water above 124⁰F.
• The DC Health handout says to set water heaters to a minimum of 120⁰F and to consider 130 to 140⁰F.
• Both sources say stagnant water is a concern. Flushing after periods of disuse, such as travel or remodeling, is the main home step they describe.
• Neither source is a complete plumbing code. Local rules on water heater settings can apply, and the EPA tells readers to check state or local codes before changing a heater.

What the EPA says about home water systems

The EPA page splits home advice into several topics: fixtures, temperature control, flushing, and the hot and cold water systems. It says that “Because Legionella bacteria occur naturally in water, it is important to maintain your home’s water system in a way that reduces the potential for bacterial growth and spread.” It also says that several strategies can be used “in combination”, and that a healthcare provider or public health professional in your area can advise you if you are unsure which steps to take.

The EPA treats the home system as a whole. Hot water, cold water, fixtures and stagnant pipes all count. The page does not give a single number for how much risk a home carries. It gives a set of steps and the reasons for them.

Temperature in the hot water system

The EPA page explains the temperature logic in simple terms. Legionella grows best between 80 and 120 degrees Fahrenheit, so the EPA suggests that keeping hot water well above that band helps. It says that “Setting the thermostat at or above 140⁰F can often keep temperatures high enough (above 124⁰F) throughout the plumbing system to prevent Legionella growth.”

The EPA adds a scald warning. It says that “Hot water at the faucet or shower may present a scalding risk at temperatures above 117⁰F.” It recommends checking state or local building codes before raising the temperature, and it says that thermostatic mixing valves, placed as close as possible to water outlets, can manage the scald risk. The EPA says that installing a valve should follow the manufacturer’s instructions or be done by a professional.

The DC Health handout gives a different range. It says “Set water heaters to a minimum of 120°F and consider setting to 130-140°F.” It also says to flush and drain water heaters “per manufacturer instructions.” The two sources agree that a higher setting is part of the home plan, but they do not use identical numbers. Read both before you change a setting, and check the heater manual.

The EPA says cold water systems are unlikely to support Legionella growth, except in warm climates where the cold water stays near 80 degrees Fahrenheit for long periods. In that case the EPA suggests insulating the cold pipes.

Flushing and stagnant water

The EPA says that “Flushing the water in your home after periods of disuse (for example, while you are away or during remodeling) will remove stagnant water.” It explains that stagnant water can change temperatures, lower disinfectant levels, and create conditions for Legionella to grow. Dead legs, meaning unused pipe sections, and unused faucets such as a guest bathroom tap also count as stagnant points.

The EPA suggests running outlets that are not used often on a weekly basis. It says to open taps and let the water run for approximately two minutes, and to flush cold water before hot. The DC Health handout uses the same two-minute idea for faucets and showerheads that have not been used for more than a week, and it lists dead legs as a place to remove, shorten or flush. Both sources ask you to reduce splashing and spray during flushing.

The EPA also mentions flushing a water heater. It says that “If you decide to flush your water heater, make sure to follow the manufacturer’s instructions or have a professional do the work.” It lists examples of when manufacturers recommend flushing, including once a year and after plumbing work.

Fixtures and spray

Showerheads, sink faucets and humidifiers can release bacteria into the air as spray, according to the EPA. The EPA says “Keeping water fixtures clean will help to prevent or remove bacterial growth and reduce the amount of spray and small droplets that are easier to inhale.” The DC handout says to “Properly maintain and regularly clean all water-using devices (showerheads, faucet aerators/screens, hot tubs, etc.), per manufacturer instructions.”

The EPA suggests that when you use a shower or a sink for a long time, you open a window or run an exhaust fan. High-risk people may also consider point-of-use filters for sinks and showerheads, and those filters must be maintained and replaced on schedule.

Shared buildings and water management programs

A house is not always the only place with a building water system. The DC handout says that if you live in a building with multiple units, you should check with the building owner or manager to see whether the building has a Water Management Program. The EPA says the same idea applies to multifamily buildings, workplaces and schools: ask whether the building runs a water management program. It describes such programs as preventive plans that name the person or team responsible for the work and monitor it.

The EPA’s page also covers buildings that reopen after low use. It says standing water in premise plumbing “can lead to a change in water temperatures as well as lower levels of disinfectant residual.” If you have been away for a long time, the flushing steps above are the home version of that concern.

For readers who also compare shower heads for flow and pressure, see our high pressure and water-saving shower head guide and the water-efficient hand-held shower head roundup. Those comparisons do not cover Legionella, and neither do the sources in this post.

Mistakes to avoid

• Changing the water heater setting without a scald plan. The EPA warns about scalding above 117⁰F and points to thermostatic mixing valves.
• Assuming a cold water line is always safe. The EPA says cold water is unlikely to support growth except in warm climates, where the water may stay near 80⁰F.
• Skipping flushing after a trip. Both sources describe flushing after disuse as a separate step.
• Ignoring the building owner in a multiunit home. The DC handout says to check whether the building has a water management program.

Frequently asked questions

Is Legionella a normal part of a home water system?

The EPA says Legionella is found naturally in fresh water and can be found in building water systems. The sources do not say how often a particular home has it, and they do not present a home test result.

Should I test my home water for Legionella?

The EPA and the DC Health handout do not recommend a home test. They ask you to contact a healthcare provider or a public health professional if you have questions about prevention.

Does a water filter solve the problem?

The EPA says point-of-use devices must be maintained and replaced on a schedule, and it does not present them as a full home solution. The DC handout lists filter replacement as a maintenance step.

Who should I call about my plumbing?

The EPA advises checking local codes and consulting a professional before adding treatment or changing a heater. A licensed plumber or the local building department is the right contact for that work.

How we chose these sources

We used the EPA page “Legionella in the Indoor Environment” as the federal source. We used the DC Health handout “Preventing Legionellosis at Home” (2025) as a local government handout for home steps. The CDC prevention page returned a 404 error and the CDC site returned 403 errors to our fetch tool, so no CDC text is quoted. Search results from county and state handouts were not used as sources.

What we could not verify

• The exact water heater setting that a particular local code requires. The sources point to local codes without listing them.
• How often a home water system grows Legionella, or how much risk a given layout carries.
• Whether a particular filter or treatment device reduces the bacteria in a home. The sources do not test products.
• The CDC’s home water guidance. The CDC pages were not available to us.

Sources

• US EPA, “Legionella in the Indoor Environment”, https://www.epa.gov/node/271246 (opened 2026-10-29). Saved text: research/sources-20261029/B4/legionella-in-home-water-system-guidance–epa-indoor-environment.txt
• DC Health, “Preventing Legionellosis at Home” (2025), https://dchealth.dc.gov/sites/default/files/dc/sites/doh/Preventing%20Legionellosis%20at%20Home_2025.pdf (opened 2026-10-29). Saved text: research/sources-20261029/B4/legionella-in-home-water-system-guidance–dc-health-home-handout.txt