Warning! Legionella Bacteria Can Grow in Your Water Heater

By Missy Nolan

April 22, 2025

Have you ever wondered what's inside your water heater (besides water, of course)? A water heater's warm, wet tank can hold numerous surprises, including infection-causing bacteria. You may even discover a harmful bacterium known as Legionella in your water heater.

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Legionella lurks in water systems, including hot tubs, fountains and water heaters. Maintaining a safe water heater temperature helps deter Legionella and other harmful pathogens from making themselves comfortable in your plumbing system. Learn more about choosing the best temperature for your water heater to thwart the spread of Legionella bacteria.

What Is Legionella Bacteria?

Legionella bacteria lurk in freshwater systems and man-made plumbing systems. Exposure to Legionella can cause numerous health concerns, including pneumonia, Pontiac fever and Legionnaires' disease. However, not everyone who's exposed to Legionella gets sick.

People with compromised immune systems or a history of smoking may be more susceptible to Legionella-triggered issues. Adults who are at least 50 years old and children who are 6 months or younger also have a higher risk of developing illness after Legionella exposure. You can protect yourself and your loved ones by keeping your water heater temperature high and cleaning plumbing equipment regularly.

Can Legionella in a Water Heater Cause Legionnaires' Disease?

Legionnaires' disease is a form of pneumonia caused by contact with Legionella bacteria. This bacterium can spread via airborne droplets or direct contact, such as washing your hands in Legionella-contaminated water. Legionella bacteria travel through plumbing fixtures and equipment, including sinks, cooling towers and water heaters.

This illness-inducing bacteria prefers temperatures below 120 degrees F (45 degrees C). Legionella generally forms when your water heater temperature is 68 to 113 degrees F (20 to 45 degrees C). Keep your hot water temperature above 140 degrees F (60 degrees C) to help kill bacteria such as Legionella in your home.

What Are the Symptoms of Legionnaires' Disease?

Legionnaires' disease presents differently in people with weakened immune systems versus healthy individuals. Folks with compromised immunity due to cancer, diabetes or HIV may experience more severe symptoms. Fortunately, Legionnaires' disease can typically be treated with antibiotics.

Here are some symptoms you may experience with Legionnaires' disease:

Coughing
Wheezing
Fever
Sore muscles
Headache
Shortness of breath
Confusion

Talk to your health care provider immediately if you're experiencing symptoms of Legionnaires' disease. Left untreated, Legionnaires' disease from water can cause lung failure or even death. Your doctor can order a chest X-ray or check your urine and phlegm for signs of a Legionella infection, such as Legionnaires' disease or Pontiac fever.

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What Temperature Should a Water Heater Be Set at to Kill Legionella Bacteria?

Set your water heater temperature at 140 degrees F (60 degrees C) or higher, and make sure it doesn't drop below 120 degrees F (45 degrees C). Legionella in a water heater is possible if the temperature falls below 120 degrees F (45 degrees C). Maintaining a hot tap water temperature and water heater temperature helps stop Legionella from spreading through your plumbing system.

You can typically adjust your water heater temperature via the thermostat. Your water heater's thermostat may be located on the top, bottom or side of your tank. Stay safe and turn off the water heater's electricity or disconnect the gas valve before changing the appliance's temperature. A plumber can help you locate the water heater's thermostat or safely disconnect your plumbing equipment.

Can You Still Catch Legionnaires' Disease If Your Water Heater Is Sanitized?

Legionnaires' disease comes from water-loving Legionella bacteria. This bacterium often hides in water heaters, but you can still get sick even if you don't have Legionella in your water heater. Legionella lurks in potable water sources, including hot and cold water you drink. You can find Legionella in neti pots, humidifiers, sinks, bathtubs, drinking fountains and CPAP machines. Storing your contact lenses in water is also risky, as it may encourage the growth of Legionella.

Stagnant water inside or outside can attract Legionella bacteria, including water from your vehicle's windshield wiper fluid tank. Keep your windows up when you run the windshield wipers, and avoid touching a wet windshield. Disinfect surfaces exposed to water regularly, and consult with a plumber if you have concerns about Legionella in your pipes or drains.

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