How To Prevent Legionella In Water Heaters - A Technician'S Guide To

We all turn on the hot water tap without a second thought, trusting it for a relaxing shower, washing dishes, or cleaning our homes. We assume it’s clean and safe. But have you ever wondered what might be quietly growing inside that warm, dark tank that sits in your basement or utility closet?

Don’t worry, I’m here to walk you through it. As an HVACR technician, my job is to ensure your home systems run safely and efficiently, and that includes your water heater. My goal is to give you the peace of mind that comes from knowing your hot water is safe for your family. This guide will provide clear, actionable steps on how to prevent legionella in water heaters.

We’ll break down what Legionella bacteria is, why it loves water heaters, and cover the simple temperature adjustments and maintenance routines you can perform. We’ll also be clear about when it’s time to put down the tools and call in a professional. Let’s make sure your hot water is as safe as it is comforting.

What is Legionella and Why Should You Care?

Before we dive into the “how,” let’s quickly cover the “what” and “why.” Legionella is a type of bacteria found naturally in freshwater environments like lakes and streams. The problem isn’t its existence in nature, but what happens when it finds its way into man-made water systems.

When Legionella gets into a building’s plumbing, it can multiply rapidly under the right conditions. If people then breathe in small droplets of water (aerosols) containing the bacteria—say, from a showerhead or a faucet—they can develop a serious type of pneumonia called Legionnaires’ disease.

While anyone can get sick, some people are at higher risk, including:

Adults over 50
Current or former smokers
Individuals with chronic lung disease
People with weakened immune systems

The good news? It’s highly preventable. Taking a few proactive steps with your water heater is one of the most effective ways to protect your home. This is the core benefit of learning how to prevent legionella in water heaters.

The Ideal Breeding Ground: Why Legionella Loves Your Water Heater

Think of your standard tank-style water heater: it’s a large container of warm, stagnant water. For Legionella bacteria, this is a five-star resort. The bacteria thrive in water temperatures between 95°F and 115°F (35°C to 46°C)—a range many water heaters are set to by default to save energy and prevent scalding.

But temperature isn’t the only factor. The inside of your tank can accumulate sediment, rust, and scale over time. This gunk settles at the bottom, creating a protective layer of sludge, also known as biofilm.

This biofilm acts as both a shelter and a food source for Legionella, shielding it from heat and disinfectants like chlorine that are present in municipal water. This is one of the most common problems with how to prevent legionella in water heaters; if you don’t address the sediment, you’re not addressing the root cause.

Your First Line of Defense: Setting the Right Water Heater Temperature

The single most important step in preventing Legionella growth is controlling the temperature of the water stored in your tank. This is where we, as technicians, always start.

The “Magic Number” for Safety

To effectively kill Legionella bacteria, you should set your water heater’s storage temperature to a minimum of 140°F (60°C). At this temperature, Legionella cannot survive. Most bacteria are killed within minutes, and consistent storage at this level ensures your tank remains a hostile environment for growth.

You can adjust the temperature dial on your water heater yourself. For an electric model, you may need to turn off the power at the circuit breaker and remove an access panel to find the thermostat. For a gas model, the dial is usually visible on the front gas control valve.

The Scalding Risk and How to Manage It

Now, I know what you’re thinking: “Isn’t 140°F dangerously hot?” And you’re absolutely right. Water at that temperature can cause a serious burn in just a few seconds. This is a critical safety trade-off we must manage.

The best solution is to install thermostatic mixing valves (TMVs) at key points of use, like your shower, or a master TMV right at the water heater’s hot water outlet. This simple device is a game-changer.

A TMV automatically mixes the 140°F water from your tank with cold water before it ever reaches your faucet or showerhead. This allows you to deliver safe, comfortable water—typically around 120°F (49°C)—to your family while keeping the tank hot enough to kill bacteria. It’s the ultimate win-win for safety.

How to Prevent Legionella in Water Heaters with Regular Maintenance

Temperature is your primary weapon, but maintenance is your long-term strategy. A clean tank is a safe tank. This section is your essential how to prevent legionella in water heaters care guide, giving you actionable tips to keep your system in top shape.

Flushing Your Water Heater Tank Annually

Remember that sludge and sediment we talked about? Flushing your tank once a year is how you get rid of it. This removes the biofilm that Legionella loves to hide in. It sounds intimidating, but if you’re comfortable with basic DIY tasks, it’s very manageable.

Here’s a simplified step-by-step guide:

Turn Off the Power or Gas: For an electric heater, shut off the corresponding breaker in your electrical panel. For a gas heater, turn the gas control knob to the “PILOT” or “OFF” position.
Turn Off the Water Supply: Find the cold-water inlet pipe on your water heater and turn the shutoff valve clockwise until it stops.
Connect a Garden Hose: Attach a standard garden hose to the drain valve located at the bottom of the water heater tank. Run the other end of the hose to a floor drain, utility sink, or outside.
Open a Hot Water Tap: Go to the nearest sink and turn on the hot water. This will release the vacuum in the tank and allow it to drain properly.
Drain the Tank: Carefully open the drain valve on the water heater. The water will be hot and may contain sediment. Let the tank drain completely, or until the water runs clear.
Refill and Restart: Close the drain valve, disconnect the hose, and turn the cold water supply back on. Let the tank fill completely (you’ll know it’s full when water flows steadily from the open hot water tap). Once full, turn off the tap and restore power or relight the pilot.

This single task is one of the most effective how to prevent legionella in water heaters tips you can follow.

Checking and Replacing the Anode Rod

Inside your water heater is a “sacrificial” anode rod. Its job is to corrode so that your tank doesn’t. Over time, this rod gets completely eaten away. When it’s gone, the tank itself starts to corrode, creating more rust and sediment for bacteria to thrive in.

Checking this rod every few years is a pro-level move that can significantly extend the life of your water heater and improve water quality. This task can be difficult, as the rod is often screwed in very tightly, so it’s a common reason to call a professional plumber or HVACR tech.

Special Considerations for Different Systems and Situations

Not all homes are the same, and your prevention strategy might need a few tweaks depending on your setup and lifestyle. This is a complete how to prevent legionella in water heaters guide, so let’s cover a few common scenarios.

Tankless Water Heaters: Are They Safer?

Yes, generally. Tankless (or “on-demand”) water heaters are much less susceptible to Legionella growth because they don’t store a large volume of hot water. Water is heated instantly as it passes through the unit.

However, the risk isn’t zero. Bacteria can still colonize plumbing lines and faucets if water stagnates for long periods. The key takeaway is that the primary risk—the warm storage tank—is eliminated with a tankless system.

What to Do After a Vacation or Long Period of Non-Use

If your home has been vacant for a week or more, the water in your pipes and tank has been stagnant, creating a potential risk. Before you use any hot water, it’s wise to perform a thermal disinfection and flush.

Turn your water heater up to its highest temperature setting (at least 140°F) and let it sit for about an hour. Then, go through your house and systematically run every hot water tap (showers, sinks, tubs) for 5-10 minutes to flush the super-heated water through the entire system. Just be careful not to scald yourself during this process.

Sustainable and Eco-Friendly How to Prevent Legionella in Water Heaters

Keeping water at 140°F does use more energy than keeping it at 120°F. However, you can offset this by adopting some eco-friendly how to prevent legionella in water heaters practices. Installing thermostatic mixing valves is a great first step, as you’re not wasting energy overheating water that you immediately cool down with cold water at the tap.

Additionally, ensuring your water heater tank and hot water pipes are well-insulated will reduce standby heat loss, making it more efficient to maintain that higher, safer temperature.

When to Call a Professional HVACR Technician

While some of these tasks are DIY-friendly, your safety should always come first. There are times when calling a licensed professional is the smartest and safest choice.

You should call a pro if:

You are not comfortable working with your home’s gas or electrical systems.
Your water heater’s drain valve is stuck, leaking, or looks corroded.
You want to have thermostatic mixing valves installed.
You need your anode rod inspected or replaced.
Your water heater is making unusual noises like popping or rumbling, which can indicate a severe sediment problem.

We have the right tools, training, and experience to handle these jobs safely and ensure your system is operating according to the how to prevent legionella in water heaters best practices.

Frequently Asked Questions About Preventing Legionella in Water Heaters

Is 120°F hot enough to kill Legionella?

No. While 120°F (49°C) is the standard recommended setting to prevent scalding, it is not hot enough to reliably kill Legionella. In fact, it falls within the temperature range where the bacteria can multiply. This is why keeping the tank at 140°F (60°C) and using a mixing valve is the safest approach.

How often should I flush my water heater for Legionella prevention?

For most homes, flushing your water heater once a year is a great routine. If you live in an area with particularly hard water (high mineral content), which creates more sediment, you might consider doing it every six months. This is a key part of any good how to prevent legionella in water heaters care guide.

Can I get Legionnaires’ disease from drinking my tap water?

It’s highly unlikely. The primary way Legionnaires’ disease is contracted is by inhaling aerosolized water droplets—a fine mist—that contain the bacteria. This typically occurs in showers, hot tubs, or from decorative fountains. The risk from drinking the water is very low for most healthy individuals.

Do water filters remove Legionella bacteria?

Some specialized point-of-use filters are designed to remove bacteria, but this should be seen as a secondary line of defense, not a primary prevention method. The most effective strategy is to prevent the bacteria from growing in your tank in the first place through proper temperature control and maintenance.

Protecting your home’s water system from Legionella is a powerful and proactive step toward ensuring a healthier environment for you and your family. By understanding the risks and following these simple maintenance steps, you can have confidence every time you turn on the hot water.

Stay safe, stay comfortable, and don’t ever hesitate to reach out to a trusted professional if you need a hand!

Author
Recent Posts

David Clark

Founder & Lead HVAC Specialist at HVACRLY

David Clark is the founder of HVACrly and a certified HVAC specialist with 10+ years working on residential and light-commercial systems. He turns complex heating, cooling, ventilation, and refrigeration topics into clear, actionable guides focused on safety and energy efficiency. When he’s not on a service call, he’s testing tools and writing step-by-step tutorials.