Legionella hot water temperature ranges: 8 key points

Key takeaways

• Keep cold water below 77°F.
• Ask a plumber to confirm your heater is set to at least 140°F, so the water at the tap sits between 113°F and 120°F.
• Keep any hot water loop between 120°F and 140°F.
• Flush rarely used taps at least weekly.
• Ask a plumber whether you have a mixing valve, where it is, and whether it works.

This guide explains eight points from those numbers. Most of them come from the Vermont Department of Health's guidance on Legionella in building water systems. That page is written for people who run buildings. Treat it as a list of questions for your plumber, not as orders for your home. The final point comes from Washington State on how the illness spreads and who is most at risk.

This guide is general information, not medical or plumbing advice for your home. Nothing here replaces a licensed plumber's check or a doctor's care.

If you read only two parts, read point 8 for the health facts and the questions at the end for what to do next. Everything in between explains the numbers.

1. Germs grow best between 77°F and 113°F

The Vermont Department of Health puts the danger zone in one plain sentence: "Water temperatures of 77°F to 113°F are the ideal range for Legionella growth." Think of that range as the germ's comfort zone. Your job is to keep your water out of it wherever you can.

Still water gives the germ more time to grow. A pipe or tank that holds water and goes unused for long periods is one example. For a hidden leak, see how to find a water leak location. That is one reason the guidance asks for regular flushing (see point 7).

Some homes have parts of the plumbing that are used rarely: a guest bathroom, a basement sink, or an outdoor hose. Those are the places where water can sit still for a long time. Keep them in mind as you read.

Here is a simple way to think about it. Lukewarm water from a tap sits inside the band, so it is in the zone the germ likes. Water at 120°F sits above it, which is where the tap guidance is aiming.

A range is a guide, not a line you can measure to a single degree. Use the numbers to judge whether your system is roughly in the right place, then let a plumber make any adjustments.

The rest of this guide shows how to keep hot and cold water out of that band.

2. Keep cold water below 77°F

The Vermont guidance is short and direct: "Make sure cold water stays below 77°F." Cold water matters too. If it warms up into the 77°F to 113°F band, the germ can grow in it.

Cold pipes can warm up in a few ways. A pipe that runs next to a hot water pipe, or through a warm basement or attic, can pick up heat. Ask your plumber to check where your cold water lines run.

A simple check helps. Run the cold tap for a while, then fill a cup and test it with a kitchen thermometer. If the reading is above 77°F, tell your plumber.

A cold reading below 77°F is what the guidance asks for, but it is only one check. Keep the reading in your notes next to the heater setting, so your plumber can see both at once.

You use cold water for drinking, cooking and washing, so it is worth checking as carefully as the hot. It is easy to check only the hot tap, which leaves half of the picture missing.

3. Set your heater to at least 140°F

The Vermont guidance for buildings says: "Set hot water heaters to at least 140°F so water at the taps is between 113°F and 120°F." This is the figure to look for on your heater's dial or in its manual.

Do not change the setting on your own. Ask a plumber to tell you what it is set to and whether 140°F is safe for your home. Very hot water can scald, so point 6 explains the safety part to ask about.

If you own a rental, ask a plumber to check the setting between tenants and write the number down.

Some heaters show a setting label, not a real temperature. Ask the plumber to confirm what the label means in degrees and to check it at the tap.

Hot water heater settings can be confusing, so write down exactly what the plumber tells you, including the date. That note is the starting point for every later check.

Some people set heaters lower to reduce burn risk. The guidance quoted above asks for at least 140°F in the heater, so a lower setting is below that guidance. Talk to a plumber about how to meet both goals, rather than picking one alone.

4. Water at your tap should be 113°F to 120°F

The Vermont guidance connects the two numbers: a heater set to at least 140°F is what keeps the water at the taps between 113°F and 120°F.

The bottom of that tap range, 113°F, is the top edge of the germ's ideal band from point 1.

The numbers in this section are for water at the tap, not for the water inside the heater tank. Keep the two ideas apart when you talk to your plumber.

Your tap is the place to test. Run the hot tap until the water stops changing temperature, then fill a cup and measure it with a kitchen thermometer. Write the number down. A reading under 113°F is worth raising with your plumber.

Measure at more than one tap, such as the kitchen sink and the upstairs shower, because the water can differ from place to place.

Write the readings on a sheet and keep it with the heater's paperwork, so the next person who checks the system can see what you found.

A single reading is only a snapshot, so repeat the check on a few different days before you decide anything.

Illustrative example: one cold tap and one hot tap

Suppose your cold tap reads 85°F. That is above the 77°F line, so tell your plumber and ask where the cold line runs. Suppose your hot tap reads 120°F. That is at the top of the 113°F to 120°F range, so write it down and check again on another day.

5. Circulating hot water should stay 120°F to 140°F

Some buildings keep hot water moving through a loop of pipe, so hot water is ready at the tap without a long wait. The Vermont guidance calls this circulating hot water and gives the range: "Make sure circulating hot water maintains a temperature between 120°F and 140°F."

This loop idea matters for landlords and for anyone who manages a building with more than one unit. The loop is part of the water system, so a single tap reading does not tell you what the loop is doing.

If your home has no loop, this point does not apply to you. Ask your plumber to confirm, because a loop can be hidden inside walls.

If there is a loop, ask a plumber to measure the water where the loop returns to the heater and at a few taps. Vermont's range is the target: 120°F to 140°F.

Ask the plumber to sketch the loop's route on paper. A simple sketch makes the next conversation shorter.

Keep in mind that the guidance is for buildings. A landlord who runs a loop should write down the readings and keep the plumber's report with the building file.

6. Hot water this hot can scald, so ask about a mixing valve

The heater water is hotter than the water you want at a tap, so something has to cool it on the way. A mixing valve is there to prevent burns from hot water. The temperature numbers in points 1 to 5 still apply to the water in your system.

It may sit near the heater, under a sink, or at a shower. A plumber can find it for you.

Ask your plumber three things: whether a mixing valve is installed, where it sits, and whether it still works.

Showers and tubs are the places where hot water reaches the body, so ask about the valve at those taps first.

If a child or an older adult lives in the home, tell your plumber when you ask.

A valve that is set wrong or stuck can leave the tap at the wrong temperature. After any work on the heater or valve, measure the tap again before you assume the water is right.

7. Run rarely used taps at least weekly

The Vermont guidance says to flush rarely used taps and other outlets at least weekly. Any place water comes out counts, such as a tap, a shower head, or a hose connection. To flush, let the water run so the old water in the pipe is replaced.

Which taps count as rarely used? Think of the guest bathroom, the basement sink, the outdoor hose, and any shower or tap that you use only for visitors or during holidays. Give each one a turn.

Make it a habit. Pick one day each week, write it on the calendar, and walk the house with a list of every tap and shower you rarely use.

Landlords should add this to the checklist for any unit that sits empty between tenants, because water in unused pipes sits still for longer.

Keep a note of the date you ran each tap. A note takes a minute, and it lets you show a plumber or a health office what you have done.

Do not treat flushing as a cure. It is one habit that keeps still water moving, alongside the temperature checks in points 1 to 6.

Flushing also helps you notice problems. If a tap runs rusty, smells strange, or sputters, tell your plumber.

8. How the illness spreads and who is most at risk

The Washington State Department of Health explains how people catch Legionnaires' disease: "People get Legionnaires' disease when they breathe in a mist or vapor." In plain words, you breathe in the germ from water that has turned into mist or vapor.

Washington's guidance is clear: "The bacteria are NOT spread from one person to another person." So a sick family member does not mean others caught it from them.

Who is most at risk? The same guidance says: "People most at risk of getting sick from the bacteria are older people (usually 65 years or older)." If you are in that group, or care for someone who is, take the water rules here seriously. A household with an older adult should keep these numbers close at hand.

Timing matters too. Washington's guidance says: "Symptoms usually begin 2 to 10 days after being exposed to the bacteria." That gap is why the illness can be hard to connect to water.

The seriousness is real. The same guidance says the illness "can be fatal in 5 percent to 30 percent of cases." That is why the water numbers in this guide are worth your time.

If you rent out a home, tell your tenants what you have checked and how to reach you if they notice hot water that runs cool or cold water that feels warm.

Keep a copy of the links to the health agency pages with your notes, so the next person can read the original words.

Keep your own notes simple: the date, the question, and the answer. You may need them if a doctor or health office asks about the water in the home.

What to do next

Use these eight points as a checklist for a short visit with a plumber. Bring a note with three kinds of reading: the heater setting, the hot water at one or two taps, and the cold water at one tap. Then ask these questions:

• What is my heater set to, and is it at least 140°F?
• Is my hot water between 113°F and 120°F at the tap?
• Is my cold water below 77°F?
• Is there a mixing valve, where is it, and does it work?
• If I have a hot water loop, is it between 120°F and 140°F?
• Which taps are rarely used, and who runs them each week?

These numbers come mainly from one state guide written for building owners, so treat them as a guide to the questions, not a verdict on your home. A plumber who sees the whole system can give you the answers that fit. For readers interested in well water, see Well water standards: 5 FAQs on private and public supply.

If someone in your home gets Legionnaires' disease and you suspect the water, tell their doctor about it. It helps the doctor decide what to test for.

Use this guide as a conversation starter. Bring it to a plumber or health office, and let them tell you what applies to your home.

Your next step today: find the heater setting on its dial or in the manual, write it down, and ask a plumber to confirm it. Then run the rarely used taps this week, and talk to a professional before you change anything.

Frequently asked questions

What range do health agencies give for Legionella growth?

The Vermont Department of Health says 77°F to 113°F is the ideal range for Legionella growth.

What heater setting does the Vermont guidance give?

The guidance says to set hot water heaters to at least 140°F so water at the taps is between 113°F and 120°F. Ask a plumber to confirm your current setting before you change it.

Can Legionnaires' disease spread from one person to another?

No. Washington State's guidance says the bacteria are not spread from one person to another person.