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Set a household water heater to 120°F. That is the figure the US Department of Energy recommends, and most manufacturers ship units preset near it. It is hot enough for showers, laundry and a dishwasher with an internal booster, cool enough that a scald takes minutes rather than seconds, and it slows scale and tank corrosion. Homes with an immunocompromised occupant are sometimes advised to store at 140°F to suppress Legionella — but only with a thermostatic mixing valve tempering the delivered water back to 120°F.
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How quickly does hot water cause a burn?
This is the argument for 120°F, and the numbers are stark. Burn-center data cited by the American Burn Association and by state health departments shows that adult skin exposed to water at 120°F takes around five minutes to sustain a third-degree burn; at 140°F it takes about five seconds; at 150°F, roughly two seconds. Children and older adults burn faster because their skin is thinner. Scalds from tap water are one of the more common preventable household burn causes in the US, and turning the dial down is the cheapest control available.
| Water temperature | Approx. time to a third-degree burn (adult skin) |
|---|---|
| 120°F | About 5 minutes |
| 125°F | About 2 minutes |
| 130°F | About 30 seconds |
| 140°F | About 5 seconds |
| 150°F | About 2 seconds |
| 160°F | Under 1 second |
What about Legionella at 120°F?
This is the real trade-off and it deserves an honest answer rather than a slogan. Legionella pneumophila multiplies in warm stagnant water, broadly between 68°F and 122°F, and dies quickly above about 140°F. A tank held at 120°F therefore sits at the top of the growth range rather than outside it. In practice most healthy households run 120°F without incident, and public health guidance from the Centers for Disease Control and Prevention focuses on higher-risk settings — hospitals, care homes, large buildings with long recirculating loops.
If someone in the house is immunocompromised, over 65, or has chronic lung disease, the safer configuration is storing at 140°F and installing an ASSE 1017 thermostatic mixing valve at the tank outlet to blend the delivered water down to 120°F. That gets both the bacterial control and the scald protection, and typically costs $150 to $400 installed.
Does plumbing code set a maximum temperature?
Not at the tank, generally — codes regulate what comes out of fixtures. Under both the International Plumbing Code and the Uniform Plumbing Code as commonly adopted, shower and tub-shower valves must be pressure-balancing or thermostatic types meeting ASSE 1016, with the maximum outlet limited to 120°F, and many jurisdictions apply similar limits to bathtub fillers via ASSE 1070 devices. Adoption differs from state to state and city to city, so confirm the version in force with your local building department rather than assuming. If your shower can deliver scalding water, the mixing valve stop is likely misadjusted — that is a fixture-level fix, not a reason to change the tank.
How do you actually change the setting?
On a gas tank, the thermostat is the dial on the control valve at the bottom of the tank. Turn it, wait three hours, then measure at the nearest hot faucet after running it a full minute, and adjust from the measurement rather than the label.
On an electric tank, there are usually two thermostats behind insulated access panels, upper and lower. Switch the breaker off before removing a panel — those terminals carry 240 volts. Set both thermostats to the same figure; leaving the lower one cooler creates a stratified tank and inconsistent output. If you are not comfortable working near live terminals, this is a reasonable thing to have a licensed plumber or electrician do.
A cheap kitchen thermometer in a glass of running hot water is accurate enough. Expect the faucet to read a few degrees below the tank setting because of pipe losses.
Related answers: how long a water heater lasts, how often to flush a water heater, and why hot water runs out so fast — the last one is frequently a thermostat set low rather than a failing tank. For the tools worth owning first, see best plumbing tools.