What Temperature Should a Water Heater Be Set To?

Is 120°F hot enough to stop Legionella in a water heater?

No. The review we cite puts “no growth” above 55°C (131°F) and a killing effect above 60°C (140°F) [2], which is why the CDC wants stored hot water above 140°F [1]. At 120°F the tank is below that line.

Legionella is a waterborne bacterium that lives in building plumbing, and temperature is the main thing a building owner controls about it. A 2004 review in the Canadian Journal of Infectious Diseases sets out the range: the optimal temperature for proliferation is between 32°C and 35°C, it proliferates readily up to 45°C, usually there is no growth above 55°C, and above 60°C the temperature is bactericidal [2].

In Fahrenheit that growth window is roughly 68°F to 113°F — which is to say, the temperature of a tank someone turned down to save money, and the temperature of the cool layer at the bottom of a tank even when the thermostat says otherwise. The same review cites a Quebec survey that found Legionella in 40% of electric water heaters despite thermostats set at 60°C, because the bottom of those tanks sat at about 30°C to 40°C [2].

That detail is worth keeping. A thermostat setting is not a tank temperature; it is a setting. Electric tanks in particular stratify, and the lower element sees the coldest water.

How fast can hot water from a heater burn?

Fast. Most adults suffer a third-degree burn in about six seconds at 140°F and two seconds at 150°F [3].

The Consumer Product Safety Commission publishes the other half of the arithmetic, and the numbers are short enough to memorise. Most adults will suffer third-degree burns if exposed to 150°F water for two seconds. Burns also occur with a six-second exposure to 140°F water, or a thirty-second exposure to 130°F. Even at 120°F, a five-minute exposure could result in third-degree burns [3].

The CPSC notes that the majority of injuries and deaths involving tap water scalds are to the elderly and to children under five [3] — the two groups least able to pull a hand away or climb out of a tub.

This is why the CPSC's advice to a household is simply 120°F [3]. If the same water that reaches the tank reaches the tap, that is the right answer.

Can the tank be at 140°F and the taps at 120°F?

Yes — that is what a thermostatic mixing valve does, and it is the arrangement the CDC itself describes [1].

The two recommendations only conflict if one number has to do both jobs. It does not have to.

The CDC's own guidance says to install thermostatic mixing valves as close as possible to fixtures, because this prevents scalding while permitting circulating hot water temperatures above 120°F [1]. The 2004 review reaches the same place from the other direction: new water heaters preset at 60°C and equipped with antiscald devices to deliver water at 49°C to the household [2].

A thermostatic mixing valve blends stored hot water with cold until what leaves it is the temperature you set, not the temperature the tank happens to be. The tank stays outside the growth window; the tap stays outside the burn window.

What should I do with my own water heater?

You have a mixing valve already, or you can have one. Many houses with a tank do not have one at the heater. Adding one is a small job on the hot outlet, and it is the difference between choosing a risk and removing both.

A tankless heater is a different case. There is no stored volume sitting in the growth window, so the set point is the delivery temperature, and 120°F at the unit is the ordinary answer.

Check with a thermometer, not with a dial. The CPSC's own instruction is to hold a thermometer under the faucet first thing in the morning, or at least two hours after water has been used [3]. The dial and the tap disagree often enough that the two minutes are worth it.

Never take the temperature for granted with a small child. The CPSC asks for a hand test before bathing an infant, every time [3].

What this page is not

This is not medical advice, and we are not going to tell you that a plumbing setting protects anyone's health. If someone in the house is immunocompromised, that is a conversation with a doctor; on the plumbing side it is a conversation about stored temperature, stagnation and delivery.

Nor is it a code citation. What we describe here is what the CDC and the CPSC publish and what we do on our own jobs. The requirements that apply to a specific building are for the permit and the inspector, not for an article.

What this means in Tampa Bay

Two local habits push the wrong way. Water heaters here are often in a garage or an unconditioned utility room, so the tank loses heat into a hot room and the thermostat cycles — and the cool bottom layer the Quebec survey describes is not theoretical [2]. And a great many houses sit empty for the season, which is stagnation added to warmth.

So on a heater we replace, we look at two things beyond the tank itself: whether there is a mixing valve on the outlet, and what the tap actually measures afterwards. If you have been away for months, run the hot taps before you use them.

FAQ

Sources

We review and cite. We do not reproduce a paper or a manual — follow the links for the originals.

1. [1]Controlling Legionella in Potable Water Systems — U.S. Centers for Disease Control and Prevention · 2025Government or standards body · quoted: store hot water above 140°F; circulating hot water not below 120°F; install thermostatic mixing valves as close as possible to fixtures
2. [2]Residential water heater temperature: 49 or 60 degrees Celsius? — Benoît Lévesque, Michel Lavoie, Jacques Joly · Canadian Journal of Infectious Diseases 15(1):11–12 · 2004Peer-reviewed · open access; growth ranges, the 40% Quebec electric-heater finding, and the preset-plus-antiscald recommendation
3. [3]Avoiding Tap Water Scalds (Publication 5098) — U.S. Consumer Product Safety CommissionGovernment or standards body · quoted: burn times at 150, 140, 130 and 120°F; the 120°F recommendation; measure with a thermometer at the faucet