For most homes, set the water heater to 120°F (49°C). That is the US Department of Energy’s recommendation and the Consumer Product Safety Commission’s too: it cuts standby heat loss, sharply reduces scalding risk, and slows mineral scaling. The exception is a household with someone at high risk of Legionnaires’ disease, where storing water at 140°F (60°C) makes sense, ideally with a mixing valve to bring the tap temperature back down. The two numbers are not a disagreement between experts. They are answers to two different hazards, burns and bacteria, and the right choice depends on who lives in the house.
Water heating is worth getting right because it is a big line on the bill. According to the Department of Energy, it accounts for about 18% of a home’s energy use, typically the second-largest expense after heating and cooling.
The trade-off in plain numbers
| Setting | Energy | Scald time for adults (third-degree burn) | Legionella |
|---|---|---|---|
| 120°F / 49°C | Lowest standby loss | About 5 minutes | Survives, but grows slowly |
| 130°F / 54°C | Middle | About 30 seconds | Killed slowly, over several hours |
| 140°F / 60°C | $36–61/year more standby loss than 120°F | About 6 seconds | Killed in about 30 minutes |
| 150°F / 66°C | Higher still | About 2 seconds | Killed quickly |
The scald times come from the Consumer Product Safety Commission, and they are for healthy adults; young children and older adults, whose skin is thinner, burn faster. The CPSC attributes roughly 3,800 injuries and 34 deaths a year in US homes to scalding from excessively hot tap water, mostly among children under five and the elderly. At 140°F the margin for error with a small child at a tap is a few seconds.
The standby figure is the Department of Energy’s estimate of what a tank held at 140°F wastes compared with 120°F, before counting the hot water you actually use.
Is 120°F hot enough to kill Legionella?
No, and that is the whole reason the debate exists. According to the CDC, Legionella bacteria grow best between 77°F and 113°F (25–45°C). The CDC’s guidance for building water systems is to store hot water above 140°F (60°C) and keep circulating hot water from falling below 120°F (49°C). A tank held at 120°F sits just above the bacteria’s preferred range: they survive but do not multiply well.
Energy Vanguard, a building-science consultancy that has reviewed the research on residential tanks, summarizes the kill times as roughly 30 minutes at 140°F and much longer at lower temperatures. Given the CDC’s growth range, the conditions to watch for in a house are water that sits still and lukewarm: a tank that has been switched off for weeks, a rarely used guest bathroom, or an electric tank whose lower element has failed so the bottom section runs cool. After a long absence, running each hot tap for a few minutes flushes standing water out of the pipes.
For a healthy household, the CDC and DOE positions are compatible. The CDC guidance is written for large buildings with long pipe runs and vulnerable occupants, such as hospitals and care homes. In an ordinary house with regular hot water use, the tank turns over daily, and 120°F is the setting the Department of Energy and CPSC recommend.
Which setting fits your household
Set 120°F if: your household is generally healthy, and especially if there are young children or older adults who could be scalded. This is the majority case and the DOE default recommendation.
Consider 140°F if: someone in the home has a suppressed immune system, chronic lung disease, or is otherwise at elevated risk of Legionnaires’ disease. The bacterial risk outweighs the energy saving in that situation, and it is worth asking their doctor.
The proper fix for both: store hot at 140°F and fit a thermostatic mixing valve (also called a tempering valve) that blends it down to 120°F before it reaches taps and showers. You get bacterial control in the tank and safe temperatures at the fixture. This is standard practice in commercial buildings and increasingly required by plumbing codes in new residential work. It is a plumber job, not a dial adjustment, but it is the answer if you cannot comfortably pick a side.
Dishwashers are a common worry. Most modern models have their own heating element that boosts incoming water, so they do not need a 140°F tank. Check the manual if yours is old.
How much does turning it down actually save?
Two separate savings, and the second is larger than most people realize.
Standby loss is heat leaking from the tank into the room, all day, every day, whether or not you use hot water. A tank held at 140°F loses heat faster than one at 120°F because the temperature difference with the surrounding air is greater. The Department of Energy puts this at $36 to $61 a year.
Consumption is the bigger one. You mix in more cold at the shower when the tank is hotter, but the heater still had to bring every gallon up to the higher temperature, and pipes lose more heat on the way. The DOE says the demand-side savings from a lower setting can exceed $400, and that every 10°F reduction can cut water heating costs by up to 5%. The lower setting also slows mineral buildup and corrosion in the tank and pipes, according to the same guidance.
Neither is a dramatic number on its own. It is a set-and-forget change that pays a small amount forever, which is a fair description of most worthwhile energy measures. If you want to confirm it on an electric heater, take a meter reading each morning for a week before and after the change.
How do you change the temperature?
| Heater type | Where the control is | Tools and safety |
|---|---|---|
| Electric tank | One or two thermostats behind access panels, under insulation | Breaker off first; flat screwdriver |
| Gas tank | Dial on the gas control valve at the bottom | No tools; dial is often unmarked |
| Tankless (gas or electric) | Digital panel or remote | Set the number directly |
| Heat pump water heater | Digital panel or app | Set the number; check the mode too |
Electric tank heaters have one or two thermostats behind removable access panels, under insulation. Turn off the power at the breaker first. These are live at 240 volts and the panel gives direct access. Set the upper and lower thermostats the same, then replace the insulation and panels before restoring power.
Gas tank heaters have a dial on the control valve at the bottom of the tank, often marked with vague settings — “warm”, a series of dots, “hot” — rather than numbers. The manual usually gives the approximate temperature for each mark, and “hot” is frequently higher than 120°F.
Tankless and heat pump units usually have a digital display and a set point you can adjust directly. Because tankless heaters do not store water, the standby argument for lowering them is weaker, but the scalding argument is the same.
How do you check what it’s actually set to?
The dial is frequently wrong, so measure rather than trust it:
- Don’t use hot water for a few hours.
- Run the hot tap furthest from the heater for a couple of minutes.
- Fill a cup and read it with a cooking thermometer.
Adjust in small increments and re-measure after a few hours, since tanks take time to settle. Aim for the temperature at the tap, because there is always some loss through the pipe run. If the tap reads well above the dial setting, the thermostat may be faulty, which is worth having checked.
What else is worth doing while you’re there
- Insulate the first six feet (about 2 m) of hot and cold pipe at the tank. Cheap foam sleeves reduce heat loss from the pipes nearest the heater.
- Fit a tank blanket if the heater is older and feels warm to the touch. According to the Department of Energy, adding insulation to a tank below R-24 can cut standby heat losses by 25–45%, saving around 4–9% on water heating. Do not cover the top of a gas heater, the burner access, the thermostat panels or the flue.
- Drain a few gallons from the bottom valve annually to clear sediment, which otherwise insulates the burner from the water and wastes fuel.
- Lower it when you travel. Most gas valves have a “vacation” setting that holds the pilot without heating the tank fully.
If you are working through household energy more broadly, the water heater sits just behind heating and cooling. That makes this setting and getting the most out of ceiling fans two of the few adjustments that cost nothing and keep paying.