Set most home water heaters to 120 degrees Fahrenheit. The U.S. Department of Energy says 120F generally meets most household needs, and the CPSC urges 120F to cut scald risk and save energy. Check your dial or tap temperature, and turn it down if it runs hotter.
Why 120F is the recommended setting
The 120F recommendation balances three things at once: comfort, safety, and cost. At 120F you still get a hot shower and clean dishes, but the water is not hot enough to scald skin in seconds. DOE's water heating fact sheet advises setting the thermostat "low enough to minimize safety concerns (scalding) yet high enough for washing (dishes, clothes) and bathing," and adds that "a setting of 120°F generally meets most household needs."
Running hotter than you need is a quiet drain on your bill. DOE's Building America Strategy Guideline cites water heating at about 13.1 percent of the energy delivered to residential buildings (2010 Building Energy Data Book), and DOE's own guidance on cutting hot water cost starts with "lowering the thermostat." A storage tank also loses standby heat, the energy it burns just keeping water hot between uses, so a lower setting means less to lose. The CPSC says lowering the heater to 120F will "conserve energy and save money." Turning the dial down is one of the cheapest energy fixes a homeowner can make.
There is a longevity payoff too. DOE's Building America guideline notes that higher water temperature settings can accelerate the accumulation of sediment in a tank, and Phoenix tap water is hard, often 10 to 17.6 grains per gallon (City of Phoenix, 2025 Water Quality Report). Scale settles on the tank bottom and heating surfaces, which cuts efficiency and shortens the heater's life. Holding the temperature at 120F helps slow that process. If your hot water already runs out fast or never gets warm enough, the cause may be sediment or a worn part rather than the dial. See why is my water not getting hot enough for that diagnosis.
The scald risk: how fast hot water burns
The strongest safety reason to choose 120F is burn time. Skin damage depends on both temperature and how long the water touches you, and the jump from 120F to 140F is dramatic. The U.S. Consumer Product Safety Commission (CPSC) warns that water heaters set too high cause thousands of scald injuries each year, many to young children and seniors whose skin is thinner and whose reaction time is slower.
The table below shows how long water at each setting takes to cause a third-degree burn in an adult, based on CPSC figures.
| Water temperature | Time to a third-degree burn |
|---|---|
| 120F | About 5 minutes |
| 130F | About 30 seconds |
| 140F | About 5 to 6 seconds |
| 150F | About 2 seconds |
At 140F a serious burn happens faster than a child can pull a hand away. At 120F there is real time to react. That gap is why the CPSC urges a 120F setting, and the case is even stronger in homes with young kids, older adults, or anyone with reduced mobility or sensation. If you have a household like that, 120F is not just the efficient choice, it is the safer one. For the rest of the bathroom, from limit stops to toilet latches, see how to make bathroom plumbing safer for kids.
The Legionella tradeoff and the pro fix
There is one honest counterpoint to a low setting, and it is worth understanding. Legionella, the bacteria that causes Legionnaires' disease, can grow in warm water. The CDC notes that Legionella multiplies in the range of roughly 77F to 113F, which sits just below a 120F tank. Water held too cool can let the bacteria establish, while very hot storage suppresses it.
This is the tension: scald safety pushes the setting down, while bacteria control pushes it up. The professional answer is not to pick one and lose the other. The standard solution is to store the water hotter and mix it down at the point of use with a thermostatic mixing valve, also called a tempering or master mixing valve, built to the ASSE 1017 standard. The tank holds water hot enough to discourage bacteria, and the valve blends in cold so the water reaching your faucets and tub is at or below 120F. You get the safety of a low tap temperature and the bacteria control of hot storage at the same time.
A second wrinkle is the dishwasher. Some older dishwashers without an internal booster heater clean and dry better with incoming water around 130F to 140F. Most newer models heat their own water and run fine on a 120F feed. If you have an older unit and spotty dishes, that is the usual reason people consider a higher setting, and a mixing valve is the clean way to do it without exposing the whole house to scalding water.
Do not raise the setting to fight a rotten-egg smell in the hot water. Manufacturers and health agencies describe other fixes, and we cover them on smelly hot water and whether it is safe.
A kitchen Smart Faucet's requested temperature is a separate control question. The covered Moen faucet's learned limits and app cap are not a reason to raise the tank setting or a substitute for shower or tub scald protection.
The same scald math matters most where guests use the water. Our page on the best water heater for a short-term rental covers setting and locking the temperature in a rental. Boiler-fed tanks have their own mixing valve rule, covered in what an indirect water heater is.
How to adjust your water heater thermostat
Setting the temperature takes a few minutes, and the method depends on whether your heater runs on gas or electricity. Work safely and give the tank time to settle between checks.
For an installed Navien NR-21DU controller, review the specific NR-21DU temperature-lock function before treating a locked adjustment as a fault.
- Gas water heaters. Look for a dial on the gas control valve near the bottom of the tank. It is usually marked with letters, numbers, or a warm-to-hot range rather than exact degrees. Turn it toward the lower end and use a thermometer at the tap to dial it in.
- Electric water heaters. These usually have two thermostats, one behind an upper access panel and one behind a lower panel. Turn off the breaker to the heater first, because the terminals carry live voltage. Remove each panel, fold back the insulation, and set both thermostats to the same temperature so they work together. Replace the insulation and panels, then restore power.
To find the real temperature, run the hot water at the tap closest to the heater for a minute or two, then catch a stream in a cup and check it with a cooking thermometer. Adjust, wait a few hours for the tank to stabilize, and measure again. Aim for 120F at the tap. If you are away from home for several days, check your heater's manual for a vacation or low setting, and return it to 120F when you get back. See whether to turn off your water heater on vacation for the full routine. If the tap still reads hotter than the dial suggests, our page on why water is too hot at the faucet covers stacking, hot garages and failed controls.
For a supported Rinnai controller that will not accept a setting change, the Priority-button guide addresses control authority. That button is not scald protection or a reason to raise the chosen temperature.
A clear recommendation
Set your water heater to 120F. For the large majority of Phoenix homes that is the right number: it delivers comfortable hot water, trims your energy bill, slows the sediment buildup that hard water drives, and cuts the scald risk to a level where a child or older adult has time to react. If yours is set hotter, turning it down is a quick win.
Choose a hotter setting only with a deliberate plan to manage the tradeoffs. If you need hotter storage for an older dishwasher or to keep Legionella in check, store hot but install an ASSE 1017 thermostatic mixing valve so the water at every tap still arrives at or below 120F. That setup is the safest way to get the benefits of hot storage without the burn hazard.
If your water heater struggles to hold any setting, leaks, rumbles, or is more than eight to twelve years old, the thermostat may not be the real issue. A failing tank is a replacement decision, not a dial adjustment, and our guide on whether to repair or replace your water heater walks through the call. HQ Plumbing & Air can set your temperature, add a mixing valve, and check the tank's health in one visit.
If your hot water still feels too hot after turning the heater down, attic-run pipes may be the reason. See why hot water is so hot and cold water isn't cold after an Arizona repipe.
For larger buildings, Legionella risk changes the temperature advice; see whether your building needs a Legionella water management plan.
Buildings that store water hot often add mixing valves to control what reaches the tap; see what a thermostatic mixing valve is and why commercial buildings use one.
Our water-heater maintenance checklist brings sediment, anode condition, pressure and expansion control together without promising a fixed lifespan.
Some facilities have a written range instead of a setpoint; Arizona assisted living, for example, must hold resident-area hot water between 95 and 120 degrees F, as covered in the Arizona assisted living hot water rule.
A mixing valve changes the answer; see what temperature a mixing valve should be set at.