What temperature should my water heater be set to?

The DOE settled on 120F because it 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. The DOE states plainly that "although some manufacturers set water heater thermostats at 140F, most households usually only require them to be set at 120F."

Running hotter than you need is a quiet drain on your bill. Water heating is about 18% of a typical home's energy use, the second-largest energy expense in most houses. Every degree above 120F adds standby heat loss, the energy the tank burns just keeping water hot between uses. The DOE estimates that a 140F setting wastes roughly $36 to $61 per year in standby losses alone, plus up to $400 a year in demand losses on some systems. Turning the dial down is one of the cheapest energy fixes a homeowner can make.

There is a longevity payoff too. Hotter water speeds up mineral scaling and corrosion inside the tank, and Phoenix tap water is hard, often 10 to 17 grains per gallon. Scale settles on the tank bottom and heating surfaces, which cuts efficiency and shortens the heater's life. Holding the temperature at 120F slows that process and helps the tank reach its full service life. 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 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.

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.

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.

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.

• 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, the DOE notes you can turn the heater to its lowest setting or "vacation" mode to save energy, then return it to 120F when you get back.

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 scale and corrosion 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 at the factory 140F, 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.