Water heater temperature, scalds, and Legionella
CPSC burn times at 120, 130, 140, and 150°F, how to measure the tap, and why a mixing valve exists for the bacteria question.
The temperature at the tub is a safety setting, not a comfort preference you discover by getting burned. The U.S. Consumer Product Safety Commission has urged households to keep water heaters near 120°F for decades. Its publication on tap-water scalds gives the times that make the reason obvious.
Most adults can receive a third-degree burn in about two seconds in 150°F water, in about six seconds in 140°F water, and in about thirty seconds in 130°F water. At 120°F the same publication says a third-degree burn can still happen, after about five minutes.
Children and older adults, who have thinner skin, are injured faster and are the majority of the serious and fatal cases the CPSC highlights. “We like it hot” is not a rebuttal to those numbers.
This page is the setting and the measurement. It is not medical advice, and it is not a Legionella control plan for a hospital. The disclaimer says that in formal language.
If someone in the house is at high risk from a waterborne infection, talk to a clinician about risk and to a plumber about a mixing valve. Do not freelance a tank temperature from a paragraph.
Measure the tap, not the dial
Gas controls often show a warm-to-hot arc with no degrees. Electric thermostats show numbers and are still several degrees off the water that arrives at the faucet. A mixing valve, long piping, and a heat-trap nipple all sit between the stat and the tub. The CPSC’s own instruction is to measure with a thermometer under a faucet, first thing in the morning or at least two hours after a heavy use, and to adjust from that reading.
Use a simple probe or candy thermometer you trust. Run the hot tap nearest the heater, and also the tub you actually bathe in, until the reading stops rising. Write the number down. If the tub is at 130°F or above, treat that as urgent.
Lower the set point, wait, and measure again. On an electric tank, kill the breaker before you remove the access panel to move a thermostat. The CPSC says the same thing: shut the power off before you adjust electric thermostats, and set both the upper and the lower thermostat to the same temperature.
On a gas tank, follow the manual for which way the dial moves. A dial turned the wrong way because the markings are faint is a common way to make a scald worse while trying to fix it.
Check a child’s bath with your own wrist every time, even after you have set the heater. A caregiver’s hand is the last device. Anti-scald tub valves, listed to a plumbing standard such as ASSE 1016, limit how far a cartridge can run toward full hot.
They are a second layer. They are not a reason to leave the tank at 150°F. They fail, and they are not present on every lavatory.
Why 120°F is the household default
At 120°F the stored energy in a given tank is lower, recovery has less work to do, scale forms more slowly, and the scald time is measured in minutes rather than seconds. The Department of Energy has also published a modest annual saving for turning a set point down to 120°F, on the order of a few tens of dollars in that article’s prices. The safety case does not depend on the dollars. The dollars are a side effect.
Dishwashers are the usual objection. Many current machines heat their own water and are satisfied with a 120°F supply. The Department of Energy’s sizing guide mentions 140°F for dishwashers that do not have an internal heater.
Read the dishwasher manual. Do not heat an entire house to 140°F for a machine that already contains a booster.
If the manual truly requires a hotter supply, a plumber can sometimes feed that one appliance differently.
The whole-house set point is still a scald decision.
The bacteria tradeoff, without a home experiment
Legionella bacteria, which cause Legionnaires’ disease and Pontiac fever, grow in warm water. CDC descriptions put the favorable growth range at roughly 77°F to 113°F, with better survival as water sits in that band, and with heat used as a control in building water systems, often by storing water hotter than the temperature delivered at the tap.
A mixing valve at the heater outlet is what makes those two temperatures possible: the tank can be hotter, and the pipe to the shower can still be at 120°F. That arrangement is common in larger buildings with a water-management plan. It is not a weekend project of “turn the dial to 140 and hope.”
A mixing valve has to be listed for the job, installed in the orientation the maker specifies, and checked with the same thermometer. A valve installed backward or set to the hot stop does not protect anyone. The tank at 140°F without a working valve is the CPSC scenario: six seconds at the tap.
If you do not have a valve, do not raise the tank into that range because a summary of CDC guidance worried you. If a clinician has told a specific household that a hotter store is warranted, hire a plumber to install and set the valve, and confirm the tap temperature after the work.
This site will not choose a storage temperature for an immunocompromised person. That is the clinician’s conversation, with the plumber as the person who can make the tap safe.
Other organisms and slime at an aerator are not Legionella and are not solved by scalding the household. Clean the fixture.
The discoloration guide and the smell guide separate those complaints. Sulfate-reducing bacteria and Legionella are different. Raising the set point is the wrong first move for a rotten-egg odor.
Who gets hurt, and where
The CPSC notes that the majority of serious tap-water scald injuries and deaths involve young children and older adults. A grandparent’s house with a gas dial turned to high, a rental with no thermometer in the bathroom, and a tub without an anti-scald valve are the usual scenes.
Landlords in many states have duties about hot-water temperature. Tenants who measure 140°F at the tap should say so in writing and should not “fix” a gas control they do not own. Owners should measure, not argue from the dial.
Water heaters with a vacation or pilot setting should be returned to the measured set point when people come home, not to the maximum because the tank was cold and someone was impatient. After any repair that disconnected a mixing valve, measure again the same day. Repairs are when safe temperatures become unsafe without anyone touching the dial on purpose.
Checklist: temperature
- Thermometer at the heater outlet and at the tub, after a long enough run.
- Target near 120°F at the tap unless a mixing valve is confirmed to be holding the tap there while the tank is hotter.
- Electric thermostats adjusted with the breaker off, both elements set alike.
- Gas dial moved in small steps, remeasured the next morning.
- Dishwasher manual checked before anyone argues for 140°F.
- No household set to 140°F or above without a listed mixing valve and a confirmed tap reading.
- Children and older adults assumed to be in the house, because guests count.
- Alarming readings in a rental reported, not adjusted on someone else’s gas valve.
What this page will not tell you to do
It will not tell you to disable a mixing valve to get a hotter shower. It will not tell you to test a relief valve by raising the thermostat until the valve opens.
That test is dangerous and belongs nowhere. The relief-valve test uses the lever, if it uses anything. It will not give a disinfection procedure for a domestic hot-water system.
Building water-management plans are a professional practice with responsibilities this site does not have. It will not diagnose a burn. Cool a burn with cool running water and get medical care. The times above are prevention.
If the measured temperature will not come down, the control is failed closed or the valve is not the one you think it is. Shut the heater off if the water is in the immediate-scald range and you cannot reduce it.
An electric high limit that does not open, or a gas burner that does not shut off, is a service emergency. When to call a pro includes runaway heat on that list. A shower is not worth a control that has already shown it will not stop.
Sources
U.S. Consumer Product Safety Commission, “Avoiding Tap Water Scalds,” publication 5098 (PDF): 120°F recommendation, burn times at 150°F, 140°F, 130°F, and 120°F, and the instruction to cut power before adjusting electric thermostats. U.S. Department of Energy, Energy Saver, “Which Water Heater Is Right for You?,” for the illustrated saving from a 120°F set point, and “Sizing a New Water Heater,” for the 140°F note about dishwashers without internal heaters.
Centers for Disease Control and Prevention, Legionella growth-temperature descriptions in the agency’s Legionella materials (growth emphasized through roughly 77°F to 113°F, with heat as a building-system control). Those CDC materials are not a personal medical recommendation. ASSE 1017 and ASSE 1016 are the usual listing families for mixing valves and tub/shower compensating valves; the installed valve’s own instructions govern the set point.