THE SHORT VERSION
- Start at 120°F. CPSC urges every household to set water heaters there, and A.O. Smith ships its electric tanks at about 120°F. At 120°F a serious burn takes more than 5 minutes of contact; at 140°F it takes less than 5 seconds.
- 140°F is a health trade-off, not a comfort setting. CDC tells large buildings to store hot water above 140°F to hold back Legionella. DOE says 120°F is considered safe for most people, and that someone with a weakened immune system or chronic lung disease may consider 140°F with mixing valves.
- If the tank goes hotter, add a thermostatic mixing valve. A master valve at the heater (ASSE 1017) blends it back to about 120°F; shower and tub valves keep their own 120°F limits, which plumbing codes require.
- Lower is cheaper. DOE puts the standby loss of a tank kept at 140°F at $36–$61 a year.
- Measure, don’t trust the dial. Check with a kitchen thermometer at the tap farthest from the heater, adjust in small steps and recheck after a couple of hours.
120°F is the right setting for most homes
The U.S. Consumer Product Safety Commission (CPSC) urges all households to lower their water heaters to 120°F. Its reasoning is scald risk: most tap-water scald injuries and deaths happen to older adults and children under five, and the time it takes hot water to burn skin drops from minutes to seconds as the temperature climbs.
Manufacturers agree. A.O. Smith factory-sets its residential electric tanks to about 120°F and calls 120°F, the “HOT” mark on its gas dials, the preferred starting point. Rinnai’s Sensei tankless units default to 120°F and only reach 140°F after a parameter change. DOE says some manufacturers have set thermostats at 140°F but that most households only need 120°F, which also slows mineral buildup and corrosion in the heater and pipes.
| Tank setting | Time to a serious burn | Good for | Watch for |
|---|---|---|---|
| 120°F | More than 5 minutes | Most homes; households with young children or older adults | Slightly less hot water per tank; a small Legionella trade-off (below) |
| 125–130°F | About 30 seconds to 2 minutes | Stretching an undersized tank a little | Scald risk rises fast; a dishwasher without a booster heater may want 130°F or more |
| 140°F | Less than 5 seconds | A weakened immune system or chronic lung disease, only with mixing valves; stretching hot water | Needs a thermostatic mixing valve; higher standby losses and faster corrosion |
| 150°F and up | About 1½ seconds | Not a residential setting | Rheem’s table puts a serious burn at about 1 second by 155°F |
Burn times: Rheem owner’s manual scald table, credited to the Shriners Burn Institute. Recommendations: CPSC, DOE Energy Saver (archived) and A.O. Smith manuals. The “good for” column summarizes those sources; it is not a medical recommendation.
Scalding: why a few degrees matter so much
Burn time doesn’t fall in a straight line. Rheem’s manual, using a table credited to the Shriners Burn Institute, lists more than 5 minutes to a serious burn at 120°F, 1½–2 minutes at 125°F, about 30 seconds at 130°F and less than 5 seconds at 140°F. CPSC’s fact sheet gives nearly the same steps: 5 minutes at 120°F, 30 seconds at 130°F, 6 seconds at 140°F and 2 seconds at 150°F.
Those times are for adult skin. CPSC says the people most often hurt are older adults and children under five, and Rheem’s manual says households with small children, disabled or elderly people may need 120°F or lower. CPSC’s advice on supervision is blunt: always hand-test water before bathing an infant, and never leave a young child alone in the bathroom.
Common ways people get burned even with a sensible setting:
- Stacking after many short draws, which A.O. Smith says can push tap water 30°F above the setting.
- A tank turned up to stretch hot water and never turned back, or a thermostat that fails. A.O. Smith lists both as causes of hotter-than-expected water.
- A mixing valve wearing out. Rheem’s technical trainer says master mixing valves typically fail gradually and let more hot water through, so you may not notice.
- Draining or flushing the tank, which Rheem’s manual warns can scald. See how to flush a water heater.
Legionella: the case for hotter storage
Legionella is the bacterium behind Legionnaires’ disease, a serious pneumonia. CDC says people catch it by breathing in mist that contains the bacteria, such as shower spray, not by drinking water, and that most healthy people who are exposed don’t get sick. Those at higher risk are people 50 or older, current or former smokers, and people with chronic lung disease, cancer, diabetes, kidney or liver failure or a weakened immune system.
| Source | What it says | Who it is written for |
|---|---|---|
| CDC | Legionella grows best at 77–113°F. Store hot water above 140°F; keep circulating hot water from falling below 120°F. | Building water management programs (hotels, healthcare and similar buildings) |
| WHO | Grows at 20–50°C (68–122°F). Keep hot water above 50°C (122°F), which means water leaving the heater at 60°C (140°F) or more. | Water systems generally |
| ASHRAE Standard 188 | Requires a Legionella risk management program; temperatures are set in that program and in its companion Guideline 12. | Commercial, institutional, multi-unit residential and industrial buildings. Excludes single-family homes. |
| DOE Energy Saver | A very slight risk at 120°F, considered safe for most people. With a suppressed immune system or chronic respiratory disease, you may consider 140°F, with mixing valves on bathing and washing taps. | Homeowners |
| OSHA | Water heaters kept below 140°F that contain scale and sediment may foster Legionella growth. | Workplaces |
Sources: CDC “Temperature guidance for Legionella control” (reviewed March 15, 2024) and “Causes and spread” (June 9, 2025); WHO legionellosis fact sheet (September 6, 2022); ASHRAE Standard 188-2021 fact sheet; DOE Energy Saver “Lower water heating temperature” (archived May 2026); OSHA Legionnaires’ disease control and prevention page.
So the strict 140°F numbers come from rules written for big buildings with long pipe runs and many occupants, and ASHRAE’s standard leaves single-family homes out entirely. For a typical house, DOE and CPSC land on 120°F. CDC itself notes the tension: temperatures hot enough to control Legionella can be higher than scald rules allow, and it points to mixing hot and cold water at or near the point of use as the way to get both.
Mixing valves let you store hot and deliver safe
A thermostatic mixing valve blends cold water into hot water to hold a set outlet temperature. Rheem’s technical trainer describes the common setup: the tank is turned up to 140°F, and a valve at the heater outlet sends the house whatever temperature the homeowner picks, usually 120°F. Because each gallon of 140°F water is cut with cold, the tank goes further.
- ASSE 1017 (master mixing valve)
- Installed at the water heater to temper the water going to the house. Watts’ LFMMV, for example, sets anywhere from 80°F to 120°F. Watts warns that a 1017 valve at the heater can’t by itself replace the fixture-level devices codes require.
- ASSE 1016 (shower valve)
- The pressure-balancing or thermostatic valve behind a shower handle. Codes require a limit stop set so the shower can’t deliver more than 120°F.
- ASSE 1070 (point-of-use limiting device)
- A small valve under a tub, sink or bidet that caps the temperature at that one fixture.
Living with a mixing valve:
- It needs care. Rheem’s trainer says master valves need periodic cleaning and typically last 5–6 years.
- It fails hot. They tend to wear so that more hot water gets through, gradually. Check the temperature at a tap a few times a year.
- Some programs require one. Rheem’s manual says that when its electric tank is enrolled in a utility’s “Advanced Load-up” program, which stores extra heat when power is cheap, an ASSE 1017 valve must be installed.
- It changes the hot-water math. At 140°F, each shower takes less from the tank; running out of hot water works through the numbers.
What plumbing codes limit: the fixture, not the tank
US model codes put their temperature limits on fixtures. The fixture rules don’t set the tank temperature, but they assume someone will cap what comes out of the shower and tub.
| Fixture | IRC (2021) | IPC (2024) | UPC-based (2025 California Plumbing Code) |
|---|---|---|---|
| Shower and tub/shower valve | 120°F limit, ASSE 1016 valve (P2708.4) | 120°F limit, ASSE 1016 valve (412.3) | 120°F at the showerhead (408.4.2) |
| Bathtub and whirlpool filler | 120°F, ASSE 1070 device (P2713.3) | 120°F, ASSE 1070 device or listed heater (412.5) | 120°F (409.4) |
| Bidet | 110°F (P2721.2) | 110°F (408.3) | — |
Sources: 2021 IRC chapter 27 (Colorado adoption) and 2024 IPC chapter 4 (Maryland adoption), read on UpCodes; 2025 California Plumbing Code §408.4 and §409.4. Section numbers change between editions, and your city may amend them. Your local permit office has the final word.
Codes also note that an in-line thermostatic valve at the heater can’t be used to meet the shower rule (IRC P2708.4, IPC 412.3), which matches Watts’ warning. A.O. Smith’s manuals add that, under ASSE 1070 and most local codes, the water heater’s thermostat shouldn’t be the sole means of preventing scalds. If you’re remodeling a bathroom, the inspector will look at the new valve’s limit stop. More code detail is in water heater code requirements.
What a lower setting saves
DOE’s Energy Saver puts the waste from a tank kept at 140°F at $36–$61 a year in standby losses alone, the heat that leaks through the tank walls whether you use hot water or not. Its project page lists savings of 4%–22% a year from turning the thermostat down, counting the hot water you use at the lower temperature as well as standby loss. Rheem adds that hotter water speeds corrosion, scale and, in electric tanks, element failures.
Why the savings are smaller than you might expect (illustrative): heat leaks out roughly in proportion to the gap between the water and the room. With a 65°F basement, a 140°F tank sits 75°F above the room and a 120°F tank 55°F above it, so standby loss drops by about 27% (20 ÷ 75). A 105°F shower uses nearly the same energy either way, because a hotter tank is simply mixed with more cold water. That is why DOE’s standby figure is modest.
Two exceptions. A dishwasher without its own booster heater may need 130–140°F for good cleaning, DOE says; the dishwasher’s manual tells you whether yours has one. And Rheem lists longer recovery times among the costs of a hotter tank, which matters most for slow-recovering heat pump water heaters. For what your heater costs to run overall, see water heater operating costs.
How to check and adjust the temperature safely
Measure first
Hold a candy or meat thermometer under the hot tap farthest from the heater. CPSC says to do it first thing in the morning or at least two hours after hot water use. DOE notes thermostat dials are often inaccurate.
Mark the starting point
DOE suggests marking the current dial position with a marker so you can return to it.
Adjust in small steps
Use the method for your heater type below. Change a little at a time.
Wait, then measure again
Give it a couple of hours (DOE’s advice) and recheck at the same tap. Several rounds may be needed.
Gas tank
The dial is on the gas control valve near the bottom of the tank. On A.O. Smith dials, HOT is about 120°F, A about 130°F, B about 140°F, C about 150°F and VERY HOT about 160°F; other brands label differently, so check your manual. Turn only the temperature dial. Leave the gas line, burner and venting alone, and if you smell gas, stop and leave the house.
Electric tank
Most have two thermostats behind upper and lower access panels, next to live 240-volt terminals. A.O. Smith’s manual says to turn the breaker off and confirm with a non-contact tester, remove the panels and fold back the insulation, then turn each dial with a screwdriver. Set both to the same temperature, and never set the upper one higher than the lower. Put the insulation and covers back before restoring power.
Heat pump and tankless
Both use a digital display or app. Tankless units heat water as it flows, so the setting is the delivered temperature; Rinnai’s Sensei defaults to 120°F. Many electronic controls, such as Rheem’s Performance Platinum electric, have a Vacation mode that drops the setting while you’re away; A.O. Smith gas tanks have a VAC setting of about 55°F.
Common questions
What is the best water heater temperature setting?
For most homes, 120°F. CPSC recommends it to prevent scalds, and DOE says most households need no more. Households where someone has a weakened immune system may consider 140°F, but only with mixing valves on bathing and washing taps.
Is 120°F hot enough to kill bacteria?
Not reliably. WHO says Legionella can grow at up to 50°C (122°F), and CDC says it grows best at 77–113°F. DOE calls the risk at 120°F very slight and safe for most people. If someone in the home is at high risk, ask a doctor and consider 140°F storage with a mixing valve.
Can I set my water heater to 140°F?
You can, but at 140°F a serious burn takes less than 5 seconds. If you do, have a plumber add a thermostatic mixing valve so taps get about 120°F, and keep your shower valves’ 120°F limit stops set.
Why is my water hotter than the thermostat setting?
A.O. Smith says repeated short draws can make tank water stack in layers up to 30°F hotter than the setting. A failed thermostat or a worn mixing valve can also send water too hot. If water is ever much hotter than the setting, turn the heater down and have it checked.
Does lowering the water heater temperature save money?
Yes, modestly. DOE estimates a tank kept at 140°F wastes $36–$61 a year in standby heat loss, and puts total savings from turning down at 4%–22%. Most of the hot water you use is mixed with cold anyway, so the biggest savings come from reduced standby loss.
What does “vacation mode” do?
It holds the tank at a low temperature while you’re away. On A.O. Smith gas tanks the VAC setting is about 55°F, low enough to save energy and keep the tank from freezing. DOE also suggests turning the heater down or off for long absences.
Do I need a mixing valve?
Not if the tank stays at 120°F; your shower valves already limit temperature. If the tank is set hotter, A.O. Smith and Rheem both recommend thermostatic mixing valves, and some utility load-shifting programs require one.
Sources
Sources checked October 5, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.
- Avoiding tap water scalds (Publication 5098) ↗ · U.S. Consumer Product Safety Commission
- Do-it-yourself savings project: lower water heating temperature (archived copy, May 2026) ↗ · U.S. Department of Energy, Energy Saver
- Temperature guidance for Legionella control ↗ · Centers for Disease Control and Prevention
- About Legionnaires’ disease ↗ · Centers for Disease Control and Prevention
- Legionella causes and spread (risk factors) ↗ · Centers for Disease Control and Prevention
- Legionellosis fact sheet ↗ · World Health Organization
- Standard 188-2021 fact sheet (scope) ↗ · ASHRAE
- Legionnaires’ disease: control and prevention ↗ · OSHA
- Performance Platinum electric water heater use and care manual (scald table) ↗ · Rheem
- How a mixing valve can get more out of your water heater ↗ · Rheem
- Residential electric water heater use and care guide (100404292, March 2026) ↗ · A.O. Smith, hosted by Lowe’s
- Residential gas water heater installation and use and care guide (Signature 100, 100341468) ↗ · A.O. Smith, hosted by Lowe’s
- Technical bulletin 32: water temperature settings ↗ · A.O. Smith
- Sensei tankless water heater installation and operation manual ↗ · Rinnai
- LFMMV thermostatic mixing valve specification sheet (ES-LFMMV) ↗ · Watts, hosted by WebstaurantStore
- 2021 IRC Chapter 27, Plumbing Fixtures (P2708.4, P2713.3, P2721.2) ↗ · International Code Council, via UpCodes (Colorado adoption)
- 2024 IPC Chapter 4, Fixtures, Faucets and Fixture Fittings (408.3, 412.3, 412.5) ↗ · International Code Council, via UpCodes (Maryland adoption)
- 2025 California Plumbing Code §408.4, shower and tub/shower valves ↗ · California Building Standards Commission, via GoCodebook
- 2025 California Plumbing Code §409.4, bathtub temperature limit ↗ · California Building Standards Commission, via GoCodebook