Water that is too hot at the faucet usually means the heater is set above 120 degrees, hot water has stacked in layers in the tank, the heater sits in a very hot space, or a control or mixing valve has failed. Measure the tap with a thermometer first. Water that keeps climbing past the setting is a repair.
How hot is it actually?
Measure before you change anything. The Consumer Product Safety Commission's scald guide says to hold a candy or meat thermometer under the faucet for the most accurate reading, first thing in the morning or at least two hours after water use. Run the hot tap nearest the heater until the temperature stops rising.
Compare what you read to these CPSC figures:
| Water temperature | Time for most adults to get a third-degree burn (CPSC) |
|---|---|
| 150°F | Two seconds |
| 140°F | Six seconds |
| 130°F | Thirty seconds |
| 120°F | Five minutes |
The CPSC urges all users to lower their water heaters to 120 degrees Fahrenheit. If you read well above that, keep going. Small kids and older adults in the house make this urgent, since the CPSC notes most tap water scald injuries and deaths are to the elderly and children under five.
Is the heater simply set too high?
Often, yes, and the dial isn't always accurate. Some tank controls use marks like "Hot" or "A, B, C" instead of numbers. If someone turned it up to recover faster during a busy week, it may never have come back down. A. O. Smith's manual says plainly that water will be hotter if someone adjusted the gas control valve to a higher setting.
Turn it down a notch, wait a few hours, and measure again with the thermometer. Repeat until the tap reads close to 120°F. Our page on what temperature a water heater should be set to covers the trade-offs, including the Legionella question.
Why can a tank run hotter than its setting?
Because of how the tank layers its water, and because of where it sits. A. O. Smith's gas heater manual lists several ways the water can come out hotter than the dial, even on a low setting.
Stacking. Repeated short draws, like a string of hand washes, can make the hot and cold water in the tank form layers. The manual says the water can then be as much as thirty degrees hotter than the gas control setting, and that this comes from usage pattern, not a malfunction. A dial at 120 could deliver close to 150 at the top of the tank. That's the case for a mixing valve, below.
A hot room. The same manual says a water heater in a hot environment can make the tank water as hot as the surrounding air, regardless of the setting. In a Phoenix garage in July, that's not a small effect. Shade, insulation and ventilation in the garage help; the setting alone can't fix it.
Preheated supply water. The manual notes that if the incoming water is preheated, for example by a solar system, the tank can run above its setting.
Phoenix adds one more twist outside the tank. In summer, the cold water line itself can come out warm, so a faucet mixing hot and cold runs hotter than it would in winter. If the hottest water shows up right after a recent repipe, see why the cold water isn't cold after an Arizona repipe.
What if only one shower or faucet runs too hot?
Then the fixture valve is the place to look, not the heater. Most single-handle showers have a limit stop inside the handle that caps how far it turns toward hot.
Moen's instructions for its PosiTemp valve say to adjust that internal stop when the water is too hot, which restricts handle movement and lowers the overall temperature range. You pull the stop, rotate it clockwise into a lower notch, and test with a thermometer until it reads no more than 120°F. Moen recommends limiting the maximum shower temperature to 120°F or less to prevent scalding. Other brands use different parts, so follow the instructions for your valve.
If a shower suddenly swings hot when someone flushes a toilet, that points to a valve without working pressure balancing, a different problem. Our comparison of pressure-balancing and thermostatic shower valves explains how each one protects you.
When is too-hot water a repair, not an adjustment?
When the temperature keeps climbing, the relief valve opens, or turning the dial down changes nothing. A. O. Smith's manual says problems with the gas control valve or other malfunctions can cause higher than expected temperatures. Its instruction is direct: if overheating occurs or the burner won't shut off, turn off the manual gas supply valve to the water heater and call a qualified person.
Signs you're in that territory:
- Water at the tap well above 140°F even after you turned the setting down.
- The relief valve on the tank discharging hot water or steam.
- Banging, hissing or a burner that runs nonstop.
- An electric heater whose reset button keeps tripping, which our page on resetting an electric water heater covers.
For a home with kids or older adults, thermostatic mixing valves at the points of use are a solid fix. The A. O. Smith manual recommends them to reduce the risk of unusually hot water reaching fixtures. We install mixing valves and repair or replace water heaters that won't hold their setting. See our water heater service.