Adjusting a water heater thermostat takes about 10 minutes and requires only basic tools. The process differs depending on whether you have a gas or electric water heater, but both involve locating a temperature dial or screw and turning it to your desired setting. The sweet spot for most households is 120°F, which the U.S. Consumer Product Safety Commission recommends to reduce the risk of scalding.
Choose the Right Temperature
Before you touch anything on the heater, decide what temperature you’re aiming for. At 120°F, water takes about four minutes of sustained contact to cause a scald burn. Raise that to 140°F and it takes just one second. At 150°F, scalding happens almost instantly. If you have young children or elderly family members in the home, 120°F offers meaningful protection.
There’s a tradeoff, though. The CDC recommends storing hot water above 140°F to prevent Legionella, the bacteria that causes Legionnaires’ disease. At temperatures below 120°F, these bacteria can thrive inside your tank. For most healthy households, 120°F is a reasonable balance between scald safety and bacterial control. If you want the best of both worlds, a thermostatic mixing valve (covered below) lets you store water hot while delivering it safely at the tap.
Adjusting a Gas Water Heater
Gas water heaters are the simpler of the two types to adjust. Look near the bottom of the tank for a control knob on the gas valve. It typically has labeled settings like “Hot,” “Warm,” “Vacation,” and “Pilot,” and many models also show temperature markings in degrees.
Turn the dial to your desired temperature. That’s it. No panels to remove, no power to shut off. Some older models use vague labels without degree markings. If yours is one of these, set it to a middle position, wait 24 hours, then test the water temperature at a faucet (instructions below). Adjust up or down from there in small increments, waiting a few hours between each change.
Adjusting an Electric Water Heater
Electric water heaters require more steps because the thermostat is hidden behind access panels on the side of the tank, and you’ll be working near live wiring if you skip the safety precautions.
What You Need
- Non-contact circuit tester or volt meter
- Phillips screwdriver or 1/4-inch nut driver to remove the access panels
- Flat-blade screwdriver to turn the thermostat dial
- Thermometer to verify the temperature at the tap afterward
Step by Step
Go to your home’s electrical panel and find the breaker labeled “water heater.” Flip it to the off position. Never work on an electric water heater without cutting the power first. Electric water heaters run on 240 volts, which is enough to cause a serious or fatal shock.
Back at the heater, remove the access panel (usually secured by two screws) on the side of the tank. Most electric heaters have two panels: one upper and one lower, each covering its own thermostat and heating element. Peel back or lift the insulation behind the panel to expose the thermostat.
Before touching anything, hold your non-contact circuit tester near the wires connected to the thermostat. Confirm the tester shows no voltage. Once you’re sure the power is off, locate the temperature adjustment dial on the thermostat. It’s a small dial or slot with degree markings. Use a flat-blade screwdriver to turn it to your target temperature.
If your heater has two thermostats (upper and lower), adjust both to the same temperature. Setting them differently can cause inconsistent heating or trigger the reset button. Replace the insulation, screw the panels back on, and flip the breaker back to the on position. The water will take one to two hours to reach the new temperature.
How to Verify the Temperature
The thermostat dial on your water heater is an approximation, not a precise instrument. The only way to know what temperature is actually reaching your faucets is to measure it directly.
Wait at least two hours after any hot water use in the house. This lets the tank fully heat and gives you a reading that reflects the thermostat’s true setting rather than partially reheated water. Then turn on the hot water at the faucet closest to your water heater and let it run for 30 to 60 seconds until it’s as hot as it gets. Hold a cooking or liquid thermometer under the stream and note the reading.
If the temperature is higher or lower than your target, go back to the heater and nudge the thermostat slightly. Wait another two hours with no hot water use, then test again. It may take two or three rounds to dial it in precisely.
Using a Thermostatic Mixing Valve
If you want to store water at a higher temperature for bacterial safety but deliver it at a lower temperature to prevent scalding, a thermostatic mixing valve solves the problem. This device installs on the hot water outlet pipe at the top of your tank. It contains a thermostatic element that automatically blends hot and cold water to produce a consistent, safe output temperature.
With a mixing valve, you can keep your tank at 140°F to kill bacteria while the valve delivers 120°F water to your faucets. The valve adjusts itself in real time, even when water pressure fluctuates from someone flushing a toilet or starting the dishwasher. Installing one is a straightforward plumbing job, though it does require cutting into the water lines, so most homeowners hire a plumber for this step.
Troubleshooting Common Issues
If you’ve adjusted the thermostat but the water isn’t reaching your target temperature, a few things could be happening. On electric heaters, one of the two heating elements may have failed. When the upper element goes out, you’ll get little to no hot water. When the lower element fails, you’ll get a small amount of hot water that runs out quickly. Either situation can make it seem like the thermostat isn’t working.
On gas heaters, a dirty or failing thermocouple can cause the burner to shut off prematurely, leaving you with lukewarm water regardless of the dial setting. Sediment buildup at the bottom of the tank is another common culprit for both gas and electric models. A thick layer of mineral deposits acts as insulation between the burner or element and the water, reducing heating efficiency and making the thermostat’s setting unreliable.
If the water coming out of your faucet is noticeably hotter than what the thermostat is set to, the thermostat itself may be failing and should be replaced. On electric models, a stuck relay can also keep the element running past the set temperature, which is both an energy waste and a scald risk.