Water Heater Thermostat: Settings, Testing and Replacement

What temperature to set

The CPSC urges everyone to set water heaters to 120°F. The DOE's Energy Saver guide gives the same number and calls it safe for most of the population. A.O. Smith also recommends 120°F as the best balance of heating speed, energy cost and safety.

Going hotter costs money and adds risk. Water heating accounts for about 18% of a home's energy use, according to the DOE. State's service handbook also notes that water stored at 160°F can be twice as corrosive as water at 140°F, which is hard on the tank.

Many electric thermostats don't show degrees. State's handbook maps its dial marks like this:

Other brands use their own marks, so check the label inside your access panel. The reliable way to know is to measure. The CPSC suggests holding a candy or meat thermometer under the faucet.

Thermostatic mixing valves

If someone in the house needs hotter stored water, a thermostatic mixing valve at the heater outlet blends in cold water so the taps deliver a safe temperature. It's a common add-on for homes with small kids or older adults, and it's a plumber's install. Some local codes require one in certain installs, so check with your local building department.

Tankless water heater temperature settings

A tankless heater has no thermostat to open up. You set the temperature on the controller or keypad, and the unit heats to that number as water flows. Start at 120°F, the CPSC's recommended water heater setting.

How to adjust an electric water heater thermostat

How the two thermostats work

Electric tanks with two elements usually heat one at a time. Bradford White describes the sequence: the upper thermostat powers the upper element first, and only when it's satisfied does it pass power to the lower thermostat and lower element. So the upper thermostat controls whether the lower one gets power at all.

That's why a failed upper thermostat usually means no hot water, while a failed lower thermostat tends to give you a short supply of hot water. The upper thermostat also carries the ECO (energy cut-off), the red reset button. State says the ECO cuts power to the elements if the top of the tank reaches 180°F.

How to tell if the thermostat is bad

Rule out the elements first. They fail more often and test with the power off. Run the water heater element test on both. If both read correctly and neither is grounded, look at the thermostats.

Signs pointing at a thermostat:

• Water that's scalding even at a normal setting. State lists a thermostat stuck calling for heat as a cause of water that's too hot.
• A reset button that trips again and again. A.O. Smith links repeated trips to a faulty lower thermostat or a grounded element.
• Good elements, a set reset button, power at the heater, and still no heat.

Testing the thermostat

Bradford White's thermostat test is a live voltage check. With the tank cold and the power on, a technician reads voltage across specific terminals on the upper thermostat. If voltage is missing there and the ECO is good, Bradford White says replace the thermostat.

That means working with the panel open and line voltage present. If you're not trained for live testing, stop here. Swap the thermostat on the strength of good element readings, or call a pro to confirm it.

How to replace a water heater thermostat

Bradford White's replacement procedure, in plain steps:

Buy a replacement that matches the old part: upper or lower, single or double pole, and the same type for your heater. You don't need to drain the tank for a thermostat. It clamps to the outside of the tank, which makes this an easier job than an element.

When it's a pro job

Call a plumber or electrician if you see burned or melted wire ends, if the reset button keeps tripping after a new thermostat, or if you're not comfortable around live 240-volt terminals. HomeGuide puts a thermostat replacement at $100 to $350 for parts and labor.