How to check the water heater’s age and set it to 120°F
Set your water heater to about 120°F, the temperature the Department of Energy recommends for most homes. Measure at the faucet farthest from the tank, adjust the dial or thermostat, then retest after at least three hours. Read the serial number on the label to find the heater's age, since brands code it differently.
Should you set water heater to 120 degrees? Why it matters
You're looking for two answers: how hot is your water, and how long has your tank been running? The dial on a tank can be off, and the last owner may have changed it, so trust a thermometer at the tap over any marking.
The Department of Energy's federal buying guide says to set most homes at 120°F. Many units ship set at 140°F, according to one energy company's guide, which also says water at 140°F can cause a third-degree burn in about five seconds. It cites DOE figures of $36 to $61 a year saved on standby heat loss, and says a lower setting slows scale in the tank and pipes. Your savings depend on your rates, tank and use.
The heater's date is on its label, in code. The rating plate shows the maker, model and serial number. Knowing the age helps you plan rather than wait for a failure. The City of Palo Alto says a gas heater more than 10 years old may be due for replacement, and the Department of Energy assumes a 13-year life when it calculates costs for heat pump models.
How to set water heater to 120 degrees, step by step
- Measure the tap
Run the hot water at the faucet farthest from the heater for at least three minutes. Fill a glass and read it with a cooking thermometer.
- Adjust a gas tank
Find the dial on the gas control valve near the bottom of the tank and turn it down a little. Some dials show 'warm' or 'vacation' in place of numbers.
- Adjust an electric tank
Turn the power off at the breaker first. Remove the access panels, move the insulation aside, set both thermostats to the same temperature, then reassemble and restore power.
- Adjust a tankless unit
Use the control panel display, then test after a few minutes.
- Retest after three hours
Wait at least three hours on a tank, measure again and repeat until you're near 120°F.
- Read the rating plate
Find the label on the side of the tank and take a clear photo of the model and serial numbers.
- Decode and record the date
Use the chart above, then write the year on the tank and in your home records.
Water heater temperature readings and age by serial number
| Tap reading | What it means | What to do |
|---|---|---|
| About 120°F | On target for most homes | Note the date and leave it |
| Well above 120°F | Scald risk, especially for children and older adults | Turn it down and retest |
| Below 120°F | Cooler water can support bacterial growth | Raise it and retest |
| Won't change after adjusting | A thermostat may be stuck | Call a plumber |
| Brand | How the serial number works | Example |
|---|---|---|
| Rheem | First four digits are month and year (MMYY) | 1209D01234 is December 2009 |
| A. O. Smith, current | First four digits are year and week (YYWW) | 2037... is about September 2020 |
| A. O. Smith, before 2008 | Second character is a month letter, skipping I, then a two-digit year | 2C06... is March 2006 |
| Bradford White | First letter is the year, second letter is the month. Year letters repeat every 20 years | HK... is October 2011 |
For other brands, check the maker's website.
What temperature should a water heater be set at if 120°F won't do?
Some homes need hotter storage. One home guide says households with elderly or immunocompromised members may keep the tank at 140°F to lower Legionella risk, which makes anti-scald protection necessary. The usual fix is a mixing valve at the tank, which the Department of Energy says to set at 120°F so the taps stay safe. Ask your doctor and a licensed plumber what fits your home.
Stop and call a licensed plumber if you smell gas, see water around the base, spot rust on the tank, or find a thermostat that won't respond. On a gas heater, leave the gas parts alone apart from the temperature dial. If the tank is near the end of its life, get quotes before it fails, so you aren't left without hot water.
How old is my water heater, and other questions
Is 120°F hot enough to be safe?
The Department of Energy recommends it for most households. One inspector quoted by This Old House calls 120°F the minimum, and homes with vulnerable members may need a different setup, so ask a plumber or doctor.
Why do I need to turn off the power on an electric water heater?
The thermostats sit behind panels next to live wiring. Turning off the breaker first keeps you from touching it. If you aren't sure which breaker it is, call a pro.
How do I know how old my water heater is?
Read the serial number on the label. Rheem and A. O. Smith use digits for the date, and Bradford White uses letters. Check the maker's website for other brands.
Sources: U.S. Department of Energy, paloalto.gov, This Old House, palmetto.com
General guidance for U.S. homes, not professional advice. For gas, electrical and structural work, call a licensed pro.