What temperature should a water heater be set to?

The two numbers, and what each one is for

Neither number is wrong. They are answers to different questions, which is why you will find both quoted confidently all over the internet without the context that reconciles them.

The case for 120

This Old House's maintenance guide gives the instruction and the payoff in two sentences: "Use a flathead screwdriver to adjust the dial to 120 degrees Fahrenheit", and "For every 10 degrees you lower the temperature, you can expect to save up to 5% in energy costs."

Their failure guide adds a second benefit that has nothing to do with the energy bill: "Setting your water heater to a lower temperature (around 120 degrees Fahrenheit) can reduce wear on the tank and components while saving energy. Lower temperatures extend the life of the water heater, help prevent scalding, and reduce energy bills."

That matters more than the saving for most households. The same guide explains why: "Excessive pressure or temperature can cause the tank to expand and contract, leading to metal fatigue and eventual failure."

Our arithmetic, marked as ours. Take the 5 percent figure at face value. Dropping a tank from 140 to 120 is two steps of 10 degrees, so up to 10 percent off the water heating portion of the bill.

Our running cost page puts average annual water heating at 574.56 dollars, a Department of Energy figure cited by Fixr. Ten percent of that is about 57 dollars a year.

Worth doing, since it costs nothing but a screwdriver, and not life-changing. This Old House publishes the percentage and Fixr the base figure; only the multiplication is ours, and "up to" is doing real work in that sentence.

The case for 140

This Old House's safety guide reports the ceiling rather than a target: "OSHA recommends setting water heaters to a maximum temperature of 140 degrees Fahrenheit (60 degrees Celsius) to prevent scalding. This temperature is still high enough to kill bacteria."

Read that sentence carefully, because it is doing two jobs. It is a scalding limit, telling you not to go above 140. And it notes in passing that 140 is hot enough to kill bacteria, which is the reason some guidance argues against going much below it.

Their plumbing expert Richard Trethewey, on the dial itself: "Now, I can set that except for about 100 and 20 right there. I can turn a little hotter, a little colder." The guide's framing is that finding the right balance prevents scalding while maintaining adequate hot water supply.

How we would decide

Our recommendation, marked as ours. Neither source gives a decision rule for choosing between the two figures, so this is our reasoning, not theirs.

• Young children, elderly or disabled people in the house: 120. Scald risk at 140 is immediate and serious, and This Old House's safety guide separately suggests anti-scald devices on faucets and showerheads as additional protection.
• Nobody vulnerable, and hot water runs short: you have headroom up to 140, which is the ceiling rather than a target.
• Anyone immunocompromised in the house, or a history of a sulfur smell: this is the case where the bacteria argument carries weight, and it is worth asking a doctor or your water utility rather than taking a number off a website, ours included.
• An unused tank, a holiday home, or a long absence: This Old House suggests lowering the temperature or using a vacation setting if the unit has one. Note that a tank sitting lukewarm for weeks is the condition bacteria like, so this is a case for turning it down and then back up on return rather than leaving it part-way.

The rotten egg connection

If your hot water smells of sulfur, temperature is part of the picture. Our rotten egg smell page covers the repair, which runs 200 to 600 dollars, and the distinction that matters first: a smell in the water is a tank problem, a smell in the air near a gas unit means leaving the house and calling the gas company.

How to change it

• Electric tanks have two thermostats behind access panels, upper and lower, and both need setting to the same figure. Cut the power at the breaker first.
• Gas tanks have the dial on the face of the gas control valve, which is also the thermostat on these units.
• Check with a thermometer, not the dial. Run the hot tap nearest the heater for a minute and measure. Dials drift, and on an older unit the marked number and the delivered temperature can be some way apart.

If the water never reaches the setting, or swings wildly, the dial is not the problem. See thermostat replacement at 100 to 250 dollars, or for an electric unit the heating element at 200 to 750 dollars.