How Hot is Too Hot? A Theoretical Dialogue on Water Temperature

In the plumbing industry, one of the things people ask all the time is how hot the temperature for the hot water heater should be. Like all things, instead of a pat answer, the answer, which is even moodier than your shower temperature dial, will likely be, “That depends.” The Arizona code is now set at 120 degrees. That is the official answer. Too hot for small children and not hot enough to prevent Legionella.

Money Savings

The first thing most people take into consideration is who is using the hot water and their needs. It is easy to say that the lowest temperature setting is best, because obviously from an economic standpoint, that will require the least amount of energy. However, when you are the second person in the shower that morning, and there isn’t any hot water left because you weren’t the first shower-taker, the enthusiasm for saving a little money quickly fizzles.


Children's skin is about 15 times thinner than an adult's.

Children's skin is about 15 times thinner than an adult's.

For advocates of hotter hot water, who just want to be able to have some hot water for that second shower, the matter of who else is living in the household may require consideration.
Children and elderly people living in the home can get a scald burn at a rate of twice that of their healthy adult counterparts. So, if the second-in-line-but-still-got-the-hot is important to you, there has to be a plan for the wee children, who are likely as not to try to pull themselves up out of the bathtub using the hot water faucet.*


Just when you thought you should probably turn the hot water heater back down and take your shower at night, another consideration is added to the mix: the potential for the bacteria which causes Legionnaires disease. When the bacteria are lurking in the water, and that water is turned into any kind of aerosol form, such as a shower, Legionnaires can be transmitted.  Legionella can hide in hot water heaters and plumbing pipes and grow in water with a temperature between 68 and 122 degrees F. There were 91 cases of suspected or confirmed Legionnaire’s disease in Arizona in 2015 (2016 figures are not yet available). 

Temperature Regulators

So, the solution that many people recommend is to set the water at 125 degrees F. This is certainly hot enough to burn an older person or a young child. People with serious concerns, but not a serious bank balance, can buy a HotStop, a temperature-regulating device which prevents too-hot water from coming out of the faucet. The water slows to a trickle until it is cool, thereby preventing an accident. Of course, if the sensitive person is standing in the shower lathered up with shampoo and body wash, the five minutes it takes for the water to resume is going to seem interminable. Not as interminable as recovery from a scald injury, however.


Finally, most hot water heaters aren’t equipped with a nifty thermostat to tell you the exact temperature of the water, so a little experimentation is in order at the faucet with a thermometer until you discover what temperature the hot water is when it arrives at your sink or shower. Each plumbing system is a little different, and those differences show themselves at the tap.

While this information may not have been helpful at assisting you with arriving at a conclusion, it does give you some concept of the problems and risks associated with setting your hot water heater at a temperature which is too hot or too cold.


*We refuse to tell terrible scalding/burn stories in this blog. If you want drama, turn on the evening news.