What is a Safe Water Heater Temperature?

There are a lot of things to think about when considering where to set your water heater temperature.

Before I get into the various issues, I will tell you that if I had to choose a single temperature based on everything I know, I would set my water heater to heat water to about 125 degrees.

(Click here to learn how to easily adjust your water heater temperature.)

Having said that, I will also say that there are many factors to consider about your circumstances before you make a decision.

Below are the two main issues/dangers that need to be considered.

Scalding –  Scalding is a serious concern – especially if there are children or elderly people in the home.

Bacteria in the water – At certain temperatures, bacteria can grow rapidly inside a water heater.

1. Some water systems have more bacteria than others.
2. Some families have certain health issues that may make someone more susceptible to the adverse effects of bacteria in the water.

Now I will discuss some specifics of possible consequences of setting your water temperature too high or too low.

1. You don’t want to set it too hot that it could potentially scald someone – especially a young child.
2. You want your hot water hot enough to keep bacteria from growing in your water heater and potentially making someone sick.
3. The hotter your water heater is set, the more money you will be paying to heat that water and to keep it heated 24/7.

Risk of Scalding from Hot Water

Scalding is a definite concern that needs to be considered.

Many people each year are scalded by hot water badly enough that they require medical attention, so this is a definite concern.

If you will look at the graph below, it shows how much time is required to be scalded by water at various temperatures.

Based on the graph, you can see that it takes about five minutes of exposure to 120-degree water before scalding occurs, so this is generally considered a safe temperature.

Bacteria

The warm, moist environment inside a water heater can be an ideal condition for certain bacteria to grow and flourish.

One of the main bacteria about which we should be concerned is Legionella, as it can be quite dangerous. It causes a lung infection which can be fatal.

The danger from Legionella bacteria comes from the bacteria entering our respiratory system.

The main opportunity for this to occur is when the bacteria is contained in very small water droplets that can be inhaled – such as when we are showering.

It does NOT come from drinking the water or getting it on your skin.

Very young children and the elderly are particularly at risk to being adversely affected by Legionella bacteria.  

The main concern with legionella and water heaters is that bacteria will reproduce in the stagnant environment of the water heater, and temperatures of 95 to 115° are ideal for these bacteria to grow and thrive.

If your home is like mine, the water does not sit in your water heater very long, so the doesn’t have much time to reproduce before the water is replaced with new water.

We seem to be constantly washing dishes or clothes, taking showers, and washing our hands. All of these activities bring fresh water into the water heater.

I’m sure that in an average day, all of the water in the water heater at my home is replaced two or three times with new water. This will obviously help to minimize the accumulation of legionella bacteria in the water heater.

A study conducted by the American Society of Sanitary Engineering showed that Legionella bacteria do not multiply when the water temperature is at least 120°.

This temperature does not kill the bacteria, it just prevents them from multiplying.

This means that if you keep the water in your water heater temperature at 120 degrees, the Legionella bacteria that are carried into the water heater by the incoming water will not be able to multiply, but they can survive to be carried to a faucet or showerhead.

Keep in mind that setting your water heater temperature to 120° will not ensure that the water in the water heater will always be 120°. As you run hot water out of the water heater, this water will be replaced with cold water. As a result, the actual water temperature in the water heater will often be lower than 120°, which will support bacterial growth.

OSHA recommends that water heater temperatures be set to at least 140°F in order to prevent or at least minimize the growth of Legionella bacteria and other microorganisms.

Energy Savings Based on Water Heater Temperature

Your water heater accounts for about 18 percent of your home’s energy bill.

Studies have shown that for every 10 degrees you turn down the temperature of the water in you water heater, you will save 3 to 5 percent on your energy bill.

This amount is certainly not insignificant, however, when safety is involved, I believe saving a few dollars is basically irrelevant and should really not be a factor in the decision.

Due to this I will not discuss the energy savings any further here.

Now, back to the discussion of the safety of your family.

Scalding and Bacteria: Conflict of Water Heater Temperature

Unfortunately, the temperatures to prevent scalding and the growth of Legionella bacteria conflict with each other.

So what should you do?

Personally, I believe that the chance of being scalded is greater than the chance of becoming ill due to Legionella. At least I hear many more stories of people – especially children – being scalded by hot water than I hear of people contracting Legionnaires’ disease.

On the other hand, Legionella can affect your health much more seriously than scalding normally can.

There is a way to protect ourselves from both scalding and from Legionella bacteria.

My recommendation is to set your water heater temperature to somewhere around 140 to 150°, and to install anti-scald devices.

You can install anti-scald devices at each plumbing fixture in your home – a fairly expensive and time-consuming undertaking.

Another option is to install a whole house anti-scald thermostatic mixing valve at your water heater. 

A whole house anti-scald thermostatic mixing valve takes the very hot water leaving your water heater and mixes it with just enough incoming cold water to bring the temperature down to a safe temperature which is preset on the device by the homeowner.

While I highly recommend keeping the water in your water heater hot enough to kill bacteria in the water and using anti-scalding devices to lower the temperature to a safe level, I realize that not everyone who reads this post is going to follow that recommendation.

In this case, I would recommend keeping the water in the water heater at about 120 – 125 degrees to prevent scalding and to minimize bacterial growth as much as possible.

There are a lot of things to take into account when it comes to the temperature of your hot water. I recommend that you look at all the factors concerning your family and what you deem as the greater risk, and set the temperature of your water heater accordingly.

If you’re willing to spend a little money, you can protect your family from multiple risks related to your water heater.

If you have any questions or comments, please leave them below, and I will do my best to answer as soon as possible.

© 2021 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at mike@morganinspectionservices.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.