Whenever I post a photo of the control panel of my heat pump water heater (like the one above), I usually hear from at least one reader who has concerns about Legionnaires’ disease. I care about my health, so I’ve dug into this topic a little bit and have made a decision about whether to keep my water heater set at 120 °F or set it higher. I’m no expert here, though, so please let me know what you think in the comments below.

Legionnaires’ disease basics

We know a whole lot about Legionnaires’ disease now that wasn’t known in 1976 when 221 attendees at the American Legion convention came down with a mysterious illness. Thirty-four of them died, setting off a furious search for the reason. The culprit turned out to be bacteria that hadn’t been known about before. Its name came from the circumstances of its discovery: Legionella pneumophila.

Legionella grows in fresh water at temperatures between 68 °F and 120 °F. It can be part of biofilms, the slimy stuff growing in a lot wet places (like the inside of a dryer vent in the photo below). You can find biofilms with Legionella in cooling towers, shower heads, misters, humidifiers, and ornamental water features. The biofilm and its Legionella population itself isn’t the problem, though.

The way a person gets Legionnaires’ disease is by inhaling aerosolized bits of Legionella. The lungs then get infected and cause symptoms similar to those of pneumonia. Since it’s a bacterial infection, doctors treat it with antibiotics.

You can find more information on Legionnaires’ disease at the Center for Disease Control, among other reputable sources. ASHRAE has a standard, a guideline, and articles on this topic, too.

Water temperature and Legionella growth

Back to the topic of this post. First, let’s look at how temperature affects the growth of Legionella. The table below is from a presentation by John Springston at the 2018 Building Science Summer Camp.

Clearly, you want to stay away from water heater settings between 77 °F and 113 °F, the red regions in that chart. And the yellow regions, too. Above 120 °F, though, there’s not much growth and even some die-off of Legionella. So what’s a person to do?

My current position

I still have my water heater set to 120 °F. According to what I’ve read about ASHRAE’s recommendation, they say the hot water in your tank should be 140 °F or higher and that your plumbing system should deliver hot water at 124 °F or higher. ASHRAE mostly deals with commercial and institutional buildings, though, so does this apply only to those larger buildings? Or to homes, too?

I’m storing it at 120 °F and delivering it at an even lower temperature. Am I doing the wrong thing? I’m eager to learn more from health experts and will set it higher if necessary.

From what I’ve read, it seems that domestic hot water isn’t where most cases of Legionnaires’ disease come from. It’s chillers and hot tubs and water features that aerosolize Legionella particles.

And there’s another reason I feel it’s not such a big a risk in my hot water system: water heater manufacturers recommend setting the temperature to 120 °F.

Lawyers are good at finding liability, yet manufacturers don’t warn you about Legionella with that setting. They do, however, warn you about the risk of scalding if you set it higher. The screenshot above shows what happens when I go even a measly one degree above 120 °F.

That’s certainly not a guarantee of a risk-free decision. It just means the risk is too small for lawyers to have changed the behavior of manufacturers . . . or that they haven’t found the connection yet. In general, though, the number of Legionnaires’ cases is pretty small.

What temperature is your water heater set at?

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Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn. Photos courtesy of author.