The Ideal, Safest Temperature to Set the Water Heater Is…

Now the daily temperatures have dipped down into the “gotta put on some socks and a sweater” range, it also means I’ve been adjusting morning showers or post-workout baths into the steamy-hot triple digits. But there can be a fine line to ride between “ahh” hot and “owww” scalding, sometimes only divided by the most minute eyelash width of adjustments…

But beside the joys of sinking into a hot bath or indulging in a relaxing shower, the temperature you set the water heater could possibly directly affect your health in an unsuspected and invisible way. You see, stagnant water inside water heaters, especially those serving multiple units in apartments or older divided buildings, and the connecting piping system from heater to taps can become breeding grounds for a plethora of pathogens…undesired guests which you might be inviting right onto your skin, into your dishwashing machine, and through your tap.

Thus, it’s important to maintain a balance between setting the water heater to a temperature reducing the risk of scalding, while also keeping a high enough setting to prevent illness, most specifically legionellosis (Legionnaires’ disease).

Tessa Miller over at Lifehacker has gathered up an excellent and informative summary about the conditions where pathogens can breed within hot water heating systems, including placing a number on the ideal water heater setting. But first it’s good to note the correlation between temperatures and how it affects possible pathogens inside household water heating systems, according to Legionella and the prevention of legionellosis from the World Health Organization website:

• Above 70 °C (158 °F): Legionella dies almost instantly
• At 60 °C (140 °F): 90% die in 2 minutes
• At 50 °C (122 °F): 90% die in 80–124 minutes, depending on strain
• At 48 to 50 °C (118 to 122 °F): Can survive but do not multiply
• 32 to 42 °C (90 to 108 °F): Ideal growth range

Ideal Maximum Temperature Is… So what’s the ideal temperature then? The Consumer Product Safety Commission (CPSC) pins the ideal balance between safety and comfort at a setting of 120 °F (49 °C).

I have to admit I set our old apartment water heater more in the 140°F range because I grew up enjoying really hot baths, and always practice the steady hand art of incrementally increasing the water’s temp with tiny adjustments throughout my shower or bath. But I can recognize the reasoning behind the CPSC’s decision to recommend a safer, though somewhat tepid 120 °F, for the sake of children and elderly, the two demographics most injured by hot water.

The Canada Safety Council recommends the following bathing habits, whether you set the temps higher or at the CPSC recommended number:

• Never leave a child alone while drawing water in a bathtub, and check the water temperature before putting your child in.
• Test the water temperature before bathing or showering.
• Turn the cold water on first, then add hot water until the temperature is comfortable.
• Teach children to turn the cold water on first, and the hot water off first.

If you’re a parent (or a very concerned and careful adult) worried about the threat of scalding, this all-digital $30 spout cover offers a color coded temperature display for peace of mind.

For more information about water heaters and health safety, check out Lifehacker’s What’s the Best Temperature for My Water Heater?