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 besides 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 the 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). There is a correlation between temperatures and how it affects possible pathogens inside household water heating systems according to the  World Health Organization.

There are two main and opposing risks:

  • Too high, and users get scalded
  • Too low, and you risk pathogens, particularly Legionella, which causes legionellosis (Legionnaires’ disease)​

Prevention of Legionellosis from the World Health Organization

  • 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

Although some manufacturers set water heater thermostats at 140ºF, most households usually only require them to be set at 120ºF, which also slows mineral buildup and corrosion in your water heater and pipes. Water heated at 140ºF also poses a safety hazard—scalding.

Savings resulting from turning down your water heater temperature are based on two components: reduced standby losses (heat lost from water heater into surrounding basement area); and consumption (from water demand or use in your home). Set too high, or at 140ºF, your water heater can waste anywhere from $36 to $61 annually in standby heat losses and more than $400 in demand losses.

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).

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.

Conclusion

Most households can have the temperature set between 120ºF and 130ºF on their water heaters. If you are concerned about energy savings and reducing your monthly energy bill then set your heater temp to 120ºF and see if that is sufficient for your needs.

If 120ºF if not hot enough, try turning the temperature up in 5º increments until you reach the perfect temperature for your needs.