Modern hot water heaters can be turned up to the point where you can get a serious burn from water that is too hot. However, water that is not hot enough can also be a heath risk.
Here are the numbers:
Water at 160F (71C) will physically damage the human body by scalding. That is way too hot.
Humans cannot stand temperatures of 140F (60C). Your body will not let you hold your hand in water that is 140F.
Water at 140F will kill 90% of the bacteria in it in 2 minutes.
Water at 122F (50C) will kill 90% of bacteria with in 2 hours.
Warm water in the range of 95-113F (35-46C) is perfect for growing Legionella bacteria. This is the bug that causes Legionnaire's disease.
What should you do?
You will see articles suggesting that for energy conservation a home hot water heater should be set in the 115-120F (46-49C) range. That may be too cool for your good health.
One U.S. government agency advises keeping your heater set at 140F for good health. Another U.S. government agency suggests setting it at 120F to save energy!
120F might not be hot enough for your health. If there are dead ends in your piping, where the flow stagnates, bacteria could grow inside your home hot water system.
A good choice for home hot water is 130F (55C). Hot enough to kill bacteria but not energy wasteful either.
Check your water temperature by putting a coffee mug in the sink. Use a candy thermometer in the mug. Let the hot water run into the mug until it is as hot as it will get. If it is below 130, adjust your heat up a little. If it is well above 130, adjust it down.
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