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What Temperature to Set a Water Heater To

This is the one setting on the appliance that can injure someone, and the numbers behind the advice are unusually stark.

Answer first

CPSC recommends a water heater setting of 120 degrees Fahrenheit, and the reason is specific: water at 150 degrees causes a third-degree burn in about two seconds. The trade-off being made is scald risk against stored capacity.

Source: US Consumer Product Safety Commission, Tap Water Scalds, publication 5098.

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The recommendation

The US Consumer Product Safety Commission advises setting a water heater to 120°F. That is not a comfort recommendation; it is a burn-prevention one.

Source: US Consumer Product Safety Commission, Tap Water Scalds, publication 5098.

Why 120 rather than 140

Because the time available to react collapses as the temperature rises.

Water temperature	Time to a third-degree burn
150°F	2 seconds
140°F	6 seconds
130°F	30 seconds
120°F	5 minutes

Who this protects

CPSC is specific that the majority of injuries and deaths from tap water scalds are to the elderly and to children under five — the two groups least able to move away from hot water quickly.

The side effect

A lower setting also reduces standing heat loss and slows scale formation, so it costs less to run and is easier on the tank. Since DOE puts water heating at 14% to 18% of a household's utility bill, that is not nothing.

Source: US Department of Energy, Which Water Heater Is Right for You?.

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