As a quick followup to my previous article about making your hot water last longer, I wanted to share some interesting numbers related to water heater safety that I found in the owner’s manual that came with our new water heater…
As you may be aware, most hot water heaters (at least those in the United States) are shipped with the thermostat set to 120°F to minimize the risk of scalds and burns from contact with excessively hot water.
Preventing burns and scalds
With that in mind, here is a table of temperatures vs. the time required to produce a 2nd or 3rd degree burn on adult skin.
|Water temperature||Time to burn|
|160°F (71°C)||About 0.5 seconds|
|150°F (66°C)||About 1.5 seconds|
|140°F (60°C)||Less than 5 seconds|
|130°F (54°C)||About 30 seconds|
|120°F (49°C)||More than 5 minutes|
Preventing Legionnaire’s disease
Another concern that you might read about has to do with Legionnaire’s disease, which is an infectious disease caused by bacteria of the genus Legionella, most commonly the aquatic Legionella pneumophila. The following table lists the effect of temperature on Legionellae (source).
|Water temperature||Effect on Legionellae|
|158-176°F (70-80°C)||Disinfection range|
|151°F (66°C)||Legionellae die within 2 minutes|
|140°F (60°C)||Legionellae die within 32 minutes|
|131°F (55°C)||Legionellae die within 5 to 6 hours|
|Above 122°F (50°C)||They can survive but do not multiply|
|95-115°F (35-46°C)||Ideal growth range|
|68-122°F (20-50°C)||Legionellae growth range|
|Below 68°F (20°C)||Legionellae can survive but are dormant|
According to most recommendations that I’ve run across, burns and scalds are much more common than Legionella infections, so you might want to give more weight to the safety vs. health concerns.