There is a common misconception that the maximum temperature allowable for a water heater is 120 degrees Fahrenheit. This idea is compounded by the safety warnings on some water heaters informing owners that temperatures in excess of 125 degrees can cause serious injuries and death. Similarly, there are plumbing codes capping the maximum temperature flowing from the fixture for showers, tubs, etc. at 120 degrees. However, the maximum temperature at the fixture is much different than the temperature applied to the water heater.
Some things to consider when setting your water heater temperature:
- The minimum temperature for hot water to prevent Legionella bacteria growth is 122 degrees. The recommended minimum disinfection temperature is 158 degrees for 5 minutes. Between 122 degrees and 130 degrees, the bacteria can survive but not multiply. At 131 degrees, the bacteria takes around 6 hours to die. At 140 degrees, the bacteria dies in around 32 minutes. It dies instantly at 151 degrees.
- There is required minimum. However, hot water is defined as water at a temperature greater than 110 degrees Fahrenheit.
- The ideal growth temperature for Legionella bacteria is between 95-115 degrees.
General recommendation is to place a temperature gauge at the point of lowest hot water temperature – usually just before the hot water return. This should be monitored and maintained at 124 degrees, and the water tank temperature adjusted accordingly. However, this technic can lead to higher water temperatures a potential risk for scalding. Thus, it is important to approach this method with caution and potentially vary the water heater temperature based on the age of the occupants (as children and elderly are most at risk) and the type of water heater in place.
Homes equipped with gas or oil water heaters should adjust to the lower end of the safe temperature spectrum – closer to 122 degrees Fahrenheit. Those sporting electric water heaters can run the temperatures toward the higher range. In both cases, it is recommended to install anti-scald devices liberally – especially in cases where there are small children and elderly in residence.