10/07/2026
This is scary to think about. Keep those tanks hot baby!
Plumbers and hospital engineers argue about one number on your water heater dial.
The money side is settled: the Department of Energy puts water heating at roughly 18 percent of a home's energy bill, second only to heating and cooling, and every 10°F you drop the tank saves another 3 to 5 percent on that line. Going from 140°F down to 120°F trims 6 to 10 percent off water heating for the price of turning a dial, and it cuts the scald risk at the same time, because 140°F water burns a child's skin in about 5 seconds while 120°F takes closer to 5 minutes. That is the advice printed on the side of most new tanks, and it is the advice almost every energy guide repeats.
The biology side is not settled at all, because Legionella multiplies fastest between 77°F and 113°F, and a tank held at 120°F keeps its bottom layer sitting right inside that window. Cold water enters at the base of the tank and the heating element sits above it, so the coolest few inches never reach the temperature on the dial. At 140°F the bacteria die off over roughly half an hour, at 158°F they are gone in seconds, and anything in between is a slow incubator with a small annual discount attached.
Drinking the water is not what gets people, breathing it is. A showerhead atomizes the stream into droplets fine enough to reach deep into the lungs, which is why outbreak investigations keep landing on showers, cooling towers and hot tubs rather than kitchen faucets. The CDC logs roughly 10,000 Legionnaires' cases in the United States each year and says the true count runs far higher, because on a chest X-ray it looks like ordinary pneumonia. About 1 in 10 diagnosed cases is fatal, and the odds get worse fast for anyone over 65 or anyone already carrying a lung condition.
You do not have to choose between the bill and the bacteria. Set the tank to 140°F and fit a thermostatic mixing valve on the hot outlet, which blends cold water into the line and delivers 120°F to every tap in the house. The tank stays hostile to Legionella, the shower stays safe to stand in, and the parts run about $100 to $200 with under an hour of plumber time. If the heater has sat unused for a week or more, run the shower hot for 3 full minutes with the bathroom door open before anyone gets in. [WCZXW]
