Why we keep hot water above 50 °C: Legionella and your water heater

What Legionella is

Legionella is a bacterium found in small numbers in most water supplies. It causes Legionnaires’ disease, a serious form of pneumonia, and a milder flu-like illness called Pontiac fever.

It multiplies in water between about 20 °C and 45 °C, fastest near body temperature, and it grows best where water sits still and where scale, rust or sludge collect at the bottom of a tank. A water heater set low, a cylinder that is too big for the household, or a pipe that is rarely used gives it all three.

People catch it by breathing in fine droplets of contaminated water, from a shower, a tap, a jacuzzi or a garden sprinkler. Drinking the water does not usually cause it. The risk is highest for people over 50, smokers and anyone whose immune system is weakened.

The temperatures a home system should keep

International guidance, including the World Health Organization’s, comes down to a few numbers. Store hot water at 60 °C or above, so anything that enters the tank is killed. Deliver it so that every tap reaches at least 50 °C, ideally 55 °C, within about a minute of running. Keep cold water below 20 °C, out of the growth range.

In a central hot water system with a return loop, the water coming back to the cylinder should also stay above 50 °C, so no part of the pipework sits in the danger range. Insulating hot pipes, and keeping cold pipes away from hot ones and out of hot roof spaces, helps both sides stay where they should.

The same applies whatever heats the water. Solar collectors and heat pumps often produce water below 60 °C, so a well-designed system includes a backup heater or a regular high-temperature cycle that lifts the whole cylinder above 60 °C from time to time.

Isn’t a lower setting cheaper?

A little, but much less than people expect. Most of the energy goes into heating the water you use, which happens at any setting. The extra cost of 60 °C over 45 °C is mainly the heat a tank loses while it waits, and good insulation keeps that small.

If hot water runs out too quickly at 60 °C, lowering the temperature is the wrong fix. The cylinder may be too small for the household, or the heater may need servicing because scale is taking up space and insulating the element.

Hot enough to be safe, without scalding

Water at 60 °C can scald skin in seconds, which is a real concern with young children and older family members. The answer is not a cooler tank. It is a thermostatic mixing valve near the shower or basin, which blends hot and cold so the water comes out at a comfortable, safe temperature, usually around 40 to 45 °C, while the tank and pipes stay hot.

Many modern shower mixers are thermostatic already. For baths and basins used by children, a separate mixing valve is worth adding, placed close to the outlet so only a short length of pipe carries mixed, lukewarm water.

Homes left empty, jacuzzis and unused pipes

Still water is the biggest risk, which matters for holiday homes on the North Coast or in Ain Sokhna that stand empty for months. Before using the house again, heat the water heater or cylinder to 60 °C or above, then run every hot tap and shower for several minutes, keeping your face away from the spray, before anyone showers. Do the same with cold taps.

Clean and descale showerheads regularly, since scale inside them shelters bacteria. Jacuzzis and hot tubs need their filters and water treatment kept up, as warm water and spray together are an ideal mix for Legionella.

After a renovation, check for dead legs: lengths of pipe that once fed a removed basin or heater and now hold still water. They should be cut back to the main pipe, not just capped.

What temperature does to Legionella