03/07/2025

Legionella Water Temperature Guide

Legionella risk assessments are mandatory for UK businesses. Building owners, landlords and facilities managers must be aware of the temperature ranges for compliance and regularly monitor their systems for risks.

Read on for everything you need to know about legionella temperatures and monitoring water systems.

What is Legionella?

Legionella is a type of bacteria that grows in warm, stagnant water. Inhaling droplets of water contaminated with the bacteria can cause Legionnaires’ disease—an uncommon but serious type of pneumonia. It is most commonly passed on from sources like showers, air conditioning and hot tubs.

What Temperature Does Legionella Grow?

Legionella bacteria thrive between 20 and 45 °C. At these temperatures, the bacteria can infest entire water systems in just a couple of days.

What Temperature Kills Legionella?

Legionella bacteria will die instantly at 70 °C.

At 60 °C, 90% of the bacteria will die on two minutes. At 50 °C, 90% will die in two hours.

Below 20 °C, the bacteria becomes dormant, but doesn’t die. If temperatures rise, the bacteria will become active again.

Temperatures for Legionella Prevention

Temperature control is the primary method for reducing the risk of legionella growth.

Businesses should maintain hot and cold water systems at the following temperatures:

Hot water tanks (also called calorifiers) should be kept at 60 °C or hotter
Hot water at taps and showers should reach at least 50 °C*
Cold water should be kept below 20 °C

*If thermostatic mixing valves (TMVs) are fitted, the temperature at the tap or shower will usually be limited to around 44 °C to prevent scalding. The 50 °C target applies to the water temperature before it reaches the TMV.

Legionella Temperature Chart

View and download our temperature chart to keep your legionella risk assessment temperatures on hand.

Legionella water temperature chart — *Download the chart*.

How to Prevent Legionella?

UK businesses are legally required to control the risk of legionella bacteria in their water systems. They should do this by:

Keeping a risk assessment that is regularly reviewed
Monitoring and recording water temperatures at tanks, pipes and outlets
Flushing unused outlets weekly
Avoiding storing water for long periods
Regularly cleaning water storage tanks, shower heads, hot tubs etc
Reduce dead legs (pipes where water doesn’t circulate or flow regularly)

Who is Responsible?

The duty holder is responsible. This could be:

The employer
A building owner
A landlord
Someone in control of the premises (e.g. a facilities manager)

How Long to Run Taps to Prevent Legionella?

You should flush a water system if it has not been in use for over a week. If a property is going to be empty for some time, you should ensure to flush the hot water system weekly.

Start with cold water:

Run all cold water outlets for at least five minutes. The water should fall below 20 °C within two minutes.

Then, check hot water:

Run all hot water outlets for at least five minutes. Turn showers up as hot as they will go—they should reach at least 50 °C within one minute.

Make sure not to breathe any water droplets in during flushing.

How to Measure Water Temperatures

Cold Water Outlets (taps, showers)

Run the tap and measure the temperature of the water
It should fall below 20 °C within two minutes
Best thermometer/probe type: Penetration probe (dip directly into the running water)

Hot Water Outlets (taps, showers)

Run the hot tap fully and measure the water temperature
It should reach at least 50 °C within one minute
Best thermometer/probe type: Penetration probe (dip directly into the running water)

Hot Water Cylinders (calorifiers—flow and return pipes)

Check the temperature of the water entering and leaving the cylinder
Stored water should be above 60 °C, return pipes above 50 °C
Best thermometer/probe type: Surface probe (press onto the pipework)

Cold Water Storage Tanks

Measure the temperature of the stored water itself
The water should be below 20 °C
Best thermometer/probe type: Penetration probe (insert into the water, ideally near the middle)

Shop Legionella Thermometers and Kits.

Summary

All UK businesses must have a legionella bacteria risk assessment. Temperature control is a key part of this, as the bacteria thrives is warm, stagnant water.

The legionella temperature range is between 20 and 45 °C. At these temperatures, the bacteria is active and can cause Legionnaires’ disease if droplets of the water are inhaled.

Duty holders must regularly record their water system temperatures, ensuring they comply with the recommended temperatures:

Cold water outlets and storage: 20 °C or below
Hot water outlets: 50 °C or above
Hot water storage: 60 °C or above

Businesses should flush water systems weekly when they are not in use. They should also take other steps to keep the systems clean and running regularly—avoiding stagnant water.