Legionella Water Temperature Guide UK

Q: Can Legionella survive in cold water?

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Q: Can boiling water kill Legionella?

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Q: What thermometer is used for Legionella testing?

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Q: Do Legionella thermometers need calibrating?

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Q: How often should Legionella temperatures be checked?

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What is Legionella and what temperature does it grow at?

Legionella bacteria grow in water, particularly at temperatures between 20 °C and 45 °C. Inhaling contaminated water droplets can cause Legionnaires’ disease, a potentially serious form of pneumonia.

Controlling water temperature is an important part of Legionella risk management. In UK water systems, hot water should typically be stored at 60 °C or above, while cold water should be kept below 20 °C.

Businesses, landlords and those responsible for water systems must assess and control the risk of Legionella, including monitoring water temperatures where appropriate.

Legionella temperature requirements at a glance

Legionella growth range: 20 °C – 45 °C

Legionella becomes dormant: Below 20 °C

Stored hot water: 60 °C or above

Hot water outlets: 50 °C or above before TMV

Legionella dies: 70 °C

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

Below 20 °C, the bacteria become dormant, but don't die. If temperatures rise, the bacteria will become active again.

At What Temperatures Does Legionella Grow and Die?

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

Legionella bacteria die almost instantly at 70 °C.

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

Below 20 °C, the bacteria become dormant, but don'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 requirements in the UK

In the UK, Legionella control is guided by the Health and Safety Executive’s Approved Code of Practice (ACOP) L8 and HSG274 guidance. These documents outline recommended control measures for managing Legionella risks in water systems.

Who is responsible for Legionella risk assessments in the UK?

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 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
Reducing dead legs (pipes where water doesn't circulate or flow regularly)

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 you 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)

Frequently Asked Questions

Can Legionella survive in cold water?

Yes. Legionella bacteria become dormant below 20 °C but can become active again if temperatures increase.

Can boiling water kill Legionella?

Yes. Legionella is killed at high temperatures, with almost instant death occurring at around 70 °C.

What thermometer is used for Legionella testing?

A calibrated digital thermometer with suitable penetration and surface probes is commonly used for monitoring water temperatures in Legionella risk assessments.

Do Legionella thermometers need calibrating?

Yes. Regularly checking the accuracy of your thermometers and probes ensures your equipment is working correctly and your risk assessments are carried out correctly.

You can send your thermometers and probes to our UKAS-accredited laboratory for recalibration to maintain accuracy.

How often should Legionella temperatures be checked?

The frequency depends on the type of water system and risk assessment requirements. Regular monitoring should be carried out according to HSE guidance.