Why temperature checks matter in a Legionella assessment

Legionella bacteria can grow in water systems where conditions allow it. Temperature is one of the main control measures used in ordinary hot and cold water systems.

In simple terms, hot water should be hot enough to control bacterial growth, and cold water should be kept cold enough to reduce the chance of bacteria multiplying.

Temperature checks are therefore a useful part of a Legionella risk assessment, especially for landlords, small businesses and simple workplace premises.

The key Legionella temperature figures

The most common figures you will see in Legionella guidance are:

• 60°C — stored hot water should normally be kept at 60°C or above.
• 50°C within 1 minute — hot water should reach at least 50°C at outlets within about one minute.
• 20°C or below — cold water should be kept below 20°C where possible.

HSE guidance on hot and cold water systems explains that hot water should be stored at least at 60°C and distributed so it reaches 50°C within 1 minute at outlets. Cold water systems should be maintained, where possible, below 20°C. Read the HSE hot and cold water systems guidance.

Hot water: what does “50°C within 1 minute” mean?

This check is about whether hot water reaches the expected temperature at the tap or outlet within a reasonable time.

To check it, run a hot tap and measure the water temperature once the water has warmed through. In most standard systems, the hot water should reach at least 50°C within about 1 minute.

A basic digital thermometer can be used for this. You do not usually need specialist equipment for a simple check, although more complex premises may need a competent professional to inspect and record the system properly.

If the hot water does not reach 50°C within about 1 minute, this should be recorded and investigated. It may indicate that the hot water temperature is too low, the outlet is too far from the heat source, or there is another issue with the system.

Stored hot water: why does 60°C matter?

If the property has stored hot water, such as a cylinder or calorifier, the stored water should normally be kept at 60°C or above.

This is different from checking the water at a tap. A cylinder may be set to store water at 60°C, while the outlet temperature is checked separately to confirm hot water reaches the taps properly.

Some simple properties, such as those with a combi boiler, may not have a stored hot water cylinder. In those cases, this part of the assessment may not apply.

Cold water: what does “20°C or below” mean?

Cold water should normally run cool after a short time. The common check is whether cold water reaches 20°C or below within about 2 minutes.

To check it, run a cold tap and measure the temperature after the water has run for a short time. If the cold water remains warm or tepid, this should be recorded.

Warm cold water can be a concern because Legionella bacteria are more likely to grow where water is warm, stagnant or poorly controlled.

Which tap should you check?

For a simple property, it is sensible to check outlets that give a useful indication of the system. This may include taps that are furthest from the boiler, cylinder or incoming cold water supply.

In larger or more complex buildings, temperature checks may need to be based on sentinel outlets and a proper monitoring schedule. If you are unsure how to select outlets in a complex system, you should use a competent professional.

HSE explains that larger buildings with more complex hot and cold water systems should consider the more detailed technical guidance. See HSE HSG274 technical guidance.

Do you need a specialist thermometer?

For a basic check in a simple property, a digital thermometer is usually enough to record a practical temperature reading.

The important thing is to record the result clearly, including:

• Which outlet was checked
• Whether it was hot or cold water
• The temperature reading
• How long the tap had been running
• The date of the check
• Any issue found

If the property is complex, has a stored water system, has vulnerable users, or has unusual pipework, a professional assessment may be more appropriate.

What should you record in a Legionella risk assessment?

A good Legionella assessment should record more than just the final answer. It should explain what was checked and whether anything needs attention.

For temperature checks, record:

• Whether hot water reaches at least 50°C within about 1 minute
• Whether cold water reaches 20°C or below within about 2 minutes
• Whether stored hot water is kept at 60°C or above, where applicable
• Any outlets that failed to reach the expected temperature
• Any limitations, such as areas that could not be accessed
• Any follow-up action needed

Our online Legionella risk assessment guides you through these checks and records the answers in a structured report.

What if the hot water does not reach 50°C?

If the hot water does not reach 50°C within about 1 minute, record the result and investigate why.

Possible reasons include:

• The boiler or cylinder temperature is set too low
• The outlet is a long way from the hot water source
• Pipework is poorly insulated
• The outlet is little used
• There is a problem with the hot water system
• A thermostatic mixing valve may be affecting the outlet temperature

If you are not sure why the reading is low, or if the system is complex, get competent advice.

What if the cold water is above 20°C?

If cold water remains above 20°C after running the tap, record it as an issue to review.

Possible causes include:

• Cold pipes running close to hot pipes
• Poor insulation
• Warm storage areas, lofts or cupboards
• Stagnant water in little-used pipework
• A cold water storage tank becoming too warm

The right action depends on the property and the water system. In some cases, improving insulation, removing redundant pipework, flushing little-used outlets or reviewing storage arrangements may be needed.

Temperature checks are not the whole assessment

Temperature is important, but it is only one part of a Legionella risk assessment.

A complete assessment should also consider:

• Whether there is a cold water storage tank
• Whether there are dead legs or redundant pipework
• Whether showers or spray outlets are present
• Whether outlets are rarely used
• Whether showers are cleaned and descaled
• Whether occupants know what to do after periods of non-use
• Whether there are any vulnerable users
• Whether the system is simple or more complex

HSE explains that employers and those in control of premises must assess and manage sources of Legionella risk. Read HSE Legionella guidance.

When should you get professional advice?

A simple guided assessment may be suitable for many straightforward rented properties, small workplaces and domestic-style water systems.

You should consider using a competent professional if:

• The building has a complex hot or cold water system
• There is a calorifier, large storage system or secondary hot water return
• The property is healthcare, care, supported living or similar
• There is a spa pool, hot tub, swimming pool or hydrotherapy pool
• There are cooling towers or evaporative condensers
• You cannot understand or access the water system
• Temperature readings are repeatedly outside the expected range
• There are vulnerable users and the controls are uncertain

If the system is outside your knowledge, do not guess. Get competent advice.

Complete your Legionella assessment online

If you are assessing a simple property or small workplace, our guided tool can help you record the key Legionella checks, including hot and cold water temperatures, showers, little-used outlets and basic management controls.

You can start an online Legionella risk assessment here.

Frequently asked questions

What temperature should hot water reach for Legionella control?

Hot water should normally reach at least 50°C within about 1 minute at outlets. Stored hot water should usually be kept at 60°C or above.

What temperature should cold water be for Legionella control?

Cold water should be kept below 20°C where possible. If cold water remains warm or tepid after running the tap, record it and review the possible cause.

Do I need a thermometer for a Legionella risk assessment?

A basic digital thermometer can be used for simple temperature checks. For complex buildings or specialist water systems, professional monitoring may be needed.

Which outlet should I test?

For a simple property, it is sensible to check outlets that represent the system, such as those furthest from the hot water source or incoming cold water supply. Larger or more complex buildings may need a formal monitoring approach using sentinel outlets.

What should I do if my readings are outside the expected range?

Record the reading, note the outlet checked, and investigate the cause. If you are unsure, or if the problem continues, get competent professional advice.