Controlling Legionella in Hot & Cold Water

This article is about the control of legionella in hot and cold water systems, an essential task if you are to protect the safety and health of people who may use such systems or come in to contact with them.

Most people have heard of Legionnaires’ disease, a serious form of pneumonia caused by the presence of Legionella bacteria in water. In order to reduce the risk of people being affected by this disease, water systems need to be kept in good condition to limit the number of bacteria present and prevent, or at least control the growth of any dangerous bacteria in the water.

What is Legionnaires’ disease?

Legionella bacteria can live in both hot and cold water systems, so it’s vitally important that these are managed properly and inspected regularly to minimise the risks to people.

Controlling legionella in hot and cold water systems

Here are some of the practical things you need to think about when it comes to controlling legionella in your hot and cold water systems:

Temperature control
Thermal disinfection of water is one of the primary methods for controlling legionella in hot and cold water systems.
Legionella bacteria thrives at temperatures between 20 ^oC and 45 ^oC. Below 20 ^oC, the bacteria won’t die but will remain dormant. If the temperature is over 50 ^oC, this will kill the bacteria.
In hot water systems, water should be supplied at 60 ^oC or more so that any bacteria are killed.
If the system is one with re-circulating hot water, then it’s important to make sure that the hot water never comes out of the taps at less than 50 ^oC (55 ^oC in healthcare environments), and the cold water is always below 20 ^oC.
Monitoring your water systems
Whoever in your company has responsibility for legionella control (usually called the legionella responsible person) should ensure the water systems are managed properly, control measures checked and inspected regularly and the systems kept clean and under microbial control.
It’s the company’s responsibility to ensure the responsible person is competent to do their job and this may involve specialist training. They should also have the authority to take any actions they deem necessary to manage the risks from legionella.
- Training resources for the responsible person and duty holder
When monitoring water temperatures, the hot and cold water outlets which are closest or furthest to the mains water tank or heater are the most important to watch – these are known as the sentinel outlets. Monitor at these points to make sure temperatures are kept within an acceptable range.
What to do with showers
It’s important that you only fit showers in areas where they will be used regularly, at least once a week.
If showers aren’t being used regularly, then they should be flushed through (at least once per week) to eliminate the risks of stagnant water and the growth of bacteria. It’s also good practice to have a programme for dismantling, cleaning and descaling all shower heads at least quarterly, as this will help to prevent bacterial growth.
The problem of stagnant water
Water stagnates when it stands unused for prolonged periods. Stagnating water, especially when the temperatures are between 20 ^oC and 45 ^oC can create ideal conditions for legionella and other bacteria to start multiplying rapidly. Stagnation is more likely in dead legs and dead ends of pipes where outlets have been removed but the pipework left in place, or in places where water isn’t used often.
Large volume water storage tanks are also a risk factor as water flow and circulation within the tank is often reduced which can lead to stagnation.
A formal legionella risk assessment should be undertaken to identify areas of concern, and then a plan developed to tackle identified risks to prevent legionella growth.
- Learn more about legionella risk assessments
Water sampling & legionella testing
The legionella risk assessment should set out how often water samples should be taken and analysed. This could be done at least annually as an additional control measure. It is particularly important where areas of stagnant water have been identified in the water system.
- Learn more about testing water for legionella

In very simple terms you should ensure that you keep your cold water cold, your hot water hot… and keep it moving!

Controlling legionella and your legal responsibilities

Cases of legionella contamination and Legionnaires’ disease infection can mean a full investigation by the Health and Safety Executive is carried out. This can affect not only those businesses suspected of causing the infection but also others close by. If companies are found to have been lax in their legionella control processes and procedures they could face prosecution under health and safety law. It’s therefore essential that everyone with responsibility for water systems is fully aware of their responsibilities, they identify any risks by carrying out a legionella risk assessment, they implement suitable control measures to manage the risks and monitor their effectiveness to ensure people are protected.

Learn more about legionella risk assessments

Legionella control specialists

Water Treatment Services offer a comprehensive range of legionella control and environmental solutions for the control of Legionella bacteria including legionella risk assessment, training and water testing for the control of legionella, pseudomonas and other waterborne pathogens in the workplace.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained engineers, in-field water treatment service specialists and technicians we can offer cost effective legionella risk management, ACOP L8 compliance and engineering support services throughout the UK and Ireland.