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Legionella In Hot and Cold Water Systems: How to Manage the Risk

Legionella bacteria can survive and multiply in both hot and cold water systems given the right conditions, so it’s vitally important that water systems are managed properly and inspected regularly to minimise the risks of Legionnaires disease.

Managing Legionella In Water

What is Legionella?

The Legionella bacteria is a naturally occurring bacteria present in the soil, which is washed into natural water sources such as rivers, lakes and reservoirs. These bacteria can find their way into domestic hot and cold water systems, but at very low levels, giving a very low risk to the end users. However given the right conditions the bacteria can multiply to levels that can be dangerous, where the bacteria are inhaled, which can cause illnesses. Legionnaires disease, a severe form of pneumonia, caused by the Legionella pneumophila bacteria, can be fatal. Anyone can develop Legionnaires’ disease, but those, particularly at risk, include the elderly, smokers, and people suffering from chronic respiratory or kidney disease.

Outbreaks of the illness occur from exposure to legionella bacteria in man-made water systems where conditions have allowed the bacteria to proliferate to dangerous levels. Some systems associated with outbreaks are cooling towers, evaporative condensers, hot and cold water systems and spa pools.

Can you see Legionella?

No, Legionella isn’t visible to the naked human eye, nor are the water droplets, called aerosol, that carry the bacteria through the air.

How It Spreads and Grows

Once the levels of Legionella bacteria have reached a dangerous level, in a water system the bacteria is then spread through the air in tiny water droplets, and if breathed in, these droplets can cause Legionnaires disease. Water temperatures between 20°C and 45°C allow for the bacteria to grow, therefore, if using temperature as a control,  it is essential that your water system is monitored for water temperature so that this temperature range can be avoided. 

Legionella In Hot and Cold Water Systems

How can water supply systems become contaminated with Legionella?

All man-made hot and cold water systems are likely to provide an environment where Legionella can grow. Water systems can become contaminated with Legionella when proper maintenance isn’t carried out. Legionella grows best within a certain temperature range, which makes temperature control the primary method used to control the risk of Legionella. It’s important that water systems are designed and maintained to avoid reaching temperatures that encourage the growth of Legionella. All water systems should operate at temperatures that prevent the growth of Legionella, so hot water systems should be kept hot, and cold water systems should be kept cold!

Stagnant water systems assist the growth of Legionella, so it’s vital to ensure your water system is designed so the water within the system is regularly turned over, ideally the entire system should be turned over every 24 hours. 

Hot Water Systems

Legionella bacteria stop growing at 45°C and will start to die off at 50°C within a couple of hours and die at temperatures above 60ºC within minutes, so it’s important that hot water systems are kept hot –  the recommended temperature for stored hot water is 60ºC. Hot water storage vessels (Calorifiers) should be internally inspected, if possible, and purged to drain to remove any buildup of debris at least once a year. 

Cold Water Systems

Temperatures below 20ºC will cause the Legionella bacterium to become dormant, but the bacteria will not die. Cold water storage tanks should be suitably sized so that the entire contents are turned over every 24 hours during normal use. The tank should be installed so the water has to flow the entire length of the tank from the inlet to the outlet, not allowing any stagnation. The tank should have a tight-fitting lid and all overflow and vents should be screened so as not to allow contamination by insects and vermin. The tank should be insulated to reduce the thermal gain during the hot summer months and should not allow the water to reach temperatures greater than 20°C. The tank should be internally inspected cleaned and disinfected as and when required and the temperature should be monitored at least twice a year, once in the summer and once in the winter,

How long does legionella take to develop in the water?

Under the right conditions, low levels of Legionella bacteria can reach dangerous levels within days, so it’s vitally important that you manage and monitor your water systems regularly. 

Acceptable levels of legionella in the water

There is no ‘safe level’ of Legionella in water, and there have been cases of Legionnaires disease linked to very low levels of Legionella. The HSE provides guidance on what actions to take if Legionella is found in the water system. The HSG274 part 2 states at levels >1000 cfu/l  The system should be resampled and an immediate review of the control measures and risk assessment carried out to identify any remedial actions, including possible disinfection of the system. Retesting should take place a few days after disinfection and at frequent intervals afterwards until a satisfactory level of control is achieved. 

Testing For Legionella In Water Systems

How do I test for legionella in water?

Where monitoring for Legionella is considered appropriate, the sampling method should be carried out in accordance with BS7592 and the biocide, if used, neutralised where possible. Water samples should be tested by a UKAS-accredited laboratory that takes part in a water microbiology proficiency testing scheme such as that run by Public Health England. The laboratory should also apply a minimum theoretical mathematical detection limit of <= 100 Legionella bacteria per litre of sample for culture-based methods.

How often should I test water for Legionella?

This is dependent on the system that you have and the outcome of your risk assessment. For open systems, such as cooling towers, evaporative condensers and spa pools etc, routine testing should be carried out at least quarterly. However, there may be circumstances where more frequent sampling may be required.

For hot and cold water systems, which are generally enclosed, i.e. not open to the elements and significant contamination in the same way as cooling towers, microbiological monitoring is not usually required. However, there may be circumstances where Legionella risk assessment is necessary, for example where there is doubt about the efficacy of the control regime or where recommended temperatures or disinfection concentrations are not being consistently achieved.

Further reading: How Often Should Legionella Testing Be Done [Answered]

Who can be appointed to test or monitor Legionella?

Testing of water quality may be carried out by a service provider, such as a water treatment company or consultant or by the operator, provided they are trained to do so and are properly supervised.

How do I interpret Legionella test results?

When it comes to interpreting your test results, you should consider what they mean in the context of your water system as your subsequent specific actions will depend on your risk assessment. Here at Franks Portlock we can interpret the results and provide you with the best course of action to not only rectify the immediate issue but what to do to prevent a recurrence.