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How to manage Legionella bacteria in hot and cold water systems

By: Jon Greaves on Feb 23, 2021

Legionella bacteria can easily proliferate in hot and cold water systems where conditions are right and appropriate controls are not in place. Employers and landlords responsible for building management have a legal obligation to control this risk, through proper assessment, planning and prevention measures. 

Legionnaires’ disease is a dangerous condition spread by inhaling microscopic droplets of water contaminated with the legionella bacterium. The bacteria lives in water, thrives in temperatures between 20 oC and 45 oC and grows fast where stagnation is able to occur and biofilm allowed to build up. The disease can be deadly for anyone, but it is particularly dangerous for those in high-risk groups such as the sick and elderly.

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So, how to do you manage and treat your hot and cold water systems in order to prevent its growth and proliferation?

How to manage systems and treat water for the legionella bacterium

The range of measures that can and should be taken include:

Temperature control

Legionella growth is typically controlled in water systems through consistent temperature control.

  • Hot water storage cylinders should store water at 60°C or higher
  • Hot water should be distributed at 50°C or (55°C in healthcare environments)
  • Cold water should be stored and distributed below 20°C.
  • Competent individuals should check the hot water storage cylinder temperatures every month and cold water tank temperatures at least every six months to ensure the safety of users.
  • Sentinel outlets, those closest and furthest away from each tank or cylinder should be checked monthly for their distribution temperatures. A competent person should routinely check, inspect and clean the system, in line with the risk assessment.

Design and maintenance measures

Those designing and maintaining systems should seek to minimise Legionella growth, by:

  • Keeping pipe work as short and direct as possible;
  • Adequately insulating pipes and tanks
  • Using materials that do not promote the growth of Legionella (avoid fibre washers, hemp, natural rubber, linseed oil-based compound jointing).
  • Preventing contamination by fitting tanks with lids and insect screens
  • Hot water vessels should have a valve to remove accumulated sludge at the base of the tank, for annual removal of build up.
  • Avoid multiple linked storage tanks because of potential operational difficulties, possible unequal flow rates and resulting stagnation.

Stagnation control

Water stagnates when it stands unused for sustained periods and this encourages the growth of legionella bacteria.  To prevent this it is recommended those managing water systems:

  • Remove dead legs and dead ends in pipe-work.
  • Flush out infrequently used outlets (especially showerheads and taps) at least weekly
  • Clean and de-scale shower heads and hoses every quarter
  • Cold-water storage tanks should be cleaned periodically and water should be drained from hot water cylinders to remove debris or signs of corrosion.

Legionella water treatment

Depending on the level of risk of bacteria growth and the vulnerability of those using the building, some premises may require additional or alternative risk controls for their hot and cold water systems. These include treating water with:

  • Copper and silver ionisation,
  • UV radiation,
  • Chlorine dioxide,
  • Ozone

In these cases there are specific regulatory requirements for maintaining treatment regimes, including weekly, monthly and annual testing to prevent over or underdosing.

The requirements for documenting, monitoring and controlling the risk of legionella growth in hot and cold water systems can be extensive and onerous. Individuals responsible for these duties need the right training to ensure watertight and demonstrable compliance.

What are my legal duties?

Employers, landlords and others with responsibility for managing buildings are required by the Health and Safety at Work Act 1974 to carry out legionella risk assessment and depending on the outcome of that assessment, manage hot and cold water systems in order to control the growth of the legionella bacterium.  

The size of your premises, the complexity of your water systems and the vulnerability of the people who use them, will all dictate the extent of the control measures you need to take.

For example, to fulfil these duties in a care home settings, a water safety plan tailored to your specific water system is essential. This plan should include risk assessment, monitoring (regular testing of water samples for Legionella), and control measures such as temperature management, biocide treatments, and physical cleaning

What technical guidelines do you need to follow?

The various assessments and controls required for different kinds of buildings’ hot and cold water systems are detailed in the following technical guidelines:

The code of practice for legionella control:

L8 ACOP Legionnaires’ disease: The control of Legionella bacteria in water systems

Guidelines for hotels, hospitals and other large premises:

HSG274 Part 2 Legionnaires’ disease: The control of Legionella bacteria in hot and cold water systems

Guidelines for those running care homes and the like:

Control of Legionella in hot and cold water systems in care services / settings using temperature


Preventing and treating Legionella in water systems is a critical aspect of building maintenance and public health. Regular maintenance, proper water treatment, and a comprehensive water safety plan are key to keeping Legionella at bay. By understanding the risks and implementing these strategies, we can significantly reduce the likelihood of Legionnaires' disease outbreaks.

Remember, while these guidelines provide a good starting point, it's essential to consult with water treatment professionals and adhere to local regulations and guidelines for managing Legionella risks in your specific context. Stay informed, stay vigilant, and keep your water systems safe.



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Topics: Legionella Control

Jon Greaves

Written by Jon Greaves

Jon has progressively worked through operational roles, account management, technical management, and senior management roles over the last 16 years within one of the group companies before moving into the role of Water and Air Managing Director. Jon has experience across multiple sectors of water and air compliance, including district energy networks; data centres; healthcare; food and beverage and facilities management. Jon acted as a corresponding steering committee member on CIBSE CP1 – Heat Networks Code of Practice for the UK released in 2020.

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