Legionella in hot and cold water systems

Legionnaires' disease is potentially fatal pneumonia caused by legionella bacteria. Infection is caused by breathing in small droplets of water contaminated by the bacteria. One person cannot pass Legionnaires' disease onto another person.

Where is Legionella found?

Legionella are widespread in the environment in water and may grow in hot and cold water systems. They survive low temperatures and thrive at temperatures between 20°C and 45°C if a supply of nutrients is present. The bacteria can be a risk if they are dispersed in water droplets such as from showers, spa baths and wet based air conditioning systems.

What are my legal duties?

Under health and safety law you have a duty to consider the risks from legionella that may affect your staff or members of the public and take suitable precautions to control the risks. You must:

  • Identify and assess the source of risks.
  • Prepare a course of action for preventing or controlling the risks.
  • Implement the action (and appoint a person to be responsible for implementing the action).
  • Keep records of your actions.

The notification of Cooling Towers and Evaporative Condenser Regulations 1992 require the notification of wet cooling towers and evaporative condensers to the local authority in whose area the equipment is situated. Notification is to the local authority regardless of whether it is the LA or HSE that is the enforcing authority for the premises concerned.

Identification and assessment of risk

  • Are conditions present, which will encourage bacteria to multiply? Is the water temperature between 20°C and 45°C?
  • Are water droplets produced eg from showers, spa baths, water misting systems, fountains and water features?
  • Can sediment, sludge, scale, organic material or biofilms build up in the water system? Is the full water system used regularly? (more than once per week)
  • Are there any outlets which are not regularly used? (Less than once a week, not including emergency showers).
  • Are there any areas of redundant pipework which will allow water to stagnate?

Preventing or controlling the risks

If a risk is identified which cannot be prevented, proper controls must be introduced.

Hot water

Water in the boiler should be kept at a minimum of 60°C and water at each outlet point should be above 50°C* within a minute of running the water.
*Separate guidance is available for residential and nursing homes.

Cold water

Water should be less than 20°C AND at each outlet point should be below 20°C within 2 minutes of running the water.


Shower heads and hoses should be dismantled, cleaned and descaled at least quarterly.

Units not regularly used

Any units not regularly used must be flushed through and purged to drain without release of aerosols, eg remove shower heads at least weekly or immediately before use.

Cold water tanks

Ensure that the tank is insulated with a closed lid.

Design of water systems:

  • Insulate hot water pipes.
  • Remove any redundant pipework.
  • Ensure that length of branch pipework serving equipment and appliances is as short as possible.
  • Individual Thermostatic Mixer Valves (TMVs) should be as near the hot water outlet as possible.
  • Hot water vessels should have a valve to remove accumulated sludge at the base of the tank, for annual removal of any build up.
  • Multiple, linked storage tanks should be avoided because of operational difficulties, due to possible unequal flow rates and possible stagnation.
  • Minimise the volume of cold water tanks.
  • Avoid fibre washers, hemp, natural rubber, linseed oil-based compound jointing.

Monitoring of Controls

Hot and cold water temperatures

Monthly monitoring

  • Check that the temperature of hot water is at least 50°C within 1 minute of running the .water at the first and last tap on the run.
  • Check that the temperature of the cold water is below 20°C at the first and last taps from the storage after running the tap for 2 minutes.
  • Check the surface temperature of pipework entering any thermostatic mixer valves is at least 50°C.
  • Check the outgoing water from the hot water cylinder is at least 60°c and incoming water is at least 50°C (some tanks have thermometer pockets for this purpose).

6 monthly monitoring

Check that the incoming cold water is below 20°C.

Annually monitoring

On a rotational basis check that at the taps:

  • the cold water temperature is below 20°C after running the water for 2 minutes;
  • the hot water temperature is at least 50°C within 1 minute of running the water.

General monitoring

Annually monitoring

Visually inspect the cold water tank. Ensure the tank is insulated and that there is a closed lid. Check for debris and if necessary, clean and disinfect at taps.

Check that all other control measures remain effective.

Record precautions and results of monitoring.

Commissioning and recommissioning - including after a business shutdown

If a hot water system feeding showers etc is newly installed or has been out of use for over 1 week, the water should be brought up to 60°C for 1 hour before being used. This should kill any legionella bacteria. Alternatively, a chemical biocide can be added for at least 3 hours then the system flushed through before use. This must be done by a competent person.

Cold water systems should be purged to drain without releasing any aerosols for example: run taps slowly, flush toilets, remove showers heads and run water. Remember Health and Safety controls. The person doing these tasks should not breathe in the aerosol. Personal Protective Equipment may be used to protect persons. Rooms should be well ventilated.

Further advice on Legionella

Go to Legionnaires' disease on the HSE website and download the Legionnaires' disease - A brief guide for duty holders Leaflet INDG458, published April 2012.

Go to Legionnaires' disease. The control of legionella bacteria in water systems on the HSE website to download the Approved Code of Practice and guidance L8 (Fourth edition) Published 2013.

Go to Health and safety in care homes on the HSE website to download the Health and Safety in care homes the leaflet HSG220.

Go to Legionnaires' disease technical guidance on the HSE website to download Legionnaires’ disease Part 2: The control of legionella bacteria in hot and cold water systems' Leaflet HSG274 Part 2 published 2014.

Go to resources on the CIEH (Chartered Institute of Environmental Health) website to download a the Legionnaire’s disease: lockdown risks and reopening safely document.

Guidance for organisations on supplying safe water supplies on GOV.UK to download Public Health England's Ensuring safe water supplies for businesses following COVID-19 lockdown published on the 14 May 2020.

Contact Environmental Health on the details below if you require any further information or advice.

Last updated: Thursday, 7 April, 2022.