Why these controls
For a small building with a simple hot and cold water system, HSE expects a simple assessment. If it shows the risks are low and properly managed, the assessment is complete; what matters after that is doing the checks and writing them down.
Temperature is the main control. Hot water stored at 60°C and reaching the taps at 50°C within a minute, and cold water kept below 20°C, keeps the water out of the range where the bacteria grow. The monthly checks at the nearest and furthest outlets show whether that is still true.
The shower is the weak point. It is used by two cyclists in summer and almost nobody in winter, so the water in its pipe stands for days and the spray makes exactly the fine droplets that carry the bacteria. It is flushed every week and the head is descaled every quarter.
Who has to do it
Employers, and anyone in control of premises, must identify and assess the risk from legionella, prevent or control it, keep records, and review what they do. That includes landlords who rent out a property, or even a room. The law is the Health and Safety at Work etc. Act 1974, the Management of Health and Safety at Work Regulations 1999 and COSHH; HSE's Approved Code of Practice L8 says how to meet it.
For landlords of homes, HSE says the duty does not require an in-depth assessment: the risk from most domestic hot and cold systems is low, because the water is used and replaced every day.
What the assessment should include
- Who is responsible for the water system, and the name of the competent person if you use one
- A description of the system: where the water is stored and heated, and which outlets it feeds. A simple sketch is enough for a small building
- Where the risks are: water between 20 and 45°C, water that is stored or stands in pipes, rust, sludge and scale, and outlets that make spray
- Who could be at higher risk
- The controls in place, the checks that show they are working, and how often
- How the checks are recorded, and when the assessment will be reviewed
Routine checks for a simple hot and cold water system
These are the checks HSE lists for hot and cold water systems. Sentinel outlets are the taps nearest to and furthest from the cylinder and the tank. Use a digital thermometer with a probe, and write every result down.
| Check | How often | What you are looking for |
|---|---|---|
| Hot water at the sentinel outlets | Monthly | At least 50°C within one minute of running |
| Cold water at the sentinel outlets | Monthly | Below 20°C after running for two minutes |
| Hot water leaving the cylinder or calorifier | When the sentinel outlets are checked | At least 60°C |
| Hot supply to thermostatic mixing valves, where fitted | Monthly | At least 50°C within one minute |
| Cold water tank, at the ball valve outlet | Every six months | Below 20°C |
| Cold water tank and hot water cylinder | At least annually | Clean, lid and vent in place, no sludge, scale or rust |
| Taps and showers not used every week | Weekly | Flushed through to drain without releasing spray |
| Showerheads and hoses | Quarterly, or as the assessment decides | Dismantled, cleaned and descaled |
Records
If you have five or more employees, record the significant findings of the assessment. Keep a log of who is responsible, the control scheme, whether the system is in use, and the results and dates of every check. Monitoring results should be kept for at least five years; other records for as long as they are current and two years after.
When you need specialist help
Get a competent person if you do not have the skills to assess the system yourself, if the checks keep failing, or if the building is larger or more complex. Cooling towers, evaporative condensers and spa pools carry a much higher risk and need specialist control and routine testing. In healthcare premises hot water should reach 55°C at the outlet, and where vulnerable people use the water, thermostatic mixing valves stop it leaving the tap above 44°C to prevent scalding.
Sources: HSE, Legionnaires' disease: what you must do; HSE, Hot and cold water systems: things to consider; HSE, Legionnaires' disease: monitoring and keeping records; HSE, Testing and monitoring your water system for legionella; HSE, Legionella and landlords' responsibilities; HSE L8, Legionnaires' disease: the control of legionella bacteria in water systems.
Hazard descriptions and controls follow HSE guidance: HSE, Hot and cold water systems: things to consider; HSE, Hot and cold water systems; HSE, Legionnaires' disease: monitoring and keeping records. Checked 20 October 2026.