How to prevent legionella in a water system
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Key points
- Prevention means controlling the conditions for growth, not eliminating legionella. It is naturally present in water and cannot be permanently removed from a live system.
- Four practical levers do the work: temperature (hot stored at 60°C, delivered at 50°C, cold below 20°C), movement (flush unused outlets weekly), cleanliness (descale showerheads, clean tanks) and design (remove dead legs).
- Legionella multiplies between roughly 20°C and 45°C. Temperature control keeps the water out of that band and is the single measure that prevents the most growth.
- No single measure prevents legionella on its own. Storing water at 60°C does not help if a warm, stagnant dead leg seeds the outlet next to it.
- Under ACOP L8 and HSG274, prevention must be assessed, written into a scheme of control, and recorded. Prevention you cannot evidence is not prevention you can prove.
Prevent legionella by denying the bacteria the conditions to multiply in your water system: keep hot water hot (stored at 60°C), cold water cold (below 20°C), water moving through every outlet, and pipework clean and free of stagnant dead legs.
Everything below is about the water system, not the illness. Legionella lives naturally in water, so you cannot remove it from a live system for good. Prevention is the continuous work of keeping the system in conditions the bacteria cannot multiply in, and being able to show you have done so.
The four levers that prevent legionella
Legionella multiplies when three things line up: water that sits still, a temperature between roughly 20°C and 45°C, and a biofilm on the pipe or tank wall that feeds and shelters it. Prevention works by removing one or more of those conditions everywhere in the system. In practice that comes down to four levers.
- Temperature. Keep hot water hot and cold water cold, so the water spends no time in the 20°C to 45°C growth range.
- Movement. Keep water turning over. Flush outlets used less than weekly so nothing stagnates.
- Cleanliness. Descale and disinfect showerheads and taps, and keep storage tanks clean, so biofilm and sediment have nowhere to build.
- Design. Remove dead legs and redundant pipework where water can pool.
No single lever prevents legionella on its own, which is the mistake that catches people out. You can store water at 60°C and still seed an outlet from a warm, stagnant branch a metre away. Prevention is the whole set, applied across the whole system, not one control done well.
Temperature: the control that does most of the work
Temperature is the primary control because legionella cannot survive genuinely hot water and cannot multiply in genuinely cold water. Under HSE guidance in HSG274 Part 2 and the ACOP L8 code, the targets that keep the water either side of the growth range are set out below.
The awkward part is that the same hot water that kills legionella can scald, which is why thermostatic mixing valves blend it down at the point of use. That does not change the storage and distribution targets: the water is kept hot right up to the valve, then mixed at the tap. Getting this balance right is the core of legionella temperature control, and it is the measure that prevents the most growth for the least effort.
| Part of the system | Target temperature | Why it matters |
|---|---|---|
| Stored hot water | 60°C | Hot enough to kill legionella in the calorifier |
| Hot water at the outlet | 50°C within one minute (55°C in healthcare) | Above the growth range, so legionella cannot multiply at the tap |
| Cold water | Below 20°C | Below the temperature at which legionella multiplies |
| Growth range to avoid | 20°C to 45°C | The band where legionella multiplies fastest |
| Sampling action levels | 100 and 1,000 CFU/litre | In hot and cold water systems, 100 CFU/litre prompts a review and resample, 1,000 CFU/litre prompts immediate action |
HSE HSG274 Part 2 and Approved Code of Practice L8
Keep the water moving
Still water is what turns a low risk into a growing one. Water that moves through an outlet regularly does not give legionella time to establish; water that sits does. The prevention job here is to stop stagnation wherever the building's normal use does not already do it.
- Flush infrequently used outlets. Any tap or shower used less than about once a week should be run until the water reaches the system temperature (hot to 50°C, cold below 20°C), which usually takes a few minutes. HSE guidance sets weekly as the usual minimum.
- Remove dead legs. Redundant pipework holds standing water that no amount of flushing downstream will reach. The fix is removal back to the live pipe, not a cap near the outlet.
- Do not overstock storage. Cold tanks and calorifiers should be sized so the water turns over in roughly a day, rather than holding far more than the building draws.
Long voids make this worse. After a building or a wing has stood empty, outlets should be flushed back into use before anyone is exposed to the spray, and the flushing set out in a monitoring schedule so it is not left to memory.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Keep the system clean
Legionella does not float freely in clean water so much as it shelters in biofilm, scale and sediment. Cleaning removes the habitat.
- Showerheads and spray taps. These are the highest-risk outlets, because they produce the fine aerosol that carries legionella into the lungs, and their nozzles collect scale and biofilm. Descale and disinfect them on a set schedule.
- Cold water storage tanks. Keep them lidded, insulated and free of debris, rust and sediment. Inspect them, and clean them when the inspection shows they need it.
- Biofilm. The slime layer on pipe and tank walls protects legionella from both heat and disinfectant, which is why biofilm in water systems is treated as a control target in its own right.
Prevention is ongoing, not a one-off fix
The most common misunderstanding about prevention is the belief that you can treat a system once, disinfect it, or fit the right equipment, and then consider legionella dealt with. You cannot. Legionella is present in almost all natural water sources at low levels, so a live water system can never be made permanently legionella-free.
A one-off chlorination or treatment clears the system at a moment in time. The bacteria return through the mains and the biofilm rebuilds, unless the temperature, movement and cleanliness controls keep running. That is why prevention is defined as ongoing control rather than eradication, and why a monitoring schedule of temperature checks and inspections sits at the centre of it. The checks are how you know the controls are still working.
None of this lets anyone certify a system as safe. The honest position is that you control the conditions and you record what you find. A clear monitoring log shows the controls were in place; it does not guarantee that the water is free of legionella at every outlet at every moment.
Recording your prevention measures
Prevention you cannot evidence is prevention you cannot prove. The legal framework treats recording as part of the control, not an afterthought. Under the ACOP L8 code and the HSG274 technical guidance, a dutyholder is expected to:
- Carry out a legionella risk assessment to identify where growth could occur.
- Appoint a responsible person to manage the controls.
- Write the controls into a written scheme of control.
- Record temperatures, flushing and inspections so the scheme can be shown to be working.
The four levers on this page are what a scheme of control turns into a routine. Our free legionella risk assessment template gives you the structure to record them: the assets, the controls, the temperatures and the flushing, in one place you can hand to an enforcing officer.
You cannot make a live water system legionella-free. Prevention is the ongoing work of denying the bacteria the warmth and stillness it needs to multiply, and recording that you have done so.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
How long should you run water to prevent legionella?
Run each infrequently used outlet until the water reaches the system temperature, which means hot water climbing to 50°C within a minute and cold water dropping below 20°C. In most systems that takes a few minutes per outlet. HSE guidance in HSG274 sets weekly as the usual minimum frequency for outlets used less than once a week. The point is not a fixed number of minutes but turning the standing water over and pulling the temperature back out of the 20°C to 45°C growth range.
What kills legionella in a shower head?
Heat and cleaning. Water serving a shower should be stored at 60°C and reach at least 50°C at the head (55°C in healthcare settings), which is hot enough that legionella cannot multiply and, at the stored temperature, is killed. Because showerheads and hoses collect scale and biofilm that shelter the bacteria, they should also be removed, descaled and disinfected on a set schedule. Temperature and cleaning work together: hot water alone will not clear a head that is furred with scale.
Can you get legionnaires' disease from a shower?
It is possible. A shower is a higher-risk outlet because it breaks water into a fine aerosol that can be breathed deep into the lungs, and the head can harbour biofilm and scale. The risk is not the shower itself but a shower fed by water that has been allowed to sit in the growth range. Keeping the water hot, keeping little-used showers flushed, and keeping the head clean are the measures that address it.
What is the most common way to get legionella?
Breathing in small droplets of water, an aerosol or spray, that contain the bacteria. The usual sources are man-made water systems that produce a spray and hold water in the growth range: showers, spray taps, cooling towers and spa pools. It is not spread from person to person, and you do not catch it from drinking water. This is why prevention focuses on the water system and its aerosol-producing outlets.
How often should outlets be flushed to prevent legionella?
At least weekly for any outlet used less than about once a week, which is the usual minimum in HSE guidance. Outlets in regular daily use flush themselves. The right frequency for everything else comes from the risk assessment: a little-used shower feeding a spray in a building with vulnerable people may need more than weekly attention, while a rarely used cold tap in a cool plant room may need less. Whatever frequency you set should be written down and the flushing recorded.
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