Legionella starts dying once water passes about 50°C. The kill speeds up sharply as you climb: at 60°C it dies within minutes, and at 70°C or above it dies almost instantly. Below 20°C it stays dormant. The dangerous middle is 20–45°C, where it actively multiplies [1].
That is the short answer most people are searching for. The longer answer matters more, because “kills” and “controls” are not the same thing, and the temperature you can safely run a tap at is not the temperature that sterilises a pipe.
The temperatures that matter, and what each one does
Heat is the oldest and most reliable Legionella control there is. But a hot-water system is a compromise between killing bacteria and not scalding the person at the tap. The numbers below are the ones UK guidance is built around.
| Water temperature | What happens to Legionella | What it means on your site |
|---|---|---|
| Below 20°C | Dormant; does not multiply | Keep cold water genuinely cold — below 20°C at the tap where practicable [2] |
| 20–45°C | Multiplies most readily; ideal growth band | The “danger zone” your whole regime exists to avoid [1] |
| 45–50°C | Growth slows and stalls | Marginal — too cool to trust, too warm to call cold |
| 50°C | Begins to die, but slowly | Minimum hot-water delivery target at the outlet (55°C in healthcare) [2] |
| 60°C | Killed within minutes | Standard hot-water storage benchmark in a calorifier [2] |
| 70°C and above | Killed almost instantly | The basis of thermal disinfection regimes [3] |
The split in that table is the thing beginners miss. The 50–60°C band is mostly about denying growth and slowly reducing numbers in flowing water. The genuine kill — fast, dependable — sits at 60°C and climbs from there. This is why hot water is stored hot and delivered hot: a calorifier held at around 60°C keeps the stored volume hostile to the bacteria, and water that reaches the tap at 50°C within a minute (55°C in healthcare premises) keeps the pipework from drifting into the growth band [2].
How this plays out on a real system
Picture a calorifier set to 60°C feeding a building. The stored water is lethal to Legionella within minutes. As that water travels out to the furthest tap, it loses heat. If the system is balanced and used, it still arrives at around 50°C — control holds. If it sits in a long dead leg or a low-use riser, it cools through 45, 40, 35°C, and now part of your “hot” system is sitting squarely in the growth band.
So the storage temperature and the outlet temperature are two different jobs. Storage at 60°C does the killing. Distribution at 50°C-plus stops the killed water from re-cooling into trouble before it reaches anyone. Lose either one and the other cannot fully cover for it. The relationship between heat and bacterial growth is worth understanding properly, and Temperature and Legionella growth sets it out in more detail.
Thermal disinfection is the deliberate version of this. When a system has been off, newly commissioned, or implicated in a problem, raising water to 70°C and running it through every outlet for a set period flushes a near-instant kill right to the ends of the pipework. Thermal disinfection covers how that regime is run and recorded.
The one thing beginners get wrong
The instinct, once you know 70°C kills fast, is to just run everything hotter. Do not.
Water at 60°C can scald in seconds, and at 70°C the scald risk is severe and immediate — which is exactly why thermal mixing valves (TMVs) blend hot water down to a safe delivery temperature at vulnerable outlets like care-home and healthcare basins. The control system therefore wants hot water stored and distributed hot, then blended cool only at the point of use. The mistake is mixing the water down early, or running the whole system tepid “to be safe”, which protects against scalds while building the perfect Legionella reservoir.
In my view the cleanest way to hold this in your head: store hot, distribute hot, blend at the tap. Never average the two into a lukewarm compromise.
There is also a trap in the word “kills”. A single hot flush does not make a system permanently safe. Bacteria recolonise from biofilm and sediment whenever water cools back into the growth band, so temperature control is a continuous discipline, not a one-off event. The 60°C calorifier earns its keep every day, not once.
This is general guidance, not a specification for your building. The right setpoints, the outlets you monitor, and how often you check them are decisions for a competent person working from a site-specific risk assessment — a hospital, a care home and a small office will not land on identical numbers, and only an assessment of your system can say which targets and dwell times apply to it.
What to do first
Find out two numbers today: the temperature your hot water is stored at, and the temperature it reaches at your furthest or least-used outlet. If storage is sitting below 60°C, or that far tap struggles to pass 50°C within a minute, you have found where control is slipping before anyone gets ill.
Then make those readings routine and recordable. Monthly sentinel checks and stored-water temperatures only prove control if you can show the trend over time — which is precisely where a paper logbook lets people down and a digital record earns its place, flagging a calorifier that has quietly dropped to 54°C before it becomes a problem. Start with the two numbers. Build the habit around them.
FAQ
Does 60°C kill Legionella instantly?
Not instantly, but quickly — within a few minutes at 60°C [3]. The kill becomes effectively instant at 70°C and above, which is why disinfection regimes use the higher figure. At 60°C you get reliable killing of stored water plus a sensible margin against scalding, which is why it is the common storage benchmark rather than a sterilisation temperature [2].
Will Legionella survive cold water?
Yes — cold does not kill it. Below 20°C the bacteria are dormant and cannot multiply, but they survive and will start growing again if the water warms into the 20–45°C band [1]. That is why “cold” means genuinely below 20°C at the tap where practicable, not merely cooler than the hot supply [2].
Is lukewarm water more dangerous than cold?
For Legionella growth, yes. Cold water below 20°C holds the bacteria dormant, whereas lukewarm water in the 20–45°C range is the band where they multiply fastest [1]. A system run tepid across the board is more hospitable to Legionella than one kept properly cold or properly hot.
Sources
[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - ACoP and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm