What temperatures control legionella?
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Key points
- Below 20°C legionella stays dormant; between roughly 20°C and 45°C it multiplies, fastest at about 37°C; above 50°C it starts to die.
- Store hot water at 60°C so the water leaving the heater is hot enough to kill the bacteria within minutes.
- Deliver 50°C at the tap within one minute of running it (55°C in healthcare settings), and keep cold water below 20°C, reached within about two minutes.
- Check sentinel outlets monthly and rotate through the remaining outlets so every one is checked at least once a year (HSG274 Part 2).
- Temperature is a control measure to record and act on, not a certificate; a reading in range on one day says nothing about the next.
Legionella is controlled by keeping water out of the temperature band where it breeds. Cold water is held below 20°C, hot water is stored at 60°C and delivered to the outlet at 50°C or above, and the danger zone in between, roughly 20°C to 45°C, is the range you are trying to keep water out of. Those three numbers, plus a regular check that they still hold, are the backbone of temperature control.
Temperature is the single most important control for legionella in most buildings because the bacteria are so sensitive to it. Get the water either cold enough or hot enough and the bacteria cannot multiply to dangerous numbers, regardless of what else is happening in the pipework. This page explains what each threshold means, why it exists, and how often you are expected to check.
Why each temperature matters
Legionella lives naturally in water at low numbers. Whether it becomes a hazard depends almost entirely on temperature:
- Below 20°C: the bacteria are dormant. They survive but do not multiply, so the population stays low. This is why cold water systems are kept below 20°C.
- 20°C to 45°C: the growth range. Within this band legionella multiplies, and it does so fastest at around 37°C, human body temperature. Water sitting anywhere in this range, in a lukewarm tank, a warmed cold pipe or a stagnant dead leg, is the core of the risk.
- Above 50°C: the bacteria begin to die rather than grow.
- At 60°C: legionella is killed within a few minutes.
- At 70°C and above: it dies almost instantly.
| Water state or target | Temperature | What it means for legionella |
|---|---|---|
| Cold water, stored and at the tap | Below 20°C | Bacteria stay dormant and do not multiply |
| Growth range (the danger zone) | 20°C to 45°C | Legionella multiplies, fastest at about 37°C |
| Hot water stored in the calorifier | 60°C | Kills legionella within a few minutes |
| Hot water circulation return | 50°C or above | The return leg must not fall below this |
| Hot water delivered at the outlet | 50°C within 1 minute (55°C in healthcare) | Hot enough to control, then blended down at a mixing valve for scald safety |
| Thermal disinfection | 60°C to 70°C at the outlet | Used to disinfect the hot side; 70°C kills almost instantly |
Source: HSE HSG274 Part 2 (hot and cold water systems), supporting ACOP L8. The risk assessment sets the checks for your building.
So the strategy is simple to state: keep water either below 20°C or above 50°C, and minimise the time any water spends in between. Everything else, from tank insulation to flushing little-used taps, exists to protect those temperatures.
The numbers to hold, and where
The temperatures the HSE expects a typical system to achieve are precise about location, not just value:
- Hot water stored at 60°C. The calorifier or water heater should keep stored water at 60°C, and the return leg of a circulating system should not drop below 50°C.
- Hot water reaching 50°C at the outlet within one minute of turning the tap on. In healthcare premises the figure is 55°C, reflecting more vulnerable occupants (see HSG274 and HTM 04-01).
- Cold water below 20°C at the outlet, reached within about two minutes of running it. Cold water that arrives warm has spent too long in warm pipework or a poorly sited tank.
The "within one minute" and "within two minutes" parts matter. A tap that eventually runs hot or cold but takes several minutes to get there is telling you that a long run of pipe is sitting in the growth range between uses. A full set of the target figures is laid out in the legionella temperature chart.
Why 50°C at the tap, not 60°C
Water is stored at 60°C to kill the bacteria, but delivering water that hot to a basin or shower would scald. Water at 60°C can cause a serious burn in seconds, and faster still for children and older people. The compromise is to store hot, then blend down close to the outlet with a thermostatic mixing valve so the water arrives at a safe temperature.
That blending creates its own small risk: the mixed, lukewarm water sitting in the short length of pipe after the valve is in the growth range. This is why thermostatic mixing valves are sited as close to the outlet as possible, are serviced regularly, and are not treated as a reason to lower the storage temperature. The control is store hot, deliver safe, keep the blended section short.
Control legionella by keeping water below 20°C or above 50°C, and by minimising the time any water spends in the 20°C to 45°C range where it breeds.
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.
How often to check the temperatures
Holding the right temperatures is only half the job; the other half is proving they still hold. HSG274 Part 2 sets out a monitoring pattern that a written scheme should follow:
- Sentinel outlets, monthly. Check the temperature at the sentinel hot and cold outlets every month (see below for what these are).
- Calorifier flow and return, monthly. Confirm stored water is at 60°C and the return is not below 50°C.
- All other outlets, annually. Work through the remaining taps and fittings on a rotating basis so that every outlet is checked at least once over the year.
- Cold water storage tanks, typically six-monthly. Check incoming and stored cold water temperatures, ideally in the warmer months when they are at their highest, to confirm the water stays below 20°C.
- Thermostatic mixing valves and showerheads. Service mixing valves and clean and descale showerheads on the frequency set by the risk assessment, commonly annually for valves and quarterly for showers.
Every reading is recorded, dated and signed, whether it passes or fails, so a pattern of drift shows up before it becomes a failure. Out-of-range results trigger action, not just a note. Keep the record in a legionella log book so the history is in one place.
What is a sentinel outlet
A sentinel outlet is a tap or fitting chosen to represent the temperature extremes of the system, so that checking a handful of points tells you about the whole. On a hot water system the sentinels are the outlets nearest to and furthest from the calorifier; on a cold water system, the nearest to and furthest from the storage tank. If the furthest hot tap still reaches 50°C quickly and the furthest cold tap stays below 20°C, the outlets in between are very likely fine too. Monitoring these points monthly gives early warning that temperatures are slipping towards the growth range.
How this fits the guidance
These temperatures and check frequencies are not house rules; they come from the HSE's approved guidance. The duty to control legionella sits under ACOP L8, and the practical temperature figures and monitoring schedule are set out in HSG274 Part 2 for hot and cold water systems. Temperature monitoring records also feed the wider picture your sampling and testing and risk assessment build up over time. Recording the numbers is what demonstrates the control is working; a single reading in range proves the moment it was taken, and nothing more.
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
At what temperature does legionella die?
Legionella begins to die above 50°C. At 60°C it is killed within a few minutes, and at 70°C almost instantly, which is why hot water is stored at 60°C. Below 50°C the bacteria survive, and between roughly 20°C and 45°C they actively multiply.
What temperature does legionella grow at?
Legionella multiplies in water between about 20°C and 45°C, with growth fastest at around 37°C, human body temperature. Below 20°C it stays dormant and does not multiply, though it is not killed. Above 50°C it starts to die, so temperature control aims to keep water either below 20°C or above 50°C.
How often should water temperatures be checked for legionella?
Under HSG274 Part 2, sentinel outlets (the nearest and furthest hot and cold taps) are checked monthly, along with the calorifier flow and return temperatures. The remaining outlets are checked on a rotating basis so that every one is covered at least once a year. Cold water storage tank temperatures are typically checked every six months, ideally in the warmer months when they are at their highest.
What is a sentinel outlet?
A sentinel outlet is a tap or fitting chosen to represent the temperature extremes of a water system. On a hot water system these are the outlets nearest to and furthest from the calorifier; on a cold water system, nearest to and furthest from the storage tank. Checking them monthly gives an early warning that temperatures are drifting towards the 20°C to 45°C range where legionella multiplies.
What temperature kills legionella?
Legionella starts to die above 50°C, is killed within a few minutes at 60°C, and dies almost instantly at 70°C. This is why hot water is stored at 60°C. The rate of kill rises sharply with temperature, so 60°C is the practical storage target and thermal disinfection is carried out at 60°C or above held at each outlet.
What is the legionella temperature range?
The legionella growth range is roughly 20°C to 45°C, with the fastest growth at about 37°C. Below 20°C the bacteria are dormant and do not multiply, and above 50°C they begin to die. Temperature control works by keeping water either below 20°C or above 50°C so it spends as little time as possible in the 20°C to 45°C danger zone.
Does boiling water kill legionella?
Yes. Legionella is killed almost instantly at 70°C, so water at boiling point (100°C) destroys it immediately. Boiling is not how a plumbed system is controlled, because you cannot boil a whole distribution system, but it explains why stored hot water is kept at 60°C and thermal disinfection runs at 60°C or above: the hotter the water, the faster the bacteria die.
How long does legionella take to grow in water?
In the 20°C to 45°C range, legionella can double roughly every three to six hours where water is stagnant and nutrients are present, so numbers can rise significantly over days rather than months. This is why stagnation matters so much: water that sits still and warm in a dead leg or a little-used outlet gives the bacteria the time and conditions they need. Keeping water moving and out of the growth range denies them both.
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