What are the legionella temperatures?
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Key points
- Hot water: store at 60°C, deliver at least 50°C at the tap within one minute (55°C in healthcare), keep circulation returns at 50°C or above.
- Cold water: below 20°C in storage and at the outlet, reached within about two minutes of running.
- The danger zone: roughly 20°C to 45°C, where legionella multiplies, fastest at about 37°C.
- The kill zone: legionella starts to die above 50°C, dies within a few minutes at 60°C, and almost instantly at 70°C.
- The storage, outlet, circulation and cold-water figures come from HSE guidance HSG274 Part 2, with HTM 04-01 for healthcare; the kill-kinetics figures (how fast legionella dies at each temperature) are HSE general guidance. A single in-range reading records one moment, it does not certify the system.
Hot water should be stored at 60°C and reach at least 50°C at the tap within one minute, 55°C in healthcare. Cold water should be below 20°C within two minutes. Between 20°C and 45°C legionella multiplies; above 50°C it dies. Those are the legionella temperatures the HSE expects.
This page collects every temperature figure in the HSE guidance into one reference: what the number is, where in the system it applies, and why it exists. If you want to check a reading you have just taken against these targets, use the legionella temperature checker; for a printable summary of the same figures there is the legionella temperature chart. For how to build these figures into a monitoring routine, see legionella temperature control.
The complete set of legionella temperatures
| Point in the system | Figure | Why it exists |
|---|---|---|
| Cold water, stored and at the outlet | Below 20°C, within 2 minutes of running | Below 20°C legionella stays dormant and does not multiply |
| The growth range (danger zone) | 20°C to 45°C, fastest at about 37°C | The band where legionella multiplies; water should spend as little time here as possible |
| Hot water storage (calorifier or cylinder) | 60°C | Kills legionella within a few minutes, so stored water is disinfected |
| Hot water circulation return | 50°C or above | Stops the return leg cooling into the growth range before reheating |
| Hot water at the outlet, general premises | At least 50°C within 1 minute | Confirms hot water arrives hot enough, fast enough, to control growth in the pipe run |
| Hot water at the outlet, healthcare | At least 55°C within 1 minute | HTM 04-01 raises the target because patients are more vulnerable |
| Thermal disinfection of the hot side | Raise the whole contents of the calorifier and circulating water for at least an hour, keeping every outlet at 60°C or above | Each tap and appliance is then run sequentially for at least five minutes at full temperature, measured and recorded (HSG274 Part 2, §2.133); carried out under scald controls |
Source: HSE HSG274 Part 2 (hot and cold water systems), supporting ACOP L8; healthcare outlet figure from HTM 04-01; kill-kinetics figures from HSE general guidance. The risk assessment for your building sets which checks apply and how often.
Why these three bands
Legionella lives in natural water at low, harmless numbers. Temperature decides whether those numbers stay low or climb:
- Below 20°C: the bacteria survive but do not multiply. Cold systems are held under this line.
- 20°C to 45°C: the growth range. Inside it legionella multiplies, fastest at about 37°C, human body temperature. Lukewarm tanks, warmed cold pipes, long hot runs that cool between uses and stagnant dead legs all land here.
- Above 50°C: the bacteria begin to die. At 60°C they are killed within a few minutes; at 70°C and above, almost instantly.
The whole temperature regime follows from that: store hot enough to disinfect (60°C), deliver hot enough to keep the pipe runs out of the growth range (50°C, or 55°C in healthcare), and keep cold genuinely cold (below 20°C). What temperature kills legionella covers the kill side in more detail.
The time limits are part of the figure
Each outlet target carries a time limit: hot water should reach 50°C within one minute of opening the tap, and cold should fall below 20°C within about two minutes. A tap that takes five minutes to run hot is telling you a long length of pipe sits lukewarm between uses, in exactly the band where legionella grows. When you record a check, record the temperature reached and roughly how long it took, not just the eventual reading.
Where the checks are taken: sentinel outlets
Checks are concentrated on the points most likely to drift first. On a hot system those are the outlets nearest to and furthest from the calorifier; on a cold system, nearest to and furthest from the storage tank. These are the sentinel outlets, checked monthly under HSG274 Part 2, with a representative selection of the other outlets considered on a rotational basis so the whole system is shown to be reaching satisfactory temperatures over a defined time, and calorifier flow and return checked monthly too.
Hot enough to control, cool enough to use
Water at 60°C scalds in seconds, so the regime stores hot and then blends down near the outlet with thermostatic mixing valves. The short blended run after the valve sits in the growth range, which is why valves are fitted close to the outlet and serviced on schedule, and why dropping the storage temperature is never the right answer to a scald concern. The balance is covered in scalding and legionella.
Three numbers run the whole regime: cold below 20°C, hot stored at 60°C, hot delivered at 50°C or above, and as little time as possible in the 20°C to 45°C band in between.
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.
When a reading is out of range
An out-of-range reading is a finding, not a catastrophe, but it is not just a note either. The written scheme should say what happens next: investigate the cause (a failed pump, a heating fault, a warmed cold pipe, a stagnating run), take the corrective action, re-check, and record all of it. A pattern of drifting readings is a trigger to review the risk assessment, because it suggests the control measure is no longer effective. Persistent problems on a complex system are a sign to bring in competent help rather than to keep logging failures.
Keep the readings somewhere they build a history, such as a legionella log book, so drift shows up before failure. A single in-range reading proves the moment it was taken and nothing more; the value is in the run of records over time.
Where the figures come from
These are not industry convention or house rules. The duty to control legionella sits under the Approved Code of Practice L8, and the practical temperature figures and monitoring pattern are set out in HSG274 Part 2 for hot and cold water systems, with the stricter healthcare figures in HTM 04-01. If the guidance is updated, the updated figures are the ones to follow; this page records the guidance as published at the date shown above.
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
What temperature should hot water be to kill legionella?
Hot water should be stored at 60°C in the calorifier or water heater, because legionella is killed within a few minutes at that temperature. At the outlet it should reach at least 50°C within one minute of running the tap (55°C in healthcare premises), and the return leg of a circulating system should stay at 50°C or above. These figures come from HSE guidance HSG274 Part 2.
What temperature does legionella grow in?
Legionella multiplies in water between roughly 20°C and 45°C, and grows fastest at about 37°C, close to human body temperature. Below 20°C it survives but stays dormant and does not multiply. Above 50°C it begins to die. Temperature control keeps water below 20°C or above 50°C so it spends as little time as possible in the growth range.
What temperature should cold water be kept at for legionella?
Cold water should be below 20°C, both in storage and at the outlet, and it should get there within about two minutes of running the tap. Cold water that arrives warm has usually picked up heat in poorly insulated pipework, a warm loft or plant room, or a badly sited storage tank, and it sits in the range where legionella can multiply.
Is 50 degrees hot enough to kill legionella?
50°C is where legionella starts to die, but the kill is slow at that temperature, which is why 50°C is the minimum delivery target at the outlet rather than the storage target. Water is stored at 60°C, where the bacteria die within a few minutes, so fresh hot water entering the system is effectively disinfected before it is distributed at 50°C or above.
What is the legionella danger zone?
The danger zone is roughly 20°C to 45°C: the temperature band in which legionella actively multiplies, with the fastest growth at about 37°C. Lukewarm water in a dead leg, a little-used outlet, a warmed cold pipe or a failing hot water circuit all sit in this band. The aim of temperature control is to keep water out of it.
What temperature should water be in a care home or hospital?
In healthcare premises the hot water target at the outlet is 55°C within one minute, rather than the 50°C used in general buildings, while storage remains at 60°C and cold stays below 20°C. The higher outlet figure comes from HTM 04-01, the healthcare water guidance, and reflects the greater vulnerability of patients. A care home is not classed as healthcare premises for this purpose: it follows the general regime in HSG220 and HSG274, so its outlet target is at least 50°C, while the 55°C target applies to hospitals and other healthcare premises under HTM 04-01. Scald risk is managed with thermostatic mixing valves close to the outlet.
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