Control parameters: the reference
The authoritative UK Legionella numbers in one place, each with its source and a plain-English note. The figures the whole site, and your scheme of control, point back to.
Source identities and official links checked on 16 July 2026.See what changed ›
Machine-readable JSONIndicative guidance only. Every value here must be confirmed against the current edition of the cited document and applied through a competent, site-specific risk assessment. Figures are common UK baselines, not a substitute for professional judgement or the source standard.
Temperature control
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Hot water storage temperature | ≥ 60 °C | Stored in the calorifier / hot water cylinder | Store hot water hot enough that the whole vessel stays at temperature. Confirm the figure and any healthcare variation against current guidance and your risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Hot water temperature guidelines to prevent Legionella, Calorifier maintenance for Legionella: inspection, drain-off and sludge removal |
| Hot water at the outlet | ≥ 50 °C ( ≥ 55 °C healthcare ) within ~1 min | Sentinel hot outlets, time to reach temperature | Distributed hot water should reach the listed temperature quickly at the tap; healthcare uses the higher figure. Balance against scald risk via TMVs.HSE HSG274 Part 2 (2024), “Operation and inspection…”, p.70; healthcare 55 °C per HTM 04-01See: Hot water temperature guidelines to prevent Legionella, TMV maintenance: balancing scald risk and Legionella control |
| Cold water at the outlet | below 20 °C within ~2 min | Sentinel cold outlets, after running | Keep cold water cold. Warming of cold mains in summer or near heat sources is a common failure; confirm the target against your risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Cold water temperature guidelines to prevent Legionella |
| Legionella growth range | 20–45 °C | Water temperature where the bacterium multiplies | Dormant below ~20 °C, multiplies fastest 20–45 °C, killed progressively above ~50 °C and rapidly above ~60 °C. Indicative ranges.HSE HSG274 Part 2 (2024), Introduction; WHO (2011) “Water safety in buildings”See: Pipe Scale, Corrosion and Biofilm: How Surface Condition Drives Legionella Risk, Temperature control basics for Legionella prevention |
| Time to kill at temperature | ~60 °C: minutes · ~50 °C: hours | How quickly heat inactivates Legionella | Higher temperatures kill faster. Used as the basis for thermal disinfection; confirm dwell times against current guidance.HSE HSG274 Part 2 (2024), Introduction; WHO (2011)See: Thermal disinfection: using heat to control Legionella |
Microbiology & action levels
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Legionella result: in control | ≤ 100 CFU/L | Hot & cold water systems (HSG274 Part 2) | System generally considered under control; continue routine monitoring. Indicative band: interpret with your risk assessment and a UKAS lab.HSE HSG274 Part 2 (2024), “Microbiological monitoring” (para 2.123), p.86See: Understanding lab results: CFU counts and what they mean, Legionella sampling 101: how and why to test your water |
| Legionella result: review | > 100 to 1,000 CFU/L | Hot & cold water systems (HSG274 Part 2) | Review control measures and resample; if the majority of samples are positive, act. Indicative band.HSE HSG274 Part 2 (2024), “Microbiological monitoring” (para 2.123), p.86See: What to do if you get a positive Legionella test, Understanding lab results: CFU counts and what they mean |
| Legionella result: immediate action | > 1,000 CFU/L | Hot & cold water systems (HSG274 Part 2) | Investigate, review controls and consider disinfection. Indicative band; your risk assessment and a competent person guide the response.HSE HSG274 Part 2 (2024), “Microbiological monitoring” (para 2.123), p.86See: What to do if you get a positive Legionella test |
| Cooling tower action levels | ≤ 100 · > 100–1,000 · > 1,000 CFU/L | Cooling towers & evaporative condensers (HSG274 Part 1) | Per HSG274 Part 1 Table 1.11: not detected/up to 100; >100 and up to 1,000 (low-level, reassess); >1,000 (act). Interpreted alongside the tower’s biocide and aerobic/dip-slide regime (Table 1.10).HSE HSG274 Part 1 (2024), Table 1.11 “Comments and action levels in response to legionella analysis results”, p.47See: Cooling towers and evaporative condensers: high-risk systems |
Monitoring frequency
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Sentinel outlet temperatures | Monthly | Nearest & furthest hot and cold outlets | Sentinels bracket each service at its extremes. Frequency is risk-assessment led; monthly is the common baseline.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Sentinel outlets: what they are and how to monitor them, Routine monitoring and flushing: the basics |
| Representative outlet temperatures | On rotation (all over a set period) | A representative selection of other outlets | Rotate so a representative spread of outlets is checked over the period set by your risk assessment (often a year).HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Routine monitoring and flushing: the basics |
| Calorifier flow & return temperature | Monthly | Hot water storage vessel | Record both flow and return; a low return can indicate stratification or scale.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Calorifier maintenance for Legionella: inspection, drain-off and sludge removal |
| Flushing little-used outlets | Weekly (or as risk-assessed) | Outlets not used regularly | Run little-used outlets to replace stagnant water; weekly is the common baseline, set yours by risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: How to implement a flushing programme for Legionella control, Water Age Explained: Why Residence Time Drives Legionella Risk |
Inspection & maintenance
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Cold water storage tank inspection | Annually (typical) | CWST condition, lid, screens, sediment | Inspect condition and cleanliness; clean/disinfect if needed. Interval is risk-assessment led.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Water tank inspections: checking for Legionella risk |
| Calorifier inspection / drain-off | Annually (typical) | Internal inspection and/or drain to check for sludge | Check for sludge/scale at the base; frequency depends on findings and risk assessment.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Calorifier maintenance for Legionella: inspection, drain-off and sludge removal |
| TMV service & fail-safe test | Per manufacturer (commonly 6–12 monthly) | Thermostatic mixing valves | Service, descale and fail-safe test; healthcare (HTM 04-01) and manufacturer schedules may be more frequent.HSE HSG274 Part 2 (2024), “Operation and inspection…”, p.70; plus TMV manufacturer instructionsSee: TMV maintenance: balancing scald risk and Legionella control |
| Showerhead clean & descale | Quarterly (typical) | Showerheads and hoses | Dismantle, clean and descale; more often where scaling is heavy. Risk-assessment led.HSE HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70See: Showerhead cleaning and descaling schedules |
| Risk assessment review | No fixed expiry: review on triggers | The Legionella risk assessment itself | Review regularly and whenever the system, its use, or the people exposed change. The "every two years" figure is a common prompt, not a legal expiry.HSE ACoP L8 (2013), “Carrying out a risk assessment”, p.12; BS 8580-1:2019, clause 10 (risk review and reassessment)See: Reviewing and updating your Legionella risk assessment regularly |
Disinfection
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| Shock chlorination | 50 mg/L for ≥ 1 h, with ≥ 30 mg/L free residual at the end | Chemical disinfection of a water system | HSG274 para 2.135: dose to 50 mg/L (ppm) for a minimum 1 hour contact, with at least 30 mg/L free residual chlorine remaining at the end; lower concentrations over longer contact times are acceptable per BS 8558. High-hazard work under COSHH; confirm against current guidance.HSE HSG274 Part 2 (2024), “Cleaning and disinfection” (para 2.135), p.88; lower concentrations / longer contact times per BS 8558See: Planning a water-system chlorination: safety and assurance checklist |
| Thermal disinfection (pasteurisation) | ≥ 60 °C at every outlet, defined dwell | Heat disinfection of a hot water system | Raise temperature and run each outlet for the defined period; manage scald risk. Confirm temperature/dwell against current guidance.HSE HSG274 Part 2 (2024), “Cleaning and disinfection”, p.87See: Thermal disinfection: using heat to control Legionella |
Legal & duties
| Parameter | Value | Where it applies | Source & notes |
|---|---|---|---|
| General duty of care | HSWA 1974, s.2 & s.3 | Employer duties to employees and others | The overarching duty under which Legionella control sits.Health and Safety at Work etc. Act 1974See: UK Legionella compliance 101: laws and responsibilities |
| Duty to assess the risk | COSHH Regulation 6 | Risk assessment for hazardous substances (incl. Legionella) | The legal hook for a suitable and sufficient Legionella risk assessment.COSHH Regulations 2002See: Legionella risk assessment basics: what it is and why you need it |
| Duty to prevent or control exposure | COSHH Regulations 7–8 | Control measures and their use | Underpins the written scheme of control and its maintenance.COSHH Regulations 2002See: Written scheme of control template and worked example (COSHH & ACoP L8) |
| Notify cooling towers to the local authority | Required (Notification Regulations 1992) | Wet cooling towers & evaporative condensers | Notifiable devices must be registered with the local authority (and de-registered when removed).Notification of Cooling Towers and Evaporative Condensers Regs 1992 |
| Report a workplace-linked case | RIDDOR (where reportable) | Reporting Legionnaires’ disease linked to work | Certain work-related cases/exposures are reportable; check the RIDDOR criteria and public-health duties.RIDDOR 2013 |
Need these as a printable card? Each row also feeds our record templates and calculators. For the documents behind the numbers, see standards & guidance.
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