In 2012, a cooling tower at a hotel in Edinburgh infected 35 people with Legionnaires’ disease. Two of them died. The investigation found the cooling tower water treatment programme had not been maintained. The risk assessment had not been reviewed in years. And the duty holder — the person legally responsible — had not known what their obligations were.
This is not an isolated case. Legionella outbreaks happen every year in commercial buildings, hotels, hospitals, and care homes — and in almost every case, the root cause traces back to a water system that was poorly managed, poorly monitored, or poorly understood by the people responsible for it.
A Legionella risk assessment is the starting point for controlling this risk in any building. It is not optional, it is not a one-time exercise, and it is not something you delegate to a contractor and forget about. This guide explains what a Legionella risk assessment involves, how to carry one out, and what your ongoing control obligations are — written in plain language for FM managers, building managers, and maintenance engineers who are responsible for water systems in commercial buildings.
Figure 1: Legionella Growth Conditions and Temperature Control Targets
| LEGIONELLA TEMPERATURE CONTROL — CRITICAL ZONES | |
| SAFE — TOO HOT >60°C | Legionella killed within minutes. Target: store hot water at 60–65°C at calorifier. |
| ⚠️ DANGER ZONE 20°C–45°C | Ideal growth range. Legionella multiplies rapidly. Peak risk: 30°C–40°C. Eliminate this range in all water systems. |
| SAFE — TOO COLD <20°C | Legionella dormant but not killed. Target: cold water stays below 20°C at all sentinel outlets. |
| HOT DISTRIBUTION ≥50°C at outlets | Hot water must reach 50°C at all outlets within 1 minute of running. TMV blends to 38–41°C at point of use. |
| ⚠️ Hot below 50°C at outlet = immediate corrective action. Cold above 20°C at outlet = immediate corrective action. Both are Legionella risk failures. | |
What Is Legionella and Why Does It Matter for FM?
Legionella pneumophila is a bacterium that naturally exists in freshwater environments — rivers, lakes, and soil. At low concentrations it’s harmless. The problem starts when it colonises building water systems, finds the right conditions to multiply, and is then inhaled as fine water droplets by building occupants or visitors.
The resulting illness — Legionnaires’ disease — is a severe form of pneumonia. It has a fatality rate of around 10% in the general population and significantly higher in vulnerable individuals such as the elderly, immunocompromised patients, and people with underlying respiratory conditions. It cannot be transmitted from person to person — every case is traced back to a water source.
For facilities managers, this matters because you are likely the duty holder — the person or organisation responsible for managing the risk in the building. In most jurisdictions, that responsibility comes with legal obligations, potential personal liability, and in the event of an outbreak, the possibility of criminal prosecution.
The three things Legionella needs to thrive are: water temperature between 20°C and 45°C, stagnant water with time to multiply, and nutrients from biofilm, scale, or organic matter in the pipework. Remove any one of these three conditions and you control the risk. A Legionella risk assessment identifies where those conditions exist in your building and what you need to do about them.
Who Is Responsible for Legionella Risk Assessment in a Building?
This is the question most building managers ask first — and it’s the right question to start with, because getting the answer wrong leads to gaps in accountability.
The legal responsibility sits with the Duty Holder. In the UK, this is defined under HSE ACoP L8 as the person who has control over the premises — typically the building owner or the organisation that manages the building. In a managed FM contract, the duty holder may formally appoint a Responsible Person to manage the day-to-day Legionella control programme on their behalf. But the duty holder cannot delegate the legal liability — only the operational responsibility.
In practice, for most commercial FM contracts:
• Building owner / landlord — duty holder; overall legal responsibility
• FM manager or facilities director — typically appointed as responsible person; manages the control scheme
• Maintenance team or specialist contractor — carries out the monitoring, flushing, and sampling tasks
• Legionella risk assessor — qualified independent person who conducts the formal risk assessment (should not be the same person managing the control scheme)
Important: the responsible person must be competent. This means understanding the water systems in the building, knowing what the risks are, and being able to verify that the control measures are working. Signing off a risk assessment you don’t understand does not discharge your liability — it just means you signed something.
How to Carry Out a Legionella Risk Assessment — Step by Step
A Legionella risk assessment is a systematic survey of all water systems in the building that could create a risk of Legionella exposure. Here is the process, from start to finish:
Step 1 — Compile a complete inventory of water systems
Walk the building and identify every system that could harbour or distribute water. This includes:
• Hot and cold water storage and distribution
• Cooling towers and evaporative condensers
• Spa pools, decorative fountains, and water features
• Humidifiers, air washers, and wet scrubbers
• Emergency shower and eyewash stations
• Any system that creates a water spray or aerosol
Each system needs to be logged with its location, approximate age, capacity, and current operating condition. If it’s not on the inventory, it won’t be assessed — and it won’t be controlled.
Step 2 — Review and verify the as-built drawings
Obtain the as-built plumbing drawings for the building and compare them against what you actually find during the site walkthrough. In most buildings that have been refurbished, extended, or modified over the years, the drawings and the reality are different.
Pay particular attention to dead legs — sections of pipework that no longer serve an active outlet. Water in a dead leg cannot be flushed by normal use, temperature gradually drifts into the Legionella danger zone, and biofilm accumulates undisturbed. Dead legs are one of the most common findings in Legionella risk assessments and one of the most significant risks.
Step 3 — Assess each system against the risk factors
For each water system identified, assess it against the four key Legionella risk factors:
1. Temperature — is water stored or distributed at temperatures in the 20–45°C danger zone at any point in the system?
2. Stagnation — are there outlets, sections, or tanks where water sits unused for more than 7 days?
3. Nutrients — is there evidence of scale, corrosion, organic matter, or biofilm in the system?
4. Susceptible users — does the building serve elderly residents, patients, immunocompromised individuals, or other high-risk groups? Higher vulnerability = higher required control standard.
Step 4 — Identify sentinel outlets and monitoring points
Sentinel outlets are the first and last outlets on each circuit — the hot and cold outlets that are most representative of the system’s temperature performance. These are your primary monitoring points. Every sentinel outlet must be temperature-tested at a defined frequency (typically monthly for hot water, at risk assessment for cold) and the results logged.
Identify them on your schematic drawing, label them on site, and make sure your maintenance team knows which ones they are. If a sentinel outlet fails a temperature check — hot water below 50°C or cold water above 20°C — that is an immediate corrective action, not a note for next month’s inspection.
Step 5 — Write the risk assessment report
The risk assessment report documents everything found during the survey and assigns a risk rating to each system or risk factor. A complete report includes:
• Description of all water systems assessed
• Schematic drawing showing all pipework, tanks, outlets, and sentinel points
• Risk rating for each system (High / Medium / Low)
• Identified dead legs and their locations
• Recommended control measures for each risk
• Name and competency of the assessor
• Date of assessment and next review date
Legionella Risk Rating by Water System Type
Use this table as a reference when assessing building water systems. Risk levels are based on typical conditions — actual risk on any given site depends on the specific system design, maintenance history, and user population:
| Water System / Location | Risk Factor & Control | Risk Level |
| Cooling tower / evaporative condenser | Monthly microbiological testing, continuous biocide dosing, quarterly inspection. Highest risk system on most sites. | HIGH |
| Hot water calorifier / cylinder | Maintain storage at 60°C minimum. Annual internal inspection. Monthly sentinel outlet temperature logging. | HIGH |
| Cold water storage tank (CWST) | Annual clean and disinfect. Monthly temperature checks. Confirm lid seal intact. | HIGH |
| Hot water distribution — long runs | Monthly sentinel outlet logging. Insulation review. Ensure hot reaches 50°C within 1 minute at all outlets. | MEDIUM-HIGH |
| Infrequently used outlets | Weekly flushing for minimum 2 minutes. Log every flush with date, outlet, and technician name. | MEDIUM-HIGH |
| Dead legs (redundant pipework) | Must be physically removed — cap at tee. Flushing alone cannot control temperature in a dead leg. | HIGH |
| Spa pools / hydrotherapy pools | Daily chemical testing, continuous dosing, weekly microbiological sampling. | VERY HIGH |
| Emergency showers / eyewash | Weekly activation and flushing for minimum 1 minute. Monthly temperature check. | MEDIUM |
| Domestic cold water — regularly used | Regular use prevents stagnation. Monitor cold water temperature at sentinel points quarterly. | LOW |
The Ongoing Legionella Control Programme: What You Must Do After the Assessment
A risk assessment on its own does nothing. It’s the starting point, not the finish line. What protects building occupants is the ongoing control programme that follows — and this is where most FM teams either get it right or fall short.
Temperature monitoring — the most important daily control
Hot water must be stored at minimum 60°C at the calorifier and reach 50°C at all hot outlets within one minute of running. Cold water must remain below 20°C at all sentinel outlets. These temperature targets are the primary defence against Legionella — everything else is secondary.
Log temperatures monthly at all sentinel outlets. Record the time, the outlet location, the temperature, and the name of the person who took the reading. If a reading fails — hot below 50°C or cold above 20°C — raise a corrective action immediately and retest within 24 hours after the issue is resolved.
Weekly flushing of low-use outlets
Any outlet that is not used by occupants in the normal course of building operation must be flushed for a minimum of 2 minutes every 7 days. This includes spare bedrooms in hotels, guest bathrooms in serviced offices, unused showers in sports facilities, and any outlet in an area that is unoccupied or rarely used.
Do not outsource this task verbally. It must be on a written PPM schedule, completed by a named person, and signed off in a logbook with the date, outlet location, and duration of flush. If your team is flushing 50 outlets weekly, they should be completing 50 signed entries per week. If they’re not, the flushing isn’t happening — it’s just being assumed.
Microbiological sampling
Water samples from sentinel outlets should be tested at a UKAS-accredited laboratory for Legionella at a frequency determined by your risk assessment — typically quarterly for high-risk systems. A positive Legionella result above 100 cfu/litre requires immediate corrective action: increased sampling, temperature investigation, and system disinfection if levels exceed 1000 cfu/litre.
One thing many FM teams don’t know: a negative result does not prove the system is Legionella-free — it only proves the sample was negative on that day. Legionella is not uniformly distributed in a water system, so sampling provides a snapshot, not a guarantee. Temperature monitoring is a more reliable continuous control.
Annual review of the risk assessment
The risk assessment must be reviewed annually as a minimum — or immediately after any of the following:
• Any Legionella-related illness associated with the building
• Any significant change to the water system (new pipework, new tanks, system extension)
• Any change in building use that affects the user population or occupancy patterns
• Any system modification during refurbishment or fit-out works
• A significant change in water quality or temperature results
| 📌 For the full plumbing maintenance routine that supports Legionella control, see the Plumbing Preventive Maintenance Checklist. Plumbing PPM Checklist → |
Legionella Compliance Records: What You Need to Keep and for How Long
If a Legionella outbreak is ever linked to your building, the first thing investigators will ask for is your records. What was monitored, when, by whom, and what was done when results were outside acceptable limits.
| Record Type | Minimum Retention Period |
| Risk assessment report | Life of building — minimum 5 years |
| Temperature monitoring logs | Minimum 5 years |
| Flushing records (date, outlet, duration, technician) | Minimum 5 years |
| Microbiological sample results and lab reports | Minimum 5 years |
| Corrective action records | Minimum 5 years |
| Cleaning and disinfection records | Minimum 5 years |
| Training records | Minimum 5 years or duration of employment |
Keep 5 years as the absolute minimum — this is the standard retention period under UK regulatory requirements and widely adopted internationally. If you’re managing a healthcare or high-risk building, keep records for the life of the building.
Frequently Asked Questions
How often does a Legionella risk assessment need to be reviewed?
A Legionella risk assessment must be reviewed at least annually. It should also be triggered immediately by any significant change to the water system, any change in building use or occupancy, any Legionella-associated illness linked to the building, or any significant change in monitoring results such as repeatedly elevated temperatures or positive microbiological samples. An annual review that simply re-dates the previous report without re-surveying the system is not an adequate review — the assessor must confirm that the system and the risks remain as previously described.
Who can carry out a Legionella risk assessment?
A Legionella risk assessment must be carried out by a competent person — someone who has the knowledge, skills, and experience to identify and evaluate the risks in a water system. This does not require a specific licence, but in practice it means someone with formal training in Legionella control (such as City & Guilds, BOHS Water Hygiene qualification, or equivalent), experience of assessing similar buildings, and access to current guidance documents including HSE ACoP L8 and HSG274. The assessor should be independent from the day-to-day management of the water system to ensure objectivity.
What is a dead leg in plumbing and why is it a Legionella risk?
A dead leg is a section of pipework that is no longer connected to an active outlet — it was part of the original system but was capped off or bypassed during a building modification without being removed. Water in a dead leg cannot be flushed by normal building use, so it sits stagnant indefinitely. Temperature drifts into the Legionella danger zone (20–45°C) and biofilm builds up undisturbed. A dead leg cannot be adequately controlled by flushing — the only proper solution is to remove it at the tee, which means cutting out the redundant pipework back to where it branches off the main.
What temperature should hot water be to control Legionella?
Hot water should be stored at a minimum of 60°C at the calorifier or hot water cylinder and distributed so that it reaches 50°C at all outlets within one minute of running. In practice, most sites target 60–65°C storage with TMVs blending down to 38–41°C at point of use to prevent scalding. If any hot outlet is returning water below 50°C after one minute of running, that is a Legionella control failure that requires immediate investigation — check insulation on the supply pipe, TMV operation, and calorifier thermostat setting.
What should I do if a Legionella water test comes back positive?
The response depends on the concentration found. Below 100 cfu/litre: the system is considered satisfactory — continue normal monitoring. Between 100 and 1000 cfu/litre: review your control measures, increase monitoring frequency, and investigate possible causes (temperature issues, dead legs, inadequate flushing). Above 1000 cfu/litre: this requires immediate action — consider restricting use of the affected outlets, carry out a full system disinfection, notify your appointed competent person, and increase sampling frequency until consecutive negative results confirm the system is under control. Document everything.
Conclusion
Legionella risk assessment is not paperwork — it is the foundation of a control programme that protects your building occupants from a genuinely serious health risk. Carry out a thorough assessment, implement the control measures it identifies, monitor consistently, and keep records that prove you did it.
The buildings that have Legionella outbreaks are almost never the ones with no water system at all — they are the ones where someone assumed the risk was being managed without verifying it was. For the plumbing maintenance tasks that support Legionella control day-to-day, see the Plumbing PPM Checklist → . You can also use the PPM Planner tool → to schedule your weekly flushing, monthly temperature checks, and quarterly sampling tasks.