How to Conduct a Legionella Risk Assessment: A Step-by-Step Guide

A legionella risk assessment is the foundation of any water hygiene management programme. It identifies the hazards within a building's water systems and provides the basis for all ongoing control measures. This guide walks through the practical process of conducting a suitable and sufficient assessment for hot and cold water services.

Before You Arrive on Site

Confirm Competence

The risk assessment must be carried out by a competent person. This means having sufficient training, experience, and knowledge to identify legionella risks and recommend appropriate control measures. For complex systems, this typically requires formal training (often a three-day course covering hot and cold water systems, and potentially cooling towers). For simple mains-fed systems, a person with appropriate awareness training may suffice -- but they must understand temperature control principles and the conditions that promote legionella growth.

Prepare Your Equipment

Before attending site, ensure you have:

- Calibrated thermometer -- This is non-negotiable. Thermometers should be calibrated annually to ensure accuracy. Inaccurate readings undermine the entire assessment. A red flag to watch for: if temperature records show the same reading (e.g., exactly 12 degrees) throughout the entire year, either the thermometer is faulty or the monitoring is not being carried out properly.
- Torch -- Essential for inspecting cold water storage tanks in roof spaces, following pipe runs, and examining internal conditions of tanks and plant rooms.
- Sample bottles -- If you encounter a situation that warrants immediate testing (e.g., heavily contaminated tank conditions), you need to be able to take a water sample on the spot for laboratory analysis.
- Digital camera -- Photographic evidence of conditions found on site is critical. Document areas of concern, equipment conditions, dead legs, tank interiors, and any issues that support your recommendations.
- Recording device -- Whether you use a clipboard and paper or a voice recorder, you need a reliable method to capture your findings as you move through the building. Some assessors use digital inspection apps to record findings, take photos, and generate reports in real time.
- Suitable PPE -- Hard hat, hi-vis, and any additional PPE required by the site.

Gather Background Information

Before beginning the physical inspection, have a thorough conversation with the site contact. This is your opportunity to:

- Understand what control measures (if any) are currently in place.
- Check whether there is an existing management structure (Duty Holder, Nominated Responsible Person, deputies).
- Review any existing records: flushing logs, temperature monitoring records, previous risk assessments.
- Assess the level of training and understanding of the responsible persons.
- Identify any water system schematics or diagrams that should be available.

Review existing temperature records critically. If cold water storage tank temperatures have been recorded at exactly 12 degrees for an entire year with no seasonal variation, something is wrong -- either with the equipment or the person doing the monitoring.

The Site Assessment: Follow the Flow

The fundamental principle of a legionella site survey is to follow the flow of water. Start at the incoming mains supply and trace the water through every system, noting conditions, temperatures, and risks along the way.

Step 1: Locate the Incoming Mains

Find where the mains water enters the building. This is your starting point. Note the location and trace the supply forward.

Step 2: Assess Cold Water Storage Tanks

Cold water storage tanks are one of the highest-risk components in any water system. For each tank, assess and record:

Can it be removed? The first question to ask is whether the cold water storage tank can be bypassed entirely, connecting the system directly to mains water. This significantly reduces the risk of legionella proliferation. If a tank only feeds a minimal load (e.g., two toilet flushes), it is almost certainly unnecessary and should be recommended for removal.

Physical condition:
- Is the tank made of approved materials (e.g., GRP -- glass reinforced plastic) or is it an old galvanised steel tank?
- Does it have a close-fitting lid? An uncovered tank is a serious risk -- debris, insects, and even animals can enter.
- What are the internal conditions? Look for corrosion, debris, scale, sediment, sludge, or any signs of microbiological activity.
- Is the tank adequately insulated to prevent heat gain from ambient temperatures in the roof space?

Hydraulic arrangement:
- Are the inlet and outlet opposed (on opposite sides of the tank)? This ensures a good cross-flow of water and prevents stagnation zones.
- What is the temperature at the inlet and outlet? There should be no more than a two-degree difference between the incoming mains temperature and the cold water storage tank temperature. A larger difference may indicate poor cross-flow or an oversized tank.
- Is the water turned over within 24 hours? If not, the tank may be too large for the building's demand. You can verify this with a load test: mark the water level, turn off the incoming supply, and measure how quickly the water level drops during normal usage. Alternatively, install a water meter on the incoming supply.

Multiple tanks:
- If tanks are connected in parallel, check that the inlets are balanced so one tank does not take all the flow while the other stagnates.
- If tanks are connected in series, check for stagnation in the middle tanks, particularly where there is no break in the water flow to create aeration.

Step 3: Assess Hot Water Systems

Identify how hot water is generated and distributed. Common systems include:

Calorifiers (indirect cylinders):
- The highest risk stored-water system. Cold water enters at the bottom, hot water exits from the top, and the area in the middle passes through the legionella growth range (20-45 degrees).
- The water leaving the calorifier must be a minimum of 60 degrees Celsius. Legionella cannot survive above 60 degrees.
- If the system has a flow and return (circulating) system with a secondary pump, the return water temperature must be a minimum of 50 degrees.
- If the calorifier has an inspection hatch, open it and inspect internal conditions. If not, take a drain sample and check for debris or discolouration.
- Check whether a shunt pump is installed on larger cylinders. A shunt pump circulates water within the calorifier during periods of low demand (e.g., overnight), de-stratifying the water to maintain 60 degrees throughout the entire vessel rather than allowing a cold layer to form at the bottom.
- Check the vent pipe arrangement. Vent pipes from calorifiers should discharge to a separate tundish, not into a cold water storage tank. If a calorifier vents hot water into a cold water tank, it turns the cold tank into a radiator -- a serious risk.

Combi boilers:
- Lower risk than calorifiers as there is no stored water. Water is heated on demand.
- Ensure the boiler is operating within manufacturer's specifications.

Point-of-use heaters (instantaneous units):
- Lowest risk as there is no stored water. These small units typically serve individual taps (e.g., above a sink for hand washing).
- No internal inspection is required. Simply ensure they operate within the manufacturer's recommended temperature range.
- Small point-of-use units serving hand wash basins do not necessarily need to deliver water above 50 degrees -- they function as point-of-use devices with minimal risk.

Step 4: Check Temperatures at Outlets

Temperature control is the primary weapon against legionella. At every outlet:

- Cold water must reach below 20 degrees within two minutes of turning on the tap. You do not need to run it for the full two minutes -- if the temperature plateaus, that is your reading.
- Hot water must reach above 50 degrees within one minute of turning on the outlet.

Test the sentinel outlets -- these are the nearest and furthest outlets from the water source (incoming mains for cold; cylinder/boiler for hot). Sentinel outlets represent the extremes of the system and are the most likely to show failures in temperature control.

Step 5: Assess Thermostatic Mixing Valves (TMVs)

TMVs blend hot and cold water to deliver safe temperatures (typically around 40 degrees) at outlets where there is a scald risk. When assessing TMVs:

- Take a surface temperature reading from the hot water pipe feeding the TMV. It must be above 50 degrees.
- Take a temperature reading from the cold water feed. It must be below 20 degrees.
- Note the location and type of each TMV for the recommendation schedule.

Step 6: Identify Dead Legs

Dead legs are sections of pipework that are no longer connected to an active outlet -- water sits in them with no flow. They are a significant risk because:

- Stagnant water within the dead leg provides ideal conditions for bacterial growth.
- Bacteria can leach back from the dead leg into the live system, contaminating it.

All dead legs must be noted on the system schematic and recommended for removal as a priority.

Step 7: Note Little-Used Outlets

Any outlet not used within a seven-day period is classified as "little used." These outlets require weekly flushing as part of the ongoing control scheme. During the assessment, identify all such outlets and include them in the flushing schedule.

Step 8: Check Backflow Prevention

Note the backflow prevention devices throughout the system. These range from simple check valves (fluid category 1) through to air gaps (fluid category 5), which you will commonly see on cold water storage tanks in laboratories and similar environments. Ensure appropriate backflow prevention is in place to prevent contamination of the mains water supply.

Documenting the Risk Evaluation

Based on your findings, assign a risk rating to each element of the system. Whether you use a Low/Medium/High scale or a numerical scoring system (e.g., 1-25 based on likelihood and severity) is a matter of professional preference. A traffic-light system (green/amber/red) is often the most immediately understandable for clients.

Critically, you should assess the risk both before and after control measures -- showing the inherent risk of the system and the residual risk once your recommended controls are in place. This demonstrates the value of the control measures and highlights the consequences of not implementing them.

What the Final Report Must Include

A suitable and sufficient legionella risk assessment report should contain:

1. Review schedule -- When the assessment should next be reviewed (e.g., every two years, or sooner if significant changes occur).
2. Executive summary -- Overview of the building, the water systems, the management structure, and who is at risk.
3. Asset register -- Detailed checklists covering all hot and cold water system assets, including sentinel outlets.
4. Risk evaluation -- Assessment of risk before and after control measures for each system element.
5. Schematics and diagrams -- Up-to-date water system diagrams showing the path of water through the building. These do not need to be engineering drawings -- they simply need to show where the water goes and how it gets there.
6. Photographic evidence -- Images of areas of concern (e.g., dead legs, uncovered tanks, corroded internals, poor insulation).
7. Management structure -- Details of the Duty Holder, Nominated Responsible Person, and deputies, along with their roles and responsibilities.
8. Recommendations -- All required works, separated into:
- Ongoing control measures (regular maintenance tasks such as flushing and temperature monitoring).
- Remedial works (one-off corrective actions such as removing dead legs, fitting tank lids, or installing insulation), with priority ratings indicating urgency. Items like dead legs and uncovered tanks should be classified as immediate priorities.

Common Pitfalls to Avoid

- Relying on uncalibrated equipment. If your thermometer is not calibrated, your temperature data is unreliable and your entire assessment is compromised.
- Accepting existing records at face value. Suspiciously consistent temperature readings (the same number every month, no seasonal variation) indicate the monitoring is not being done properly.
- Ignoring the question of tank removal. Always ask whether a cold water storage tank can be eliminated entirely before recommending controls to manage it.
- Failing to establish the management structure. Without clearly appointed and documented responsible persons, there is no accountability for implementing the control measures.
- Treating the assessment as a one-off. A risk assessment is a living document that must be reviewed and updated as circumstances change.

Conclusion

A legionella risk assessment is a systematic, structured process. By following the flow of water through a building, checking temperatures, assessing storage conditions, and identifying areas of stagnation and risk, you build a comprehensive picture of the hazards present and the controls required.

The assessment is not the end -- it is the beginning. It generates the control scheme, the monitoring schedules, and the remedial works programme that keep the building's water systems safe on an ongoing basis.