Legionella Risk Assessment: A Practical Guide for Property Owners and Businesses

A water system can look clean and operate normally while still presenting potential Legionella risks. For homeowners, landlords and businesses, one of the most effective ways to understand those risks is to assess the system properly and establish appropriate control measures.

Legionella bacteria are widespread in natural water sources. The concern arises when conditions within purpose-built water systems allow the bacteria to multiply. Temperatures between approximately 20°C and 45°C, combined with nutrients such as scale, rust, sludge, organic matter and biofilms, can support bacterial growth.

A Legionella risk assessment is therefore not simply a box-ticking exercise. It should help identify how a particular water system operates, where risks may exist and what practical controls are appropriate.

1. What is a Legionella risk assessment?

A Legionella risk assessment is a structured examination of a water system to identify foreseeable risks associated with exposure to Legionella bacteria.

The assessment should consider the building, its water system, how that system is used and the people who may be exposed.

For businesses and landlords, this can mean reviewing:

Hot water storage
Cold water storage
Pipework
Taps and showers
Temperature controls
TMVs
Infrequently used outlets
Areas of stagnation
Tank condition
Scale and sediment
Previous maintenance
Changes to the plumbing system

HSE guidance is aimed at dutyholders, including employers and people who control premises and have health and safety responsibilities for others.

2. Know who is responsible

A major weakness in water hygiene management is uncertainty over responsibility.

In a commercial property, several parties may be involved, including:

Property owners
Landlords
Managing agents
Facilities managers
Employers
Maintenance contractors
Plumbing contractors
Tenants

Responsibilities should be clearly established so that important checks are not assumed to be someone else's job.

Where a written scheme or control programme is required, it should clearly explain who is responsible for carrying out each task and how the controls are to be maintained.

3. Create an accurate picture of the water system

You cannot effectively manage a system that you do not understand.

A suitable assessment should establish how water enters, is stored, heated, circulated and delivered throughout the building.

For more complex buildings, a schematic diagram can be particularly useful.

The diagram can help identify:

Storage tanks
Calorifiers
Hot water cylinders
Primary and secondary circulation
Branches and loops
Taps
Showers
TMVs
Low-use outlets
Potential dead legs
Other relevant system components

An accurate schematic also makes future maintenance and troubleshooting considerably easier.

4. Monitor the temperatures

Temperature control is one of the traditional methods of reducing Legionella risk.

HSE guidance recommends maintaining cold water, where possible, below 20°C. Hot water should be stored at a minimum of 60°C and distributed so that it reaches at least 50°C within one minute at outlets.

For larger or more complex systems, monitoring should not be limited to one convenient tap.

HSE guidance refers to sentinel outlets and representative outlets when monitoring hot and cold water systems. Flow and return temperatures can also provide useful information about the operation of circulating systems.

The purpose of these checks is to establish whether the system is maintaining the intended temperature regime throughout the areas being assessed.

5. Investigate stagnation and unused outlets

Unused outlets are easy to overlook.

A landlord may have a property with an unused bathroom. A business may have an office wing that is rarely occupied. A commercial building may have showers or facilities that are only used occasionally.

Where water remains stagnant, the risk profile can change.

A suitable control programme should identify infrequently used outlets and establish how they will be managed. HSE guidance recommends regular flushing, commonly on a weekly basis, for infrequently used outlets where appropriate.

This is particularly important when a building has recently reopened after being vacant.

6. Inspect and maintain tanks and hot water equipment

Storage vessels should be incorporated into the wider water hygiene programme.

Cold water storage tanks should be kept clean and appropriately covered. HSE guidance recommends checking tanks to ensure they remain clean at least annually, with remedial work where necessary.

Hot water cylinders and calorifiers should also be considered.

Scale, sludge and debris can affect the condition of the equipment and may contribute to conditions that support bacterial growth.

The required frequency of inspection and cleaning should be determined by the condition of the system and the findings of the risk assessment.

7. Maintain showers, taps and TMVs

Outlets are where people interact directly with the water system, so their condition matters.

Showerheads and hoses can accumulate scale and deposits. HSE guidance recommends cleaning and descaling them quarterly or according to the risk assessment.

Thermostatic mixing valves also require attention. They may be used where there is a significant scalding risk, but their maintenance should be incorporated into the wider water system management programme.

A neglected outlet can undermine an otherwise well-maintained system, which is why maintenance should consider the complete water network rather than focusing only on the main plant.

8. Take action when the assessment identifies a problem

A risk assessment has little value if its recommendations are not implemented.

If the assessment identifies a poorly maintained tank, low hot water temperatures, stagnant pipework or other concerns, the appropriate remedial action should be planned and completed.

Depending on the findings, this could include:

Tank cleaning
Chlorination
Pipework disinfection
Water testing and sampling
Temperature control adjustments
Removal of redundant pipework
Showerhead cleaning and descaling
TMV servicing
Plumbing modifications
Improvements to insulation or circulation

The correct treatment depends on the system and the identified cause. There is no single treatment that is suitable for every property.

9. Keep evidence of completed work

Documentation is an important part of effective water hygiene management.

Businesses and landlords should retain appropriate records of inspections, temperature checks, cleaning, testing, servicing and remedial work.

Records can help demonstrate:

What was checked
When it was checked
What was found
Whether a problem was identified
What corrective action was recommended
Whether the corrective action was completed

This is particularly useful where a property has multiple contractors or changes hands between management teams.

10. Review the assessment when the property changes

A risk assessment should not be considered permanently finished.

Changes to a building can change its water system and therefore its risk profile.

This can happen after:

A refurbishment
A new bathroom installation
Building extensions
Changes in occupancy
Installation of new hot water equipment
Changes to pipework
Long periods of vacancy
Changes in how a commercial building is used

When the system changes, the assessment and control measures should be reviewed accordingly.

What should you do if your property has never had a Legionella assessment?

Do not wait for a water quality problem before looking at the system.

Start by gathering information about the building and its plumbing. Establish the location and type of hot and cold water equipment, identify rarely used outlets and find out when tanks, cylinders, showers and other components were last inspected.

A competent professional can then assess the system and identify appropriate control measures.

Where necessary, the next stage may involve water testing and sampling, tank cleaning, chlorination, pipework disinfection or targeted maintenance.

For commercial properties and rental accommodation, having a clear documented approach can also make ongoing management considerably easier.

Water hygiene and Legionella services

London Chlorination and Water Treatment provides water hygiene and water treatment services for homeowners, landlords, commercial property owners, businesses and property managers.

Services can include Legionella risk assessments, water testing and sampling, tank cleaning, chlorination, pipework disinfection and hot and cold water system maintenance.

Whether you are managing a rental property, operating a business premises or responsible for a larger commercial building, London Chlorination and Water Treatment can help you understand the condition of your water system and identify the appropriate next steps.

Contact London Chlorination and Water Treatment to discuss your requirements and request a quotation.