Legionella Risk Assessment: What Landlords and Building Managers Need to Know

Legionella Risk Assessment: What Landlords and Building Managers Need to Know

Legionnaires' disease is a potentially fatal form of pneumonia caused by inhaling water droplets or aerosols containing Legionella pneumophila bacteria. The bacteria colonise hot and cold water systems where conditions allow — standing water at intermediate temperatures, scale, sediment, and biofilm providing ideal growth conditions. UK law places a duty on employers and those responsible for premises (including landlords) to manage the risk of legionella. Understanding this duty, what it requires, and how it is discharged through a hot water system design is essential for anyone specifying or maintaining water systems in non-domestic or rental properties.

The Legal Framework

The duty to control legionella risk arises from:

• Health and Safety at Work etc. Act 1974 (HSWA) — general duty of employers and those in control of premises to ensure health and safety
• Control of Substances Hazardous to Health Regulations 2002 (COSHH) — legionella bacteria are a biological hazard subject to COSHH assessment and control
• The Management of Health and Safety at Work Regulations 1999 — require risk assessment

The practical guidance is set out in:

• L8 ACOP (Approved Code of Practice): Legionnaires' Disease — The Control of Legionella Bacteria in Water Systems — the primary regulatory document; compliance with L8 creates a strong presumption of compliance with law
• HSG274 Technical Guidance — three volumes covering hot and cold water systems (Part 2), cooling towers (Part 1), and other risk systems (Part 3)

Who has the duty? The "duty holder" is the person in control of premises — typically the employer for workplaces, or the landlord/managing agent for rental properties. For residential properties, the Health and Safety Executive (HSE) guidance makes clear that landlords letting residential accommodation have a duty to assess legionella risk, but this does not always require a formal written assessment — for simple domestic-type systems with no significant risk factors, a brief documented mental assessment recorded in writing may be sufficient. For HMOs, commercial premises, care homes, hotels, and larger residential buildings, a formal written risk assessment carried out by a competent person is expected.

How Legionella Grows in Water Systems

Legionella pneumophila is ubiquitous in natural water sources at low concentrations. It becomes a risk when it colonises and proliferates in man-made water systems. The conditions that favour growth:

• Temperature: The bacterium multiplies most rapidly between 20°C and 45°C. Below 20°C it is dormant; above 60°C it is rapidly killed. The danger zone for hot water storage is 20–45°C — water that is not hot enough.
• Stagnation: Still water allows bacterial colonies to establish. Pipes with dead legs (sections of pipework that are not regularly flushed), infrequently used outlets, and large-volume storage tanks at inadequate temperature all create stagnation risk.
• Nutrients: Scale, sediment, biofilm, and rubber seals in pipework provide nutrients and protected micro-environments for legionella to colonise.
• Aerosol production: The risk of infection requires the bacteria to be inhaled — shower heads, spray taps, and cooling towers all create aerosols. A contaminated hot water system only poses a health risk if it can produce an aerosol.

Hot Water System Controls

Storage Temperature

HSG274 Part 2 requires that:

• Hot water storage cylinders maintain water at 60°C throughout
• Hot water is distributed so that it reaches 50°C at all outlets within one minute of running
• Cold water is stored and distributed below 20°C

A storage cylinder thermostat set to 60°C is the primary control — at 60°C, legionella is killed in approximately 2 minutes. At 50°C, kill time extends to around 80–120 minutes. Below 46°C, the bacteria survive and can multiply.

Weekly Flushing of Infrequently Used Outlets

Any outlet that is used less than once a week (spare room shower, guest bathroom tap) should be flushed weekly — running cold water for two minutes, then hot water until the outlet reaches maximum temperature — to prevent stagnation. This should be logged as a routine maintenance task.

Thermostatic Mixing Valves (TMVs)

Where the stored hot water is at 60°C, point-of-use thermostatic mixing valves are required at outlets accessible to vulnerable persons (care homes, children's facilities) to prevent scalding — the mixed temperature must not exceed 43°C (or 41°C for baths in care settings). TMVs introduce a new risk: the mixed water at 43°C is in the legionella growth range. To address this, TMVs must be:

• Located as close as possible to the outlet — minimising the volume of mixed water that can stagnate
• Flushed periodically to prevent bacterial growth in the mixed water zone
• Maintained in accordance with the manufacturer's schedule (typically annual strip-down, descale, and re-commissioning)

Showerheads

Flexible shower hoses and showerheads accumulate scale and biofilm and are a recognised source of legionella aerosol. Routine cleaning (quarterly at minimum, monthly in higher-risk settings):

• Remove showerhead and hose
• Dismantle and descale (soak in descaling solution or citric acid)
• Flush through with hot water before reassembly
• Replace the hose if it shows cracking, discolouration, or internal sediment

In premises with a legionella management plan, periodic disinfection of showerheads with chlorine-based solution or heat disinfection (running at 60°C+ for 3 minutes) may be required.

Cold Water Systems

Cold water storage cisterns (CWSC) and their distribution must be maintained below 20°C. Controls include:

• Cisterns should be insulated, covered, and located away from heat sources
• Cisterns should be inspected annually — cleaned and disinfected if sediment, algae, or biofilm are found
• The cistern overflow pipe should be fitted with an insect-proof terminal
• The volume of the cistern should not significantly exceed the property's daily demand — oversized cisterns have long water residence times, increasing stagnation risk

Risk Assessment: What It Must Cover

A legionella risk assessment under L8 must:

1. Identify and assess the water systems in the premises
2. Identify all potential sources of legionella risk (storage temperatures, dead legs, infrequently used outlets, cooling equipment)
3. Identify populations at particular risk (elderly, immunocompromised, very young)
4. Identify any existing control measures and assess their effectiveness
5. Produce a written record of the assessment and any actions required
6. Be reviewed when there is reason to believe it is no longer valid (significant system changes, new evidence, following an incident)

For a simple domestic-type hot water system (combi boiler or a single cylinder) in a single tenancy property with no dead legs, the assessment may be brief. For a system with a storage cylinder, multiple shower outlets, and TMVs, a more detailed assessment and ongoing monitoring log is expected.

Landlord Obligations in Practice

For residential landlords letting a property with a standard domestic hot water system:

• Risk assessment: Carry out and document a basic assessment. HSE provides a template for simple domestic systems. For higher-risk properties (HMOs, multi-occupancy), commission a formal assessment from a competent person.
• Cylinder temperature: Ensure the hot water cylinder thermostat is set to 60°C. Verify with a thermometer at the hot water outlet (temperature should reach 50°C within 1 minute).
• Tenant advice: Advise tenants to report any issues with hot water temperature or pressure; to flush taps and showers after periods of absence.
• Vacant properties: During void periods, flush all outlets weekly. Consider a property management protocol that covers legionella control during vacancy.
• Showerhead maintenance: Include descaling and inspection in the annual maintenance schedule.
• Record keeping: Keep a simple log of any water system maintenance, temperature checks, and risk assessment review dates.

Combi Boilers and Legionella

Combi boilers do not store hot water — they heat water instantaneously on demand. There is no hot water storage cylinder and therefore no storage temperature risk. The legionella risk from a combi boiler system is very low, since there is no volume of warm standing water. The risk assessment for a combi-boiler-only system is therefore very simple: confirm no dead legs, check cold water supply temperature, and note that no storage is present. For most single-tenancy properties with a combi boiler, this constitutes the entirety of the required assessment.

Summary

Legionella risk management is a legal requirement for anyone in control of premises with a hot or cold water system — including residential landlords. The duty is proportionate to the risk: a simple single-tenancy combi boiler property needs a brief assessment and basic precautions; a large building with storage cylinders, TMVs, and multiple shower outlets needs a formal documented scheme and regular monitoring. The core controls are simple — store hot water at 60°C, keep cold water below 20°C, eliminate dead legs, flush infrequently used outlets, and maintain showerheads. These are also basic good practice for any hot water system.

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