Legionella Risk: What Plumbers Need to Know

Legionella pneumophila is a bacterium that causes Legionnaires' disease — a potentially fatal form of pneumonia — and the milder Pontiac fever. It is found naturally in the environment, particularly in water, but becomes a public health hazard when it proliferates in engineered water systems and is inhaled as fine aerosol droplets.

Legionella thrives in warm water — typically between 20°C and 45°C — and in conditions that favour bacterial growth: sediment, biofilm, dead legs, and stagnant water. The bacterium is killed at 60°C and above, and is inhibited at temperatures below 20°C.

Plumbers are directly relevant to Legionella control because they:

• Design and install domestic and commercial hot and cold water systems
• Commission unvented hot water cylinders and vented storage cylinders
• Install and service thermostatic mixing valves (TMVs)
• Connect showers, baths, and taps — all potential aerosol sources
• Carry out maintenance work on existing water systems where Legionella bacteria may be present in biofilm or sediment
• Advise building owners and landlords about their water system safety obligations

A plumber who designs a system with dead legs, incorrect storage temperatures, or inappropriately specified TMVs may indirectly create the conditions for a Legionella outbreak. Understanding the risk is not optional — it is part of professional competence and legal compliance.

Legionella risk is regulated under several overlapping pieces of legislation:

• Health and Safety at Work etc. Act 1974: The general duty to ensure the health, safety, and welfare of employees and others. This applies to employers and to self-employed people whose work could affect others.
• Control of Substances Hazardous to Health Regulations 2002 (COSHH): Legionella is a biological agent under COSHH. Duty holders (employers and building controllers) must assess the risk from exposure to Legionella and implement controls. COSHH covers not just employees but any person who might be exposed — including building occupants and the public.
• Management of Health and Safety at Work Regulations 1999: Requires risk assessment and implementation of preventive measures.

The key guidance document is the L8 Approved Code of Practice (ACoP): Legionnaires' disease — The control of legionella bacteria in water systems, published by the HSE. The L8 ACoP does not itself create legal obligations, but failure to follow it can be used as evidence in criminal proceedings. Complying with the ACoP is accepted as meeting the underlying legal duty.

The duty holder for Legionella purposes is whoever has control of the premises and/or the water system. This is typically the building owner, employer, or landlord. However, plumbers who design or install water systems also have duties under the Health and Safety at Work Act and COSHH to avoid creating or exacerbating the risk — and they have a professional duty to advise clients about their obligations.

Understanding your client's obligations helps you to design systems correctly and advise them appropriately. The L8 ACoP requires duty holders to:

• Identify and assess the risk: Carry out a Legionella risk assessment of the water systems in the premises. This must consider: whether conditions that favour Legionella growth are present, who might be at risk (particularly vulnerable people — immune-compromised, elderly, very young), and what controls are appropriate.
• Manage the risk: Implement a written scheme for controlling Legionella. This includes temperature monitoring, regular flushing of low-use outlets, inspection of tanks and systems, and record keeping.
• Implement and manage the control scheme: Ensure the control scheme is properly implemented, that records are kept, and that the scheme is reviewed when the system changes or when results suggest controls are inadequate.
• Keep records: Records of risk assessments, control schemes, inspections, temperature readings, and any remedial action must be kept for at least five years.

For private landlords, the Health and Safety Executive confirms that the L8 ACoP applies — domestic premises are not exempt. Landlords should conduct a Legionella risk assessment before letting a property, review it when the system changes or after each tenancy, and take action to address identified risks (typically temperature controls, removing dead legs, and flushing outlets).

As a plumber, when you are working on a client's water system, you should: understand their current Legionella risk status, inform them of any design decisions that affect Legionella risk, and document the system you install or modify so they can update their risk assessment.

Temperature control is the primary Legionella prevention measure in domestic and commercial water systems. The L8 ACoP specifies the following temperature requirements that plumbers must design and commission to:

• Hot water storage: Water in hot water cylinders and calorifiers must be stored at 60°C or above. At this temperature, Legionella bacteria are killed within two minutes. Below 60°C, bacteria can survive and multiply.
• Hot water distribution and delivery: Hot water must be capable of reaching 50°C at all outlets within one minute of running. This limits the time users can be exposed to water at temperatures that favour Legionella growth.
• Cold water storage: Cold water in tanks and cisterns must be stored below 20°C. Above 20°C, Legionella can begin to multiply.
• Cold water distribution: Cold water should be distributed and used at temperatures below 20°C where possible.

These temperature requirements create a practical tension with scalding prevention — particularly in premises used by vulnerable people. Thermostatic mixing valves (TMVs) resolve this: they allow hot water to be stored and distributed at safe anti-Legionella temperatures (60°C/50°C) while blending it to a safe delivery temperature (38–43°C for most uses; the NHS recommends 41°C for care settings) at the point of use.

Key installation requirements for TMVs:

• Use TMVs rated to the appropriate NHS or WRAS standard for the application (care settings require NHS model scheme TMVs; domestic TMVs are not appropriate for healthcare)
• Commission TMVs to deliver water within the correct temperature range at the outlet
• Install TMVs as close to the outlet as possible to minimise the volume of blended water in the dead leg between the TMV and the outlet
• Document the commissioning temperature settings for the building owner's records

The best Legionella management starts at design stage. When planning or installing a domestic or commercial hot and cold water system, apply these principles:

• Eliminate dead legs: A dead leg is a section of pipe that ends in a capped-off point or a rarely-used outlet. Water in dead legs stagnates and creates ideal conditions for Legionella growth. Design systems to eliminate dead legs, or keep them as short as possible (no more than three times the pipe diameter is a common design rule of thumb).
• Minimise storage volume: The larger the water storage volume, the longer the water retention time and the greater the risk. Where possible, specify systems that minimise stored water — combi boilers and instantaneous water heaters eliminate hot water storage entirely for most domestic applications.
• Design for regular use: Outlets that are used regularly flush themselves naturally. Design layouts that give all outlets reasonable regular use; flag rarely-used outlets to the building owner as requiring a regular flushing regime.
• Material selection: Certain materials (copper, stainless steel, some ceramics) are less hospitable to Legionella biofilm than others. Avoid materials that support biofilm growth, and check that any new materials are WRAS-approved for potable water use.
• Commission correctly: Before handing over any new or altered hot water system, verify that storage and delivery temperatures meet the L8 ACoP requirements. Record the commissioning temperatures and provide a written handover note to the building owner.

Legionella awareness and risk assessment training is available through several routes for UK plumbers:

• CIPHE Legionella Awareness Training: The Chartered Institute of Plumbing and Heating Engineering offers CPD-recognised training on Legionella risk and water hygiene for plumbers and heating engineers. One-day awareness courses are widely available from CIPHE-accredited trainers.
• Legionella Control Association (LCA): The LCA provides guidance, training, and a register of companies competent to carry out Legionella risk assessments and control services. More relevant for specialist Legionella risk assessors than for general plumbers, but useful background material.
• HSE Guidance: The HSE publishes free guidance documents on Legionella control, including the L8 ACoP and the Technical Memoranda HSG274, which covers: Part 1 (evaporative cooling systems), Part 2 (hot and cold water systems — the most relevant for plumbers), and Part 3 (other risk systems). Download free from hse.gov.uk.
• WaterSafe and water company training: As part of the WaterSafe approved contractor scheme, plumbers may access water hygiene training relevant to working on the public water supply network.
• Unvented cylinder training: Any plumber installing or commissioning unvented hot water systems must hold an appropriate qualification (e.g. City and Guilds 6189, BPEC, or SNIPEF equivalent). Unvented system training covers Legionella temperature requirements as part of the commissioning syllabus.

For most plumbers, a one-day Legionella awareness course plus reading of the HSE's HSG274 Part 2 provides sufficient grounding to work safely and responsibly on domestic hot and cold water systems. More specialist training is appropriate for plumbers working in healthcare, care homes, or commercial properties with complex water systems.