Legionella and Water Hygiene for UK Plumbers and Heating Engineers — The Law and Risk Assessments (2026)

What Legionella Is and How It Spreads

Legionella is a genus of bacteria found naturally in rivers, lakes and the ground, usually in numbers too low to cause harm. The problem arises when it gets into a man-made water system and finds conditions that let it multiply. Those conditions are well understood:

• Warm water — the bacteria thrive at temperatures of roughly 20–45°C. Below 20°C they survive but stay largely dormant; above around 50°C they begin to die, and they are killed quickly at 60°C and above.
• Stagnation — still or slow-moving water gives the bacteria time to colonise and form biofilm. Little-used outlets, dead legs and oversized storage are classic culprits.
• Nutrients — scale, rust, sludge, sediment and biofilm on pipe and tank surfaces all feed bacterial growth. A clean, well-flowing system is a hostile environment for Legionella.

People catch Legionnaires' disease by inhaling contaminated aerosols — the fine spray produced by showers, taps, spa pools, cooling towers and similar equipment. You do not catch it by drinking the water, and it is not contagious from person to person. That is why systems that create spray, especially showers, are the focus of so much control work. People at higher risk include the over-45s, smokers, heavy drinkers, and anyone with a weakened immune system or existing respiratory or kidney disease.

The Law — HSWA, COSHH and the L8 ACoP

There is no single "Legionella Act". The duty comes from general health and safety law applied to this specific hazard. The key pieces are:

• Health and Safety at Work etc. Act 1974 (HSWA) — places a general duty on employers and those in control of premises to ensure, so far as is reasonably practicable, the health and safety of employees and anyone else who may be affected by their work, including from Legionella.
• Control of Substances Hazardous to Health Regulations 2002 (COSHH) — Legionella is a biological agent, so the COSHH duty to assess and control exposure to hazardous substances applies directly to it.
• Approved Code of Practice L8 — "Legionnaires' disease: The control of legionella bacteria in water systems". The ACoP has special legal status: if you are prosecuted and have not followed it, you must show you achieved compliance some other equally effective way, or a court can find against you.
• Guidance HSG274 — the detailed technical guidance that sits beneath L8, in three parts covering cooling systems, hot and cold water systems, and other risk systems such as spa pools. This is where the practical control detail lives.

The legal duty falls on the duty holder: the employer, the person in control of the premises, or whoever has control over the water system through a tenancy or contract. For a commercial building that is usually the business or the facilities manager. For rented homes the landlord is the duty holder. Landlords do have a duty to assess and control the Legionella risk in the properties they let — but note there is no legal requirement for a "legionella certificate". What the law actually requires is a risk assessment and, where the risk warrants it, controls and records. Companies selling landlords mandatory certificates are over-stating the position.

The Legionella Risk Assessment

The legal starting point under L8 is a "suitable and sufficient" legionella risk assessment. It is not a box-ticking exercise — it is the document that decides what controls a system actually needs. A proper assessment should:

• Identify and describe the system — the tanks, cylinders, calorifiers, pipework runs, outlets and any spray-producing equipment, ideally with a schematic.
• Identify who is at risk — occupants, staff, the public, and any especially vulnerable groups using the building.
• Identify the sources of risk — dead legs and blind ends, stored or stagnant water, little-used outlets, water held in the 20–45°C range, scale and sediment, and showers or other aerosol sources.
• Set out the controls and a written scheme — the temperatures, flushing regimes, cleaning and monitoring needed to keep the risk under control, and who is responsible for each.

The assessment must be carried out by a competent person — someone with the right knowledge, training and experience of water systems and Legionella control. It must be reviewed regularly and whenever there is reason to believe it is no longer valid, for example after the system is altered, extended, re-purposed or left out of use for a period. A risk assessment from five years ago for a building that has since had a new extension and changed use is not "suitable and sufficient" any more.

Control Measures — Keep Hot Water Hot and Cold Water Cold

This is the practical heart of the subject for trades, and the single most important rule is simple: keep hot water hot, keep cold water cold, and keep the water moving. Legionella cannot easily multiply outside its temperature window, so temperature control is the primary defence in most domestic and commercial systems.

On the hot side, store hot water at 60°C in the cylinder or calorifier, and design and balance the distribution so that water reaches the taps at 50°C (or 55°C in healthcare premises) within one minute of opening the outlet. On the cold side, keep stored and distributed cold water below 20°C, again ideally reaching that temperature at the outlet within a minute or two. The danger zone you are designing to avoid is the 20–45°C range where the bacteria multiply fastest.

Where outlets are fitted with thermostatic mixing valves (TMVs) for scald protection, remember that the water in the cylinder and distribution pipework must still run at the control temperatures up to the valve — the TMV blends down only at the point of use. The other practical controls every plumber and heating engineer should apply are:

• Avoid stagnation — remove redundant dead legs and blind ends, and flush little-used outlets at least weekly so fresh water moves through the whole system.
• Descale and clean shower heads and flexible hoses regularly — these are prime aerosol sources and collect scale and biofilm. They should be cleaned and descaled, typically quarterly.
• Insulate pipework so hot pipes stay hot and cold pipes stay cold; keep cold pipes away from heat sources such as airing cupboards and uninsulated hot runs.
• Keep storage sized correctly — oversized cold tanks and cylinders increase residence time and let water drift into the warm range.
• Monitor and record sentinel temperatures (the nearest and furthest outlets, and the tank and cylinder) where a written scheme calls for it.

Quick Reference: Key Legionella Control Temperatures

What Trades Must Avoid Doing

Some of the worst Legionella risks are created by well-meaning but careless plumbing and heating work. Avoid the following:

• Do not create dead legs or blind ends. When you remove, cap or isolate a redundant outlet or appliance, cut the pipe back to the live run rather than leaving a capped stub. A capped spur full of stagnant water sitting in the warm zone is a textbook breeding ground.
• Do not oversize storage. Fitting a far larger cold tank or cylinder than the demand requires lengthens the time water sits warming up. Size to the actual demand.
• Do not cross-connect potable and non-potable supplies, or leave temporary connections in place after commissioning.
• Do not leave commissioning debris — swarf, flux, jointing compound, sealant offcuts and pipe shavings — in the system. They become nutrients and harbour biofilm.
• Flush and, where appropriate, disinfect new or altered systems before handover. Pipework that has been pressure-tested and then left full of static water for weeks before the building is occupied is a known risk; thorough flushing (and chlorination on larger or higher-risk systems) before use is essential.
• Warn customers about non-use. Tell them to flush all outlets, including showers, after any period the property has been empty — void properties, second homes and holiday lets in particular. Advise running the shower at low flow into a cloth to limit aerosol, or removing the head and flushing the hose, after a long shutdown.

Domestic vs Commercial and Higher-Risk Systems

Most ordinary domestic systems are lower risk when they are used regularly, kept at the right temperatures and not left to stagnate — water that turns over daily through taps and a shower rarely gives Legionella a chance to build up. That does not remove the landlord's duty to assess, but it usually means the controls are simple.

Risk rises sharply in settings where water is stored, distributed widely, used intermittently, or sprayed. Treat the following as higher risk and apply (or arrange) more formal assessment and control:

• Care homes, nursing homes and other healthcare premises, where occupants are especially vulnerable.
• HMOs and larger let buildings with shared or complex pipework and variable occupancy.
• Gyms, leisure centres, hotels, dentists and any premises with showers, spa pools or spray systems.
• Buildings with low or seasonal occupancy where outlets sit unused for long periods.

Cooling towers and evaporative condensers are in a category of their own. They are highly efficient aerosol generators and have been behind several major outbreaks. They are notifiable — under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992 the local authority must be told of any such device on the premises. Work on these systems is firmly specialist territory.

What This Means for a Small Plumbing or Heating Business

You do not need to become a water hygiene consultant to take Legionella seriously. For most small plumbing and heating businesses the practical checklist is:

• Understand the temperatures and the dead-leg principle. Store hot at 60°C, deliver 50°C at the tap, keep cold below 20°C, and never leave a stagnant capped spur. This alone prevents most of the risk you could create.
• Follow L8 and HSG274 as your reference points for anything beyond a simple domestic job.
• Carry out or arrange risk assessments where you take on the duty-holder role — for example managing a landlord's portfolio or a commercial building's water system. If it is outside your competence, bring in a specialist.
• Keep records — risk assessments, temperature checks, flushing and cleaning logs, and disinfection certificates. Records are your defence if anything is ever questioned.
• Get legionella awareness training. A recognised one-day course gives you and your team the baseline knowledge and is cheap insurance against doing something on site that creates risk.
• Refer higher-risk commercial work to specialists — cooling towers, large healthcare systems and complex schemes are better passed to or partnered with a water treatment company than attempted alone.

Frequently Asked Questions

At what temperature should hot water be stored to control legionella?

Store hot water at 60°C in the cylinder or calorifier. At that temperature Legionella is killed within a few minutes. The distribution system should then be balanced so the water reaches the taps at 50°C within one minute of opening them — or 55°C in healthcare premises. Where scald protection is needed, fit thermostatic mixing valves at the outlets so the stored and distributed water stays hot while the temperature at the point of use is blended down safely.

Do landlords need a legionella risk assessment?

Yes. Landlords are duty holders for the properties they let and must assess and, where necessary, control the Legionella risk under HSWA, COSHH and the L8 ACoP. However, there is no legal requirement for a "legionella certificate". What the law requires is a suitable and sufficient risk assessment, sensible controls where the risk warrants them, and a review when the system or its use changes. For a simple, regularly occupied home the assessment is often straightforward, but it still has to be done and recorded.

What is a dead leg and why does it matter?

A dead leg is a length of pipework that no longer has regular flow through it — typically a section left capped after an appliance or outlet was removed, or a blind end with no draw-off. Water sits there stagnant, often drifting into the 20–45°C range, with nothing to flush it through. That makes it an ideal place for Legionella to multiply and for biofilm to form, which can then seed the rest of the system. When you alter pipework, always cut redundant branches back to the live run rather than leaving a capped stub.