Legionella tank cleaning is not optional. For building owners, landlords, and facilities managers in the UK, managing Legionella risk in water systems is a legal duty. In this article we will explain how often cleaning is required, what the regulations say, and why a properly lined tank is necessary.
What is Legionella?
Legionella bacteria are naturally present in water. In small numbers in natural water sources, they pose no real threat. The danger arises when they are allowed to multiply inside man-made water systems, including water storage tanks, cooling towers, hot and cold water pipework and spa pools.
When water containing high concentrations of Legionella is turned into a fine aerosol spray and inhaled, it can cause Legionnaires’ disease. This is a serious and potentially fatal form of pneumonia. It cannot be passed between people, but it can spread rapidly through a building’s water system if conditions allow the bacteria to thrive.
Legionella multiplies most rapidly at temperatures between 20°C and 45°C. Water that sits stagnant, contains sediment or scale or has low turnover creates exactly the right conditions for bacterial growth.
Important:
UKHSA data published in 2025 recorded 472 cases of legionellosis in England and Wales in 2024. The case fatality rate was 2.8%. Legionnaires’ disease is serious. Prevention through proper water tank maintenance is the only reliable way to protect building occupants.
Why Legionella Tank Cleaning is a Legal Requirement in the UK
Legionella tank cleaning is a legal requirement in the UK to prevent the growth of Legionella bacteria, which cause deadly Legionnaires’ disease. Finding from the UKHSA Legionellosis Report 2024 highlight the dangers of this bacteria and the importance of tank cleaning:
- 472 cases of legionellosis reported in England and Wales in 2024
- 2.8% case fatality rate for Legionnaires’ disease in 2024
- 64% of cases in people aged 60 and over in 2024
- 6 month maximum interval between cold water storage tank inspections and cleans
In the UK, the management of Legionella risk is governed by several pieces of legislation and official guidance. The most important are:
ACOP L8:
The Approved Code of Practice and Guidance issued by the Health and Safety Executive (HSE. This sets out the practical steps duty holders must take to control Legionella in water systems.
ACOP L8 applies to all workplaces and premises where water systems are present. This includes offices, hotels, hospitals, schools, leisure centres, residential blocks and manufacturing sites.
The Health and Safety at Work Act 1974:
This places a general duty on employers to ensure the health and safety of employees and others who may be affected by work activities.
The Control of Substances Hazardous to Health Regulations 2002 (COSHH):
This requires employers to assess and control biological agents, including Legionella bacteria.
HSE Technical Guidance HSG274:
This provides detailed technical guidance on managing Legionella in specific water system types.
Together, these require any person with responsibility for a building or water system to:
- carry out a Legionella risk assessment
- implement a water safety plan
- ensure that cleaning and maintenance are carried out at the required intervals
How Often Must Water Tanks Be Cleaned to Control Legionella?
How often a water tank needs to be cleaned depends on the system, the risk level identified in the Legionella risk assessment, and the condition of the tank itself. That said, ACOP L8 and HSG274 set clear minimum expectations.
| Tank Type | Frequency of Cleaning |
| Cold water storage tank inspection and clean | At least every 6 months |
| Hot water calorifier inspection and clean | At least every 12 months |
| Cooling tower inspection and clean | At least every 6 months (quarterly in high-risk settings) |
| Water temperature monitoring | Monthly minimum |
| Microbiological water sampling | As determined by risk assessment (often quarterly) |
| Microbiological water sampling | As determined by risk assessment (often quarterly) |
| Full Legionella risk assessment review | At least every 2 years, or after significant changes |
If a tank is in poor condition, has a history of contamination, or serves a vulnerable population such as care home residents or hospital patients, more frequent cleaning and testing will be needed.
What Increases the Risk of Legionella Growth?
Not all water systems carry the same level of legionella risk. Several factors make it easier for Legionella bacteria to multiply and create a health hazard.
- Water temperature between 20°C and 45°C. This is the ideal growth range. Cold water tanks that are poorly insulated or located in warm roof spaces frequently fall into this range
- Stagnant water. Low water usage, dead legs in pipework, or infrequently used outlets allow water to sit without turnover, creating ideal conditions for bacterial growth
- Sediment, scale and rust. These provide nutrients for Legionella and protect it from disinfection treatments. A degraded or corroded tank interior significantly increases this risk
- Biofilm on tank surfaces. A thin layer of organic material on internal tank walls, known as biofilm, shelters and feeds Legionella bacteria
- Poorly maintained or ageing tanks. GRP, steel, and concrete tanks that have not been relined are far more likely to harbour sediment and support bacterial growth in water tanks
Our article on how tank relining prevents contamination in water tanks explains how surface condition directly affects the level of microbial risk inside a potable water storage tank.
Seasonal risk: Legionella cases tend to rise during warmer months. UKHSA data confirms this seasonal pattern year on year. Building managers should treat the approach of summer as a trigger to review the condition of the tank and cleaning schedules.
What Does Legionella Tank Cleaning Involve?
Cleaning a water tank and preventing legionella follows a defined process set out in the relevant British Standard and HSE guidance. Here is what a compliant clean involves:
- Isolation and draining. The tank is isolated from the supply and fully drained before any inspection or cleaning begins
- Internal inspection. The water tank interior is physically inspected for sediment, scale, biofilm, corrosion, mould growth and any structural defects. This is the point at which underlying problems are identified
- Physical clean. All surfaces are scrubbed and any sediment, debris and biofilm are removed. This step is essential before disinfection, as chemical treatments cannot penetrate physical contamination effectively
- Disinfection. The tank is filled and treated with a chlorine-based disinfectant at the correct concentration and contact time, as specified in BS 8558 and ACOP L8
- Flush and refill. The disinfectant solution is flushed through the system and the tank returned to service
- Documentation. A written record of the clean, including inspection findings, disinfection concentrations, and the name of the person who carried out the work, must be kept for a minimum of five years
If the inspection reveals serious tank deterioration, a clean alone will not be sufficient. The underlying surface condition needs to be addressed.
How Tank Lining Reduces Legionella Risk Long-Term
Cleaning removes contamination. A high-performance tank lining prevents legionella from returning.
The internal surface condition of a water storage tank has a direct impact on how easily Legionella can establish itself. Rough, porous, corroded or degraded surfaces create the perfect environment for sediment to accumulate and biofilm to form.
Applying COVAC’s WRAS-approved Acothane DW coating to the internal surface of a tank solves this at the root. The coating creates a non-porous and chemically resistant barrier that gives bacteria nowhere to anchor and sediment nowhere to gather.
What a Properly Lined Tank Does for Legionella Control
- Smooth, non-porous surfaces prevent biofilm formation and sediment build-up
- Easier, more effective cleaning at each scheduled interval
- Reduced risk of Legionella contamination in water tanks between cleaning visits
- Longer intervals between full refurbishments, reducing total lifecycle cost
- Coating is WRAS listed and fully approved for use in contact with potable water
- Minimum 10-year guarantee on all COVAC lining work
Find full details on our Legionella control page, our potable water tank lining service, and our Acothane DW coating system.
For tanks that need structural work before a lining can be applied, our GRP tank repair, steel tank repair, and concrete tank repair services prepare the surface correctly before coating.
Who is Responsible for Legionella Compliance?
Under ACOP L8, the person responsible for managing Legionella risk in a building is known as the duty holder. This is typically the employer, building owner or the person in control of the premises.
In practice, this means:
- Building owners and landlords have a legal duty to ensure water systems in their properties are assessed and maintained to prevent Legionella risk
- Employers must protect employees and visitors who use or work near water systems in their premises
- Facilities managers are often appointed as the responsible person and must ensure a written water safety scheme is in place and followed
- Managing agents for residential blocks, commercial properties, and mixed-use buildings carry the same duties as direct owners if they are responsible for the common water systems
The duty holder can appoint a competent person or contractor to carry out the practical work. But the legal responsibility for ensuring it is done, documented, and completed to the required standard remains with the duty holder.
Failure to comply with ACOP L8 can result in enforcement action by the HSE, including improvement notices, prohibition notices, and in serious cases, prosecution.
COVAC works with facilities management companies, NHS trusts, hotel groups, schools and universities, and many other building types to help duty holders fulfil their responsibilities and protect the people in their buildings.
FAQs
How often does a water tank need to be cleaned for Legionella compliance?
Under ACOP L8, cold water storage tanks should be inspected and cleaned at least every six months. Hot water calorifiers require a clean at least annually. If a Legionella risk assessment identifies elevated risk, more frequent cleaning will be required.
Is Legionella tank cleaning a legal requirement?
Yes. The legal requirements for Legionella control in the UK are set out in ACOP L8, the Health and Safety at Work Act 1974, and COSHH 2002. Failure to comply is a criminal offence. Building owners, employers, and those in control of premises all carry a duty of care to manage Legionella risk in their water systems.
What temperature does Legionella grow at?
Legionella bacteria multiply most rapidly between 20°C and 45°C. They are killed at temperatures above 60°C. This is why maintaining cold water below 20°C and hot water above 60°C is a central part of any Legionella prevention strategy. Tanks in warm roof spaces or those with poor insulation are particularly vulnerable.
What records need to be kept for Legionella compliance?
ACOP L8 requires duty holders to maintain written records of all water tank inspections, cleaning and disinfection work, water temperature monitoring, microbiological test results, and risk assessment reviews. These records must be kept for a minimum of five years and be available for inspection by the HSE if requested.
Need Help With Legionella Compliance?
COVAC works with building owners and facilities managers across the UK to protect water systems through professional tank lining, repair and refurbishment.
Call us or request a quote today.
