Legionella Control Starts with Smart Building Services Design

Legionella is a hidden but serious health hazard that thrives when conditions are just right - or rather, when they are wrong. It’s a persistent yet preventable risk within plumbing and building services systems that we as M&E engineers need to manage. We take this responsibility for safety seriously, ensuring that every water system we design is focused on efficiency and prevention. Legionella control starts at the first stage, not as an afterthought once systems are installed.

Keep Systems Clean and Flowing

Effective Legionella control can be summed up in one principle: “keep systems clean, flowing, the hot hot and the cold cold.” Legionella bacteria become dormant below 20°C and are killed within seconds at 60°C. The crucial aspect is maintaining proper temperature and flow.

According to HSE guidance the following measures are essential:

• Hot water storage should maintain at least 60°C

• Hot water distribution should ensure water reaches 50°C (or 55°C in healthcare) at the outlet within one minute of turning it on.

• Cold water should be stored and distributed to reach the outlet at less than 20°C within two minutes of turning on the outlet.

In a well-plumbed system these temperature targets should be achievable in seconds. Our approach ensures that flow, insulation, and system balance are optimised to achieve these standards consistently.

Balancing Safety and Comfort

While temperature control is the primary defence against Legionella, it brings another challenge, namely the risk of scalding. This is particularly important in healthcare environments, care homes, and residential buildings.

To protect users, thermostatic mixing valves (TMVs) should be installed, which mixes hot and cold water together to control the water temperature to safe levels. They should be placed close to the point where the water comes out so that it is mixed just before discharging, to reduce the risk of having the water sit in the danger zone.

Alternative means of control may be necessary, such as the use of biocides including copper silver ionisation or chlorine dioxide, as appropriate and agreed within the Water Safety Plan.

Legal Responsibilities and Risk Management

Under the Health and Safety at Work Act all employers, landlords, and those in control of premises, including commercial, industrial, and domestic rented properties, are legally responsible for identifying and controlling Legionella risks.

This duty includes:

• Completing risk assessments in accordance with regulations

• Implementing a Water Safety Plan aligned with BS 8680:2020 Code of Practice

• Ensuring that all testing, maintenance, and recommissioning work is carried out by competent, qualified professional

These legal duties highlight the importance of system design and maintenance. Beyond compliance, engineers play a key role in reducing Legionella risks through smart plumbing layouts, and integrating these considerations early in the design phase ensures long-term safety and adherence to health regulations.

The Hidden Risk of Empty Buildings

An often-overlooked aspect of Legionella control comes when buildings are left vacant or underused. When water isn’t regularly drawn through a system, stagnation occurs, creating the perfect conditions for bacterial growth.

Signs of stagnation include unpleasant odours, discolouration, and visible sediment or biofilm buildup. Stagnant water can also lead to corrosion within pipework, particularly at joints, and cause a rise in harmful chemical contaminants such as lead.

Before reopening or reoccupying a building after an extended shutdown, it’s vital that water systems are flushed, tested, and recommissioned safely.

We recommend:

• Appointing an engineer to carry out a full risk assessment before recommissioning.

• Developing a task-specific risk assessment for flushing and testing activities.

• Conducting pressure testing and leak inspections prior to system restart.

• Following WSP procedures to confirm water quality and operational safety before occupancy.

Designing Out the Risk

The most effective control against Legionella begins long before water flows through the pipework. We design systems with proactive prevention in mind, considering flow paths, pipe sizing, insulation, and maintenance access from the start.

Our design process focuses on eliminating dead legs and areas of low use, ensuring continuous flow and proper circulation throughout the system. We specify materials that resist biofilm formation and integrate monitoring systems to support long-term compliance and performance. By engineering systems that remain balanced, efficient, and easy to maintain, we significantly reduce the risk of bacterial growth and waterborne hazards.

Our expertise spans a range of sectors, including healthcare, education, and commercial developments. From initial concept through to commissioning and maintenance guidance, we deliver complete M&E solutions designed with safety at their core. Clients can be confident their systems are designed for longevity, efficiency, and compliance. Every project we deliver reflects our commitment to creating sustainable and reliable water systems that protect both people and property.

If you’re planning a new development, refurbishment, or reviewing existing water systems, we can help you ensure safety from the ground up.