Legionella Risks in Modular Buildings: Part 1

The Journey of the Water Systems

Modular construction is becoming an increasingly important part of infrastructure delivery, where the benefits are understandable due to:

• Faster construction
• Reduced disruption on operational hospital sites
• Greater standardisation and the potential for improved quality through factory-controlled manufacture.
  
  

Many industries, particularly in healthcare, with the New Hospital Programme (NHP) and wider NHS investment increasingly considering Modern Methods of Construction (MMC), modular solutions are likely to become a much more familiar feature of the healthcare estate; however, from an Authorising Engineer (Water) perspective, there is a set of important questions we need to ask:

Are Water Safety Processes Keeping Pace?

1. The principles for controlling Legionella and other waterborne pathogens have not changed simply because the building is manufactured differently. What has changed is where the water system is installed, when it may first be exposed to water, how it reaches the site, and who is responsible for it during that journey. This thought creates some important considerations for organisations, designers, modular manufacturers and Water Safety Groups.

The Water System May Start Its Life in a Factory

1. One of the major differences with modular construction is that significant parts of the domestic water system may be installed within the manufacturing facility rather than actually on the site. Depending on the modular solution chosen by the site, this could include hot and cold-water distribution pipework, sanitary fittings, taps, thermostatic control devices (TMTs / TMVs), isolation valves, balancing valves (DRVs / TBVs), along with any other associated equipment.
   
   
   
2. There are clear advantages to this approach as installation takes place within a controlled environment, quality assurance can potentially be more consistent, and standardised designs can improve repeatability. However, a factory-controlled construction environment should not automatically be assumed to be a hygienically controlled environment suitable for everyone, particularly healthcare water systems.
   
   
   
3. During manufacture, consideration needs to be given to the protection of pipework and components from dust, debris and contamination. Open-ended pipework needs to be appropriately protected (specifically for healthcare as per HTM:04-01 Part A – sections 3.43 & 12.31), and installation practices should reflect the fact that the completed system will eventually supply water which could be within a healthcare environment.
   
   
   
4. This should form part of the Project Water Safety Plan (PWSP), along with the Project Water Safety Design Review and quality assurance process via physical audits/inspections of the installation works rather than being considered solely a manufacturer's construction issue.

Has Water Already Been Introduced?

1. This is perhaps one of the most important questions to ask when receiving a modular building: has the water system ever contained water?
   
   
   
2. A newly delivered module may contain a water installation that appears unused, but that does not necessarily mean it has always been perceived to have remained dry, as:
   
   - Water may have been introduced during factory pressure testing, functional testing, TMT / TMV testing or other factory acceptance procedures.
   
   
   
3. What happens afterwards is important, as draining the installation does not necessarily mean every
   part of the system becomes completely dry. Small quantities of water can potentially remain within valve bodies, TMTs / TMVs, tap cartridges, low points and horizontal sections of pipework. The module may then remain within the factory or a storage area for several weeks before transportation to its final stage of the journey.
   
   
   
4. Residual water combined with time, stagnation and favourable temperatures creates an obvious water hygiene consideration. Where practicable, keeping systems dry until controlled commissioning should therefore be considered.
   
   
   
5. Sections 1. to 4. should not be undertaken as wet testing is avoidable until the module(s) are at the crucial stage of introducing water via a controlled method of works which has been agreed via the Project Water Safety Group (PWSG), along with being defined in the PWSP; this should be undertaken as a final stage of works once fully constructed at the site and not at the factory where it is being built.
   
   
   
6. Where practicable, pressure and integrity testing methods that avoid introducing water should be considered during factory manufacture. Any proposed dry-testing methodology should be suitable for the system and undertaken in accordance with the manufacturer's requirements, project specification and an agreed safe system of work. Where wet testing is unavoidable, the introduction of water and subsequent management of the system should be specifically addressed within the PWSP and agreed with the Project Water Safety Group.

Then the pre-fitted module(s) are put on a truck!

1. This is where modular construction introduces a particularly interesting water safety consideration. Once manufacturing is complete, the module may be loaded onto specialist transport and moved by road, potentially over a considerable distance, before arriving at the intended site.
   
   
   
2. We therefore need to think about the water installation not simply as a static system but as something that is going to be transported. During its journey, the module may experience vibration, movement, changes in level, acceleration, braking, and changes in environmental temperature.
   
   
   
3. Mechanical consideration needs to be thought about: what is the method of pipework jointing (Solder / Compression / PressFit), how are the connections supported, movement through the journey.
   
   
   
4. Post-transport verification - a defined inspection and verification process should be undertaken following delivery to confirm that pipework, supports, connections and hygienic protection have not been compromised during transportation.
   
   

What Happens When the Module Arrives?

1. Arrival at the site does not necessarily mean immediate installation, as modules can be stored while enabling works are completed, crane operations are coordinated or construction programmes change.
   
   
   If a module has previously contained water, this storage period becomes part of its water safety history.
   

1. 1. 1. Who is responsible during that period?
         
         
      2. How long has it been stored?
         
         
      3. Are pipe ends and connections still protected?
         
         
      4. Has the module remained secure?
         
         
      5. What inspection is undertaken before it is connected?
         
         
      6. These may appear to be construction logistics questions, but from a water safety perspective they can be equally important.
         
         

Connecting the Modules Creates Another Risk Point

1. The water system installed within the factory is only part of the final system.

1. 1. 1. Once the modules arrive on site, they need to be interconnected and connected to the permanent building infrastructure; this introduces another installation stage along with potential risk.
         
         
      2. Factory-installed water hygiene controls can quickly be undermined if open pipework is left exposed during site assembly or if module-to-module connections are undertaken without appropriate cleanliness controls.
         
         
      3. There therefore needs to be continuity between factory installation standards and site installation standards.
         
         
      4. From a Water Safety Design Review perspective, the interface between the modular manufacturer and the site contractor deserves particular attention.
         
         

But the water safety journey doesn’t end when the modules are connected. The real challenge is ensuring that responsibility, assurance and water hygiene controls continue through commissioning and into occupation.

In Part 2, we’ll look at the wider water safety journey — from factory manufacture through to occupant use — and explore what Authorising Engineer (Water) reviews are highlighting about early engagement, standardisation, design assurance and commissioning.

Feel free to reach out if you have any questions about the issues mentioned above or if you would like to consult with one of our experts on water hygiene.


Editor’s Note: The information provided in this blog is correct at the date of original publication – September 2026.

© Water Hygiene Centre 2026