It’s fair to say that if things are done right in the first place, it will make things easier in the long term, and the same can be said for the design of water systems. There are several issues that teams managing our water systems encounter, which would be easier to address if the water systems had been designed and installed correctly in the first place.
Lessons learned from incorrect water system designs
This has been highlighted on several occasions in the press, where major problems have happened that, if appropriately designed, could have been avoided - most notably the recent issues at the Queen Elizabeth University Hospital in Glasgow. The issues found here have led to a full public enquiry, with the findings showing that proper investment into the design, installation, and commissioning of water systems is necessary.
There are many aspects to consider when it comes to proper design:
Water Usage:
Engaging with end users at the design stage can make a significant difference in managing water systems in the future. The last thing we want is for there to be several low-use outlets that will likely never be used, which will need an ongoing regime of flushing to manage, or capital expenditure to remove.
Another aspect of water system design is to ensure that the capacity is suitable for the use. We don’t want to have an overcapacity of water stored within the cold water tank(s), as this will lead to stagnation and an increased Legionella risk. To this end, the system should not be designed to allow for ‘potential future use’. Capacity should be sufficient for the planned occupancy levels, and if future use needs to be considered, then allowing for increased capacity at this time is advisable.
High-use outlets:
Placing high-use outlets at the end of pipe runs can help with water turnover. For example, a typical WC flush is around 4-6 litres. If this is supplied by 15mm copper pipework, which contains approximately 0.14 l/m, a single flush would purge around 28 metres of pipework, based on just a 4-litre flush.
Correct components:
There are many components that could be installed on a water system that will increase the legionella risk. All components installed on our water systems should be made of approved materials (such as following WRAS or Kiwa approval schemes). Where non-approved materials are in use, this can increase the risk of bacterial contamination.
But even if we do have approved materials, there are many that will increase the Legionella risk once installed. A simple example of this is thermostatic mixing valves (TMVs). They are typically installed to prevent scalding risk. Still, the presence of these devices does increase the Legionella risk, as they blend hot and cold water to a suitable temperature for Legionella growth and have surfaces within them that can harbour biofilm, etc. So where they are not needed, they should not be installed. It is therefore essential to conduct a scalding risk assessment during the design stage of the project so that TMVs are not installed where they are not needed, thus reducing the Legionella risk where possible.
While on the topic of TMVs, ensuring there is only one TMV device per outlet can prevent future issues and remedial work. Problems occur when a TMV is fitted on the supply to a TMT (a tap with a built-in TMV). This doubles the maintenance requirements and is unnecessary.
Minimising pipe runs:
Keeping things simple is a comment that many people make, but it makes sense when it comes to water system design. Overcomplicated systems can end up being challenging to manage and require additional monitoring points. Short and direct pipe runs will help with water flow and prevent stagnation.
Preventing heat transfer:
Properly insulating pipework will help prevent heat transfer, but it is also essential to keep hot and cold pipework away from each other so that we can further minimise heat transfer. No insulation is perfect, but avoiding cold pipework running on top of hot by ideally keeping them in separate risers wherever possible will help with heat gain in the cold water systems. Also, keeping cold water pipework away from sources of heat gain, such as radiant panels, will help keep the water cold within.
These are not the only items to consider in our designs, but they do highlight some of the common themes we observe when reviewing design projects.
Reviewing these aspects of the design at the earliest opportunity can pay dividends when it comes to managing the legionella risk of a system once it is operational. Having input from all parties throughout the design stages can help pick up these issues before they become a problem, from initial input at concept design, an initial review at RIBA stage 3, a full review at RIBA stage 4 for a complete technical design, and then oversight of the construction phase of the project to ensure that the plans are being followed. This will also help demonstrate that Legionella risk has been considered during the design process.
Having a Project Water Safety Plan and a Project Water Safety Group in place will help manage the entire process, from design and construction to handover and soft landings.
With any project getting it right first time will help keep our legionella risks as low as reasonably practicable.
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 as of the date of original publication - August 2025.
© Water Hygiene Centre 2025
