Greg Rankin, CEO of Hydrosense, explores the hidden Legionella risk associated with closed-loop systems and how installers can address it.

Commonly found in building heating systems, air-conditioning and cooling systems, and some heat pumps, closed-loop water systems are often considered low risk for Legionella because they are sealed and don’t normally aerosolise water.

However, in the face of rapidly rising Legionella counts in the UK and globally, this assumption is misleading and there are, in fact, a number of hidden threats in closed-loop systems that could increase the risk of Legionella.

The reality, after all, is that most modern closed-loop systems are designed to move heat efficiently, not to stop bacteria growing. As a result, several routine design and material choices can unintentionally create ideal conditions for biofilm and Legionella growth.

Common polymers such as EPDM and PVC leach small amounts of biodegradable organic carbon that feed early microbial colonisation. Their surface chemistry and roughness further promote bacterial attachment, allowing biofilms to establish quickly and persistently.

System geometry and flow conditions compound the risk. Oversized pipework reduces velocity, lowering the shear stress needed to disrupt biofilm. Dead legs caused by insufficiently removed pipework that previously serviced now-redundant equipment, bypass lines and poorly flushed components trap stagnant water where disinfectant residuals vanish. Expansion vessels, strainers, balancing valves and plate heat exchangers often contain warm, low-flow pockets that act as long-term microbial reservoirs.

Temperature control is another critical factor. Many low-temperature closed-loop systems operate between an ideal range for Legionella growth. The scope for poor insulation, intermittent use or standby periods escalate matters, allowing systems to drift into this zone for hours or days at a time and, in turn, giving the bacteria even more chance to multiply.

The end result is a critical and yet still largely overlooked Legionella threat which installers cannot afford to underestimate.

Best practice recommendations

But, you may ask, what proactive steps can installers and the teams they work with take to minimise this risk and maintain disinfection efficacy?

Though there may be no silver bullet solution, a large part of the answer lies in good design. The priority should be to minimise conditions that allow stagnation or nutrient accumulation. This should include reducing or eliminating dead legs, ensuring consistent circulation across all branches, and avoiding oversized pipework that lowers velocity.

Material selection also matters. Here the recommendation is to choose components with low corrosion potential and surfaces that don’t encourage microbial attachment, and avoiding fiber washers or natural rubber, which can shed nutrients into the system. Incorporating high-quality filtration and magnetic dirt separation also helps prevent particulate build-up, while dedicated flushing points, drain valves and accessible dosing ports make ongoing biofilm management far easier throughout the system’s lifespan.

During commissioning, the system should be cleaned thoroughly before filling to remove oils, debris and residues that act as early-stage biofilm nutrients. A controlled biocide treatment should be applied to neutralise any bacteria introduced during installation. Initial water quality testing is essential – not only for Legionella but also for indicators such as iron and suspended solids, which support microbial growth. Temperature control should then be verified to ensure the system does not drift into ranges that favour bacterial proliferation.

Underscoring this too, a clear management plan should be established from day one, covering regular flushing, inspection, cleaning and appropriately timed chemical dosing, ensuring disinfection remains effective over the long term.

Technological advantages

The good news in this too is that there are several emerging technologies that are transforming how to detect and control Legionella in closed-loop systems by providing faster insight, smarter dosing and better detection of microbiological changes.

Take, for example, advanced filtration and magnetic dirt-separation systems which are now able to remove finer particulates and corrosion debris, depriving biofilms of the surfaces and nutrients they need to establish. Equally, automated biocide-delivery systems are becoming more sophisticated, with some platforms adjusting dosing based on real-time indicators such as ATP or other microbial activity signals. This allows biocide to be delivered only when needed, improving efficacy and reducing chemical overuse.

Continuous temperature and flow sensors can also be used to provide a constant stream of operational data, enabling operators to spot drifts in the temperature range that favours bacterial replication. These systems also help identify stagnation zones or periods of inadequate circulation long before problems escalate. Alongside this, AI and predictive analytics are being explored to interpret temperature, flow, corrosion and water-quality data to flag when a system is moving toward high-risk conditions.

Finally, rapid on-site Legionella tests – capable of detecting all L. pneumophila serogroups, the most dangerous form of Legionella – are now widely used to verify system conditions immediately, validate corrective actions and monitor high-risk points without waiting days for lab culture results. Together, these technologies offer a more proactive, data-driven approach to keeping closed-loop systems microbiologically safe. As well as continuous monitoring they are particularly useful post-remediation to detect the viable but non-culturable forms of Legionella that Lab tests miss, which may have been created with typical remediations such as heat shock or biocide shock dosing.

Making it watertight

Legionella prevention is a shared responsibility that must begin long before a system is handed over. Manufacturers influence risk through material choices, component geometry and the guidance they provide. Engineers shape it through hydraulic design, elimination of stagnation points and specification of monitoring and access provisions. Installers affect it through commissioning hygiene, flushing quality and early biocide control.

The largest knowledge gap, as we see it, is not assigning responsibility, but ensuring each party understands how their decisions affect the next stage. Strengthening that continuity, supported by better data and rapid on-site testing, is key to raising the overall standard of Legionella control.

For further information please visit: https://hydrosense-legionella.com/