The Hidden Legionella Risk In Closed-Loop Systems
By Greg Rankin
For a long time, it’s been assumed that closed-loop water systems — those commonly found in building heating systems, air-conditioning units, and cooling systems — are at a low risk for Legionella. This is because they do not typically generate aerosols that can be inhaled, and Legionnaires' disease spreads mainly through breathing in tiny, bacteria-laden water droplets that reach the lungs.
However, the reality is that this assumption is misleading and there are, in fact, many reasons why closed-loop systems can actually inadvertently promote the risk of Legionella.
Fundamentally, 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. For example, many of these types of systems rely on pipe materials such as PEX, PVC, and EPDM, which can actually give bacteria an early foothold, while oversized pipes, dead legs, and low-flow areas create warm, stagnant zones where bacteria like Legionella can thrive.
Temperature control is another critical factor. Many low-temperature closed-loop systems operate between 68–113°F, which is the 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.
Corrosion and dirt in the system make these problems worse. Rust from carbon steel, along with magnetite and general sediment, can build up and create small protected areas where bacteria can hide. These areas can protect biofilms, allowing them to persist despite routine heat and chemical control measures.
All of this, of course, unfolds against a backdrop of skyrocketing Legionella rates in the U.S., up a staggering 1,000% over the past two decades. That's why prioritizing this critical, but often overlooked, Legionella risk has never been more urgent.
Proactive Prevention
The good news is that there are some simple, proactive steps water operators can take to minimize this risk and maintain disinfection efficacy.
Though there may be no silver bullet, in our experience, a large part of the answer lies in good design. Hereby, the priority should be to minimize 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 neutralize 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 favor 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.
An Innovation Advantage
Alongside this, there are a number of emerging technologies that are transforming how water operators can 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 into the 68–113°F range that favors bacterial replication. These systems also help identify stagnation zones or periods of inadequate circulation long before problems escalate. Alongside this, artificial intelligence 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 isn’t new and most water operators will be well versed in the compliance routines required to mitigate risk. What is new, however, is the pace and scale of Legionaires’ disease happening in the U.S..
With this, it becomes even more important to pay due diligence to risk areas that may have otherwise been overlooked — such as closed-loop systems.
By giving water quality and microbial control the same level of attention as any other building water system, and by using faster, more responsive testing alongside better system management, water operators can better protect buildings and the people who use them.
Greg Rankin is the CEO of Hydrosense. For more information, visit https://hydrosense-legionella.com/.