In today’s hydronic, radiant and closed-loop plumbing systems, the assumption that microbial risk is minimal simply because water remains confined within a loop is misleading. As Michael Cudahy of the Plastic Pipe and Fittings Association (PFFA) notes, "In a closed loop system, occupant and worker exposure to the water is typically limited, but not zero, and good design and operations can help lower or limit the risks."

Even without domestic water contact, Cudahy reminds designers that "accidental releases or other exposure is possible in closed systems for occupants or building maintenance teams."

Material selection and biofilm habitat

One of the major influences on microbiological behavior in these systems is pipe material. Cudahy advises engineers to "avoid or tightly control materials that may develop rough surfaces," because roughness, corrosion and leachables all feed biofilm development. His view is clear: "Copper may show initial suppression effects as the new material surface corrodes away, but ultimately all materials develop adherent scale and biofilms, and longer term, the surface roughness becomes a factor."

Recent research supports this. A 2024 study in npj Biofilms & Microbiomes found that new polymeric plumbing materials rapidly developed biofilms, with Legionella pneumophila reaching approximately 3.1 × 10⁴ MPN/cm² after just four weeks of exposure. A 2025 investigation of combined material-disinfectant effects similarly demonstrated that pipe material strongly shaped the active microbial community in both water and biofilms.

Greg Rankin, CEO of Hydrosense, offers an engineering-focused view of why this happens. "Common polymers such as EPDM, PEX, PVC and polypropylene leach small amounts of biodegradable organic carbon that feed early microbial colonisation," he explains. "Their surface chemistry and roughness promote bacterial attachment, allowing biofilms to establish quickly and persistently."

Rankin adds that carbon steel poses its own risks. "Carbon steel performs worst from a microbiological perspective. It corrodes whenever oxygen is present, forming tubercles and rough surfaces that create sheltered micro-niches ideal for biofilm development." By contrast, "stainless steel offers the strongest overall resistance," he says. "Copper next, plastics are in the middle, and carbon steel performs worst."

For engineers and designers, the conclusion is direct: a material chosen solely for thermal or hydraulic performance may unintentionally shape the system’s biological behavior for decades.

Flow, stagnation and temperature as risk multipliers

System geometry, hydraulics and temperature influence biofilm and pathogen risk just as strongly as material. Stagnation, oversized piping, intermittent circulation and poorly purged branches create environments where disinfectant residuals decay and microbial communities thrive.

Cudahy stresses the basics: "Avoid piping dead-ends where water may stagnate and be difficult to drain or treat." Rankin expands on the hydraulic consequences: "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."

Modern research validates these observations. A 2025 ASPE study showed that higher flow velocities correlated with higher disinfectant residuals, lower microbial ATP, lower turbidity and reduced nitrification across multiple pipe materials. Meanwhile, a 2024 model-system experiment found that daily flushing reduced culturable Legionella in biofilms compared with weekly flushing, but viable-but-non-culturable (VBNC) Legionella increased under daily flushing—revealing the complexity of microbial response within closed systems.

Temperature is another critical control point. "In a closed or open system, avoiding the warm temperature ranges where pathogens grow rapidly is important," Cudahy says. "Keep the system hot enough to kill the pathogens or keep it cool, below 77°F."

Rankin emphasizes that "temperatures in the 68–113°F (20–45°C) range create the ideal environment for Legionella pneumophila replication." Even thermal shock strategies require caution. "It is suppression, not complete sterilisation," Rankin notes, explaining that Legionella can survive as a viable but non-culturable organism after exposure to extreme heat and later recolonize the system.

Why modern hydronic loops are increasingly vulnerable

Modern building decarbonization trends exacerbate microbial risks. Low-temperature hydronic circuits operating in the 30–50°C range are now common, particularly with heat pumps. As Rankin notes, these "frequently operate between 30–50°C, the optimal range for L. pneumophila."

At the same time, modern system architecture adds complexity: expansion vessels, large manifolds, plate heat exchangers, bypass lines and oversized headers all create warm, low-flow pockets. These become persistent microbial reservoirs, difficult to purge and largely unseen during routine operation.

The dynamic nature of biofilm also creates unpredictable contamination spikes. Rankin explains that "sudden increases in flow after stagnation can shear material from the biofilm surface" and release large pulses of microorganisms, including Legionella. These sloughing events are a major source of intermittent positive results in buildings believed to be well controlled.

Closed-loop hydronic systems are dynamic water environments where material, temperature, flow and chemistry interact continuously—and often invisibly—to create or control risk.

Commissioning, monitoring and technological shifts

Design is only the first step. Commissioning hygiene and ongoing monitoring determine whether the system remains microbiologically stable over time.

Rankin outlines the commissioning priorities: "During commissioning, the system should be cleaned thoroughly before filling to remove oils, debris and residues. A controlled biocide treatment should be applied. Initial water quality testing is essential."

Cudahy underscores the importance of standards-based water-management thinking, urging designers and operators to follow ASHRAE 188 and the newer ASHRAE 514. "ASHRAE 514 includes ASHRAE 188 by reference as a requirement," he says, noting that 514 expands risk management beyond Legionella to encompass chemical, physical and other microbial hazards across the building lifecycle.

Emerging technologies are reshaping the monitoring landscape. Rankin points to "advanced filtration and magnetic dirt-separation systems," automated biocide delivery platforms that "adjust dosing based on real-time indicators," continuous flow and temperature sensors, and artificial-intelligence-driven analytics. Rapid on-site Legionella testing is particularly transformative, "capable of detecting all L. pneumophila serogroups" and essential for validating remediation events or identifying VBNC organisms that culture tests miss.

Recent modelling suggests that ignoring biofilm detachment can underestimate Legionella concentrations in building water systems by more than four orders of magnitude—an insight that underscores the value of real-time monitoring and biofilm-aware hydraulic design.

Where codes and standards are headed

Both experts agree that codes and standards will tighten. "We have seen ASHRAE 188 become a critical risk-management document," Cudahy notes, predicting it "is likely to become mandatory for all non-residential buildings with centralized water systems." He anticipates a similar gradual adoption path for ASHRAE 514.

Rankin expects codes to become more explicit about material performance, dead-leg elimination, continuous monitoring access and required flow conditions. He anticipates increased emphasis on "materials with lower biofilm affinity, reduced leachables and clearer performance data," as well as maintenance regimes supported by sensors, automated dosing and rapid testing. "Standards of maintenance will be improved," he says, "with greater use of technological systems to ensure more accurate maintenance and risk predictions."

Closed-loop hydronic systems are dynamic water environments where material, temperature, flow and chemistry interact continuously—and often invisibly—to create or control risk.

"Together, these factors mean many closed-loop systems are biologically active by design," Rankin says. And as Cudahy reminds the industry, exposure is still possible, and the responsibility to manage risk spans design, commissioning and operation.

For plumbing engineers, designers and contractors, the path forward is clear: treat closed-loop systems as living water systems, integrate microbiological thinking into every phase, and apply the same rigor to water-quality risk as to hydraulic performance. With standards evolving and awareness rising, the systems that succeed will be those designed and operated with microbiological control at their core.