Hydrosense Warns Closed-Loop Systems May Harbor Hidden Legionella Risks

Hydrosense is urging system designers, specifiers and building operators to reassess how closed-loop water systems are designed and managed, warning that common design practices and material choices can unintentionally create favorable conditions for biofilm formation and Legionella growth.

Closed-Loop Systems Often Assumed Low Risk

Closed-loop water systems are widely used in building heating systems, air-conditioning and cooling applications, and some heat pump installations. Because these systems typically do not aerosolize water, they are often considered to pose a lower risk for Legionella compared to domestic hot water systems.

Hydrosense cautions, however, that this long-held assumption can obscure a range of hidden risk factors—particularly as Legionella counts continue to rise across the United States and water systems grow more complex.

Design and Materials Can Enable Bacterial Growth

Greg Rankin, CEO at Hydrosense comments: “The reality is that modern closed-loop systems are designed to move heat efficiently, not to stop bacteria growing. Some common pipe materials such as PEX, PVC and EPDM 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 a critical factor too. Many low-temperature closed-loop systems operate between an ideal range for Legionella growth. The scope for intermittent use or standby periods makes matters worse, giving the bacteria even more chance to multiply.”

Hydrosense notes that oversized pipework, limited circulation and intermittent operation can all undermine water quality over time, particularly in systems operating at lower temperatures intended to improve energy efficiency.

Greater Focus Needed During Design and Commissioning

In response to these risks, Hydrosense is calling for increased attention to microbiological safety during the design and commissioning stages. Recommended measures include minimizing dead legs, avoiding oversized piping, selecting low-leach materials, improving filtration, and implementing robust flushing and monitoring programs.

The company also highlights the value of rapid, on-site Legionella testing to provide immediate visibility into system conditions and identify potential issues that traditional laboratory culture methods may fail to detect.

Shared Responsibility Across the System Lifecycle

"Legionella rates are rising fast worldwide as global temperatures climb and water systems become more complex," adds Greg. "That makes it more important than ever to dispel the old assumption that closed-loop systems aren’t at risk.

"Of course, Legionella prevention isn’t the responsibility of one party. It starts with manufacturers and designers, continues through installation and commissioning, and ultimately relies on operators having the right insight and tools to manage systems safely throughout their lifespan. By taking due diligence at every stage, we can reduce risks and protect building occupants from preventable exposure."

For more information, visit Hydrosense at hydrosense-legionella.com.