Hidden Threats in Closed-Loop Water Systems

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20 February 2026 | Updated 19 February 2026

Hydrosense is advising building operators to rethink how closed-loop systems are designed and managed, warning that many common design and material choices unintentionally create ideal conditions for biofilm and Legionella growth.

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 don’t normally aerosolise water.

However, in the face of rising Legionella counts in the UK, Hydrosense has asserted that there are, in fact, a number of hidden threats in closed-loop systems that could increase the risk of Legionella.

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 20- 45°C which is the ideal range for Legionella growth. The scope for intermittent use or standby periods makes matters worse, giving the bacteria even more chance to multiply.”

In response, Hydrosense is calling for a greater focus on microbiological risk during design and commissioning, including minimising dead legs, avoiding oversized pipework, selecting low-leach materials, improving filtration and ensuring robust flushing and monitoring regimes. As part of this, rapid on-site Legionella testing can provide immediate visibility of system conditions and detect issues that traditional lab culture methods can miss.

Greg adds: "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."

Picture: A close up image of Legionella.

Article written by Dave Mapps | Published 20 February 2026

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