When Legionella Keeps Coming Back: Why the Answer Is Root Cause, Not Another Clean

Introduction

A positive Legionella result triggers immediate action. Flushing. Disinfection. Repeat sampling. In most cases, that response is correct and it works.

But some buildings keep coming back. The same outlet. The same section of pipework. Month after month, the result sits outside the action level, the disinfection happens, and the next sample brings the same reading.

When that’s the pattern, the problem isn’t the disinfection. Something in the system is creating the conditions for Legionella to grow — and until that’s identified and fixed, the cycle continues.

Why Repeat Contamination Happens

Legionella bacteria thrive in specific conditions: water temperatures between 20°C and 45°C, low or no flow, and the presence of nutrients such as scale, sediment, or biofilm. A water system that regularly returns positive results usually has one or more of these conditions built into it somewhere.

The most common root causes are:

Dead legs. Sections of pipework that branch off the main distribution system with no active outlet at the end. Water sits in these sections without moving, temperature control is lost, and Legionella can colonise and multiply. Dead legs are one of the most frequently identified findings in a Legionella risk assessment — and one of the most commonly deferred remedial actions.

Thermostatic mixing valves running out of calibration. TMVs blend hot and cold water to deliver safe outlet temperatures, typically between 38°C and 44°C. A valve that has drifted out of specification may be delivering water at a temperature that falls within the Legionella growth range — or allowing cold water contamination of the hot supply. Regular TMV servicing and temperature verification is a core part of any water hygiene programme.

Cold water storage tanks running too warm. Cold water should be stored and distributed below 20°C. Tanks located in warm plant rooms, roof voids, or areas with poor ventilation can drift above this threshold, particularly during summer months or in buildings with high internal heat loads. A tank consistently running at 22°C or 23°C is not a minor deviation — it is a Legionella risk.

Insufficient flushing of low-use outlets. Outlets that are used infrequently allow water to sit in the final sections of pipework without movement. In a building with low occupancy or areas that have been closed off, this stagnation can occur across multiple points simultaneously. Flushing records should demonstrate that every outlet on the system is being reached at appropriate intervals.

Biofilm. A mature biofilm provides both a habitat and a nutrient source for Legionella bacteria. Once established, biofilm is not reliably removed by standard disinfection alone. A system with a persistent contamination problem may require physical cleaning and descaling in addition to chemical water treatment.

The Problem With Treating the Symptom

The standard response to a failed Legionella sample — flush, disinfect, resample — is the right immediate action. It addresses the contamination that has been detected and demonstrates that the building is being managed responsibly.

What it does not do is address the underlying condition.

If a dead leg is providing a reservoir of contaminated water, disinfecting the outlet downstream will produce a clean sample. But the dead leg remains. The next sample may be clean, or it may not — depending on how far the contamination has spread and how quickly conditions return to the growth range.

Each time the cycle repeats, the evidence trail becomes more difficult to defend. An auditor or an enforcing authority looking at a pattern of recurring positive results will ask why the root cause was not investigated. The answer — that disinfection was carried out each time — is not sufficient if the underlying problem was identifiable and not addressed.

What a Root Cause Investigation Looks Like

When Hydraclean is asked to investigate a building with a recurring Legionella problem, the starting point is the water system as it actually exists — not as it was designed or as the last risk assessment described it.

That means reviewing the current schematic against the physical pipework, identifying sections of the system that may have been altered, extended, or taken out of use without being properly decommissioned. It means checking TMV calibration across the site, not just at the outlets that have returned positive samples. It means reviewing temperature monitoring records to understand what the system has actually been doing between sampling visits.

In many cases, the root cause is already documented somewhere. A risk assessment flagged a dead leg. A service report noted a TMV running outside tolerance. A temperature log showed a cold water tank drifting above 20°C on warm days. The information exists — but it hasn’t been acted on.

At a London hotel, a Legionella risk assessment identified multiple dead legs across the hot and cold water distribution system. Rather than wait for those dead legs to produce a failed sample, the remedial programme was planned and scheduled as a structured project. Each identified dead leg was removed systematically, the pipework was reconfigured to ensure flow reached all parts of the system, and the works were documented and signed off.

This is the difference between reactive and proactive management. The dead legs were a known risk. Addressing them before they became a contamination event is straightforward facilities management — and it is significantly less disruptive and less expensive than managing a recurring positive result.

The Role of Documentation

A recurring Legionella problem is not just a water hygiene issue. It is a documentation issue.

Under ACoP L8 and HSG274, dutyholders are required to demonstrate that the risks in their water system have been identified, evaluated, and controlled. A pattern of recurring positive results without evidence of root cause investigation is difficult to defend — both to an external auditor and, in the worst case, to an enforcement body.

Good documentation does not prevent Legionella from growing. But it demonstrates that the building was managed by someone who understood the risks, took appropriate action, and followed through. When the investigation is carried out and the root cause is identified and addressed, that process needs to be recorded with the same rigour as the sampling results themselves.

Hydraclean clients have access to the Delta compliance portal, which holds every site visit record, sampling result, temperature log, and action point in one place. When a root cause investigation is carried out and remedial works are completed, the full record is available to download — ready for an audit the same day it’s requested.

When to Call in External Support

Most facilities managers are managing water hygiene alongside a significant list of other responsibilities. A recurring Legionella problem is not always a sign that something has been done wrong — water systems are complex, and even well-managed buildings can develop issues over time.

The point at which external specialist support becomes necessary is when the pattern of results cannot be explained by what the current management programme would predict. If the system is being flushed, sampled, and maintained in line with the written scheme, and the results are still coming back outside the action level, the investigation needs to go deeper than the programme itself.

That is a technical conversation, and it benefits from a fresh pair of eyes on the system.

How Hydraclean Can Help

Hydraclean provides Legionella risk assessments, water sampling, remedial works, and water hygiene management programmes for commercial buildings, education estates, healthcare facilities, hotels, and public sector properties across the UK.

If your building is producing recurring positive Legionella results, or if you have outstanding remedial works from a previous risk assessment that have not been completed, we can help you identify what needs doing and put a structured programme in place to address it.

To speak to our technical team, call 0161 430 5100, email Sales@hydraclean.co.uk, or visit hydraclean.co.uk.

Hydraclean Ltd, Unit 7, Cromwell Road, Bredbury, Stockport, SK6 2RF