You take a positive Legionella result seriously: disinfect the outlet, flush it through, re-sample, get a clean count, and sign the action off. Three months later the same tap is positive again. Sometimes it never really went away.
A result that keeps coming back is not bad luck, and it is rarely a lab error. It usually means the remedial work cleared the reading without clearing the thing that produced it. The bacteria you sampled live in biofilm and sediment on the inside of the system, not in the water flowing past; knock back the water and the reservoir quietly reseeds it [5]. So a recurrence is worth reading as a diagnosis. It is pointing at where control actually broke.
Read the recurrence as a symptom, not a setback
When a positive returns, the instinct is to repeat the last fix harder — a stronger shock dose, another flush, a faster turnaround on the resample. That treats the outlet as the problem. But the outlet is usually just the place where a hidden reservoir vents into water you happened to sample. Until you find and remove that reservoir, you are disinfecting downstream of the source on a loop.
This is the shape of most remedial action failures: the task was completed and recorded, but the cause was never named. A logbook full of “disinfected and flushed” entries against the same outlet is not a record of control. It is a record of the same fault being papered over four times.
Work the diagnosis in order
Recurring positives have a short list of usual causes, and it pays to check them roughly cheapest-and-most-common first.
- You re-sampled inside the disinfection window. A sample taken soon after a shock dose can read clean simply because residual biocide was still present, or because the sample was not neutralised before it reached the lab. That is a process check, not proof of durable control. HSE technical guidance sets out where sampling supports verification after remedial work [2], and the sampling code of practice covers neutralising samples taken after biocidal treatment [6].
- You disinfected the outlet, not the reservoir feeding it. A dead leg, a long flexible shower hose, a strainer, an expansion vessel, or the blended-water pipework downstream of a TMV can each hold warm, still water that reseeds the outlet within weeks.
- The control that failed first is still failing. If the original cause was a calorifier running cool or a low-use outlet left stagnant, and nothing has genuinely changed on site, the result will come back on schedule.
- Nobody named the root cause. The action says what was done, not why the outlet went positive — so the same conditions rebuild unnoticed.
- The system itself is the limiting factor. Some recurring positives are a design problem: oversized pipework, redundant branches, or blending arrangements that guarantee tepid water somewhere. Design flaws: how poor system design can cause Legionella problems covers the patterns that make a building re-grow Legionella faster than any cleaning regime can keep up.
From a returning positive to a close-out that holds
The pattern of the recurrence usually tells you which reservoir you are dealing with. Match what you are seeing to the most likely cause, confirm it with one specific check, then act on the source instead of the symptom.
| What the recurrence looks like | Most likely cause | The check that confirms it | What to do |
|---|---|---|---|
| Clear straight after disinfection, positive again weeks later | Resample taken inside the biocide window; biofilm regrew | Compare the resample date to the disinfection date; check whether the sample was neutralised | Treat the post-disinfection sample as a process check; resample only after the system is back in representative service |
| One outlet keeps returning while its neighbours stay clear | A local reservoir feeds that outlet (dead leg, flexible hose, strainer, blended-water leg below a TMV) | Trace the pipework back from the outlet; look for a length that never sees full flow or full temperature | Remove or shorten the dead leg, swap the flexible/EPDM part, and bring the branch into routine flushing |
| Several outlets on one circuit return together | The calorifier or cold store is the shared reservoir (base sediment, stratification, running cool) | Check stored vs incoming temperatures and any drain-down/inspection findings | Clean and de-sludge the vessel, correct and verify temperature, then re-disinfect that circuit |
| Positives return even though “all tasks complete” | The control that originally failed is still failing in practice | Spot-check the actual temperature or flushing at the outlet, not the logbook entry | Restore the real control first, prove it is holding, then resample |
| The whole system drifts positive again periodically | System design or the written scheme is the limiting factor | Review the risk assessment and scheme against the system as it is actually built | Bring in competent specialist help; consider design remediation |
The thread through every row is the same: a positive that recurs is telling you the source survived the last intervention. Find the source, and the resample looks after itself.
What a genuine close-out actually contains
A negative resample on its own does not close anything; it describes one outlet at one moment [3]. A close-out that holds shows four things: the root cause named in plain words, the failed control restored and shown to be holding, a resample taken once the system is back in normal service confirming the result, and a review trigger so the same drift is spotted earlier next time. That mirrors the duty chain HSE sets out — assess, control, monitor, review — applied to a single failure rather than the whole site [4].
Write down the decision, not just the deed. “Basin tap positive because it sat on a 1.2 m dead leg that never reached temperature; leg removed, outlet added to weekly flushing, resampled clear after return to normal use” is a close-out. “Disinfected and flushed” is the start of the next recurrence.
When to bring in specialist help
Escalate when several outlets on different circuits keep returning, when positives recur after a competent disinfection and a confirmed control fix, or when counts climb rather than settle. At that point you are likely looking at a systemic reservoir — a calorifier that needs cleaning, a section of pipework that needs modifying, or a written scheme that no longer matches the building. A water treatment provider working to L8 and HSG274 can pressure-test the scheme and the system together [1][2]; choosing one registered to a recognised code of conduct gives you a competence baseline to hold them to [7]. Persistent, unresolved positives are also exactly the kind of pattern an inspector reads as a management failure rather than a technical one, which is why closing the loop properly matters beyond the sample sheet — see Enforcement trends: how Legionella compliance is evolving.
A note on scope before you act: this is general guidance, not a disinfection method statement or a stand-in for a competent person’s judgement on your system. The right resample interval, the disinfection approach, and what counts as an acceptable count depend on the system, the biocide used and your risk assessment — set those with your provider, not from a guidance article.
FAQ
Why did my resample come back clear and then positive again later?
A clean result soon after disinfection often reflects residual biocide rather than a system genuinely back under control. Once the biocide clears and the water returns to normal temperature and flow, surviving biofilm regrows and reseeds the outlet [5]. Resample once the system has been in representative service, not the day after a shock dose.
How long should I wait before re-sampling after disinfection?
Long enough that the result reflects normal operation rather than leftover biocide, and timed to your risk assessment and the method used [2] — confirm the interval with your provider rather than assuming a fixed number. Sampling too soon is one of the most common reasons a positive appears to clear and then returns.
Should I just keep disinfecting an outlet that keeps testing positive?
Repeated disinfection of the same outlet without finding the reservoir is the definition of a failed close-out. If a tap returns positive after a proper disinfection, stop repeating the dose and trace the pipework behind it — dead leg, flexible hose, blended-water branch, or a vessel upstream — then fix the source [2].
Sources
[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems — Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [4] HSE, “Legionnaires’ disease — what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm [5] CDC, “Controlling Legionella”. https://www.cdc.gov/control-legionella/index.html [6] BSI, “BS 7592:2022 — Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [7] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/