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Little-used outlets and dead legs: still the most common finding on site

Year after year, the same issue tops our risk assessment findings: water sitting still. Here is why stagnation matters, how to find it, and what a workable flushing regime looks like.

Risk assessmentWater hygiene monitoring

If we had to name one finding that appears in almost every Legionella risk assessment we carry out, it would be stagnant water. Little-used outlets, dead legs left behind after refurbishments, and oversized storage tanks all give Legionella the thing it needs most: time in water at a comfortable temperature.

Why stagnation matters

Legionella multiplies between roughly 20°C and 45°C. A hot water outlet that is rarely opened cools down to room temperature in the pipework; a cold water outlet on a warm riser drifts upwards. Biofilm forms on the pipe wall, nutrients accumulate, and the disinfectant residual from the mains supply disappears. Every time the outlet is finally used, the water that has been sitting there is discharged, sometimes as an aerosol from a shower head.

Finding the problems

The obvious candidates are easy: the accessible toilet nobody uses, the shower in a store room, the outside tap. The ones that cause trouble are less obvious:

  • Capped pipework behind a wall or above a ceiling where a sink was removed.
  • Bypass loops and drain valves on plant that are never operated.
  • Empty rooms in care homes, hotels and student accommodation between occupants.
  • Seasonal buildings or wings, particularly in education.
  • Expansion vessels on hot water systems, which are often a dead leg by design.

A good schematic, kept up to date after every alteration, is the single most useful tool for finding these. That is one reason we treat schematics and asset registers as a service in their own right rather than an afterthought.

A flushing regime that will actually happen

Guidance in the UK and Ireland expects little-used outlets to be flushed weekly, for long enough to draw fresh water through to the outlet, and for that to be recorded. The regimes that fail are the ones that are unrealistic for the people doing them. A workable regime has:

  1. A definitive list of outlets, with a location description someone new could follow.
  2. A flushing duration that has been checked at the outlet, not guessed.
  3. A single record, paper or digital, signed by the person who did it.
  4. A named person who reviews the record and chases gaps.

Removing the risk rather than managing it

Where an outlet or a dead leg is not needed, removing it is nearly always cheaper over a year than flushing it every week. Capped pipework should be cut back to the live main, not just capped. Our reports prioritise removal over management wherever it is practical, because it takes the task off your team's list for good.