A tap nobody turns and a shower nobody stands under are not low-risk because they’re quiet. They are higher-risk because they’re quiet. The water inside them sits, drifts toward room temperature, and gives Legionella the still, tepid conditions it likes — and then the next person to open that outlet breathes in whatever has grown.

Flushing is how you break that cycle. Done properly it pushes the stagnant water out and brings fresh, properly hot or cold water through before bacteria can settle in. Done as a paper exercise it changes nothing, and a careless flush can fire contaminated mist straight into the room. Below is the procedure that gets it right, plus the judgement call that decides which outlets even need it.

What a good flush is for, and what to have ready

Good flushing — best practice rather than box-ticking — does two jobs at once. It turns the water over so nothing in that branch sits long enough to stagnate, and it confirms the supply still arrives at the right temperature: hot water hot, cold water cold, which is the whole basis of control in a domestic-style system [3].

Before you start a round, line up three things:

  • An honest list of every outlet and how often it is genuinely used — not how often someone assumes it’s used.
  • The acceptance temperatures and intervals your risk assessment and written scheme have set for the site.
  • Somewhere to record each flush as you do it, whether that’s a paper sheet or a digital logbook.

If any of those is missing, sort it first. Flushing without a target temperature or a record is just running taps.

Deciding which outlets go on the flushing list

Not every quiet outlet needs a scheduled flush, and a couple that look busy never actually get used. Walk each one through these branches before you commit it to a routine.

  • Is normal use turning the water over often enough — roughly at least once a week?
    • Yes: ordinary use is doing the work. Leave it off the flushing schedule, but spot-check now and then that it still hits temperature.
    • No: treat it as a little-used outlet and put it on the programme. UK guidance commonly points to flushing infrequently used outlets at least weekly [3].
  • Is it an outlet that is realistically never going to be used again — a capped basin, the supply to a removed appliance, a spur to a room taken out of service?
    • Yes: don’t flush it forever. That’s a dead leg, and the lasting fix is to remove it rather than nurse it weekly. See Neglected water systems: the danger of stagnation on why stagnation is the root cause to design out.
    • No, it’s still live: keep it on the list while it stays connected.
  • Does it spray, or serve people more vulnerable to infection — showers, spray taps, outlets in care or clinical areas?
    • Yes: rank it as higher priority. Your risk assessment may set a shorter interval and a stricter temperature check for these.
    • No: standard interval is fine.

That sort is worth redoing whenever the building’s use changes. A floor that empties out, a ward that’s mothballed, a wing closed for refurbishment — each one quietly converts ordinary outlets into little-used ones overnight.

The flushing procedure, step by step

1. Kill the aerosol before you open anything

On a shower, take the head off and let the water run from the hose into the drain, or wrap and submerge the head so it can’t spray. On spray taps, plan to open them gently rather than wide. Open a window or run the extract. Legionella reaches people through inhaled mist, not through drinking water, so flushing a colonised outlet at full blast is exactly the aerosol event you’re trying to prevent [5]. Done when: water can run without misting into the air you breathe.

2. Run the cold until it runs genuinely cold and clear

You’re purging the warmed standing water and drawing cooler, mains-fed water through the branch. A short burst won’t do it on a long run — you have to move the whole standing volume. Done when: the temperature has dropped and held to the figure your scheme sets, commonly below around 20°C within a couple of minutes [2].

3. Run the hot until it runs properly hot

This both turns the water over and tells you the hot leg is actually delivering heat to that point, not arriving lukewarm after a long dead run. Done when: it reaches the hot target in your scheme, commonly around 50°C within roughly a minute at the outlet [2].

4. Don’t skip the mixed and the hidden outlets

Mixer taps and TMV-blended outlets need both supplies exercised, not just whatever comes out at the spout. So do the second basin in an accessible toilet, the cleaner’s sink, the outside tap, the rarely-used kitchenette. These are precisely the outlets that drift onto the little-used list and then get forgotten.

5. Record it as you go

Note the date, the outlet, who did it, and any reading or problem. An unrecorded flush is, to an HSE inspector or an insurer, a flush that didn’t happen. Recording temperatures as well as ticks is what turns the round from “we ran the taps” into evidence the control is holding.

Where flushing programmes quietly fall apart

The failures are rarely dramatic. They’re small habits that hollow out the programme:

  • A two-second token run that never clears a long branch, so the stale water stays put.
  • Ticking the outlet off without checking temperature — you’ve confirmed water came out, not that control held.
  • Doing the easy, accessible outlets and quietly skipping the awkward ones, which tend to be the very ones least used.
  • Flushing a true dead leg week after week instead of cutting it out.
  • Letting the outlet list go stale while the building’s occupancy moves on without it.

Checking the programme actually works

Ticks tell you tasks happened; temperatures and trends tell you they worked. Once a flushing programme is running, look at whether outlet temperatures are landing where they should, whether exceptions are getting closed quickly, and whether the list still matches the building you walked. Monitoring and testing frequency follows the system and the risk assessment rather than a single national number, so don’t treat a fixed interval as fixed forever [4]. Re-walk the site periodically and ask the plain question: which outlets have changed how much they’re used since last time?

Before you lean on this

The weekly habit and the outlet temperatures above are general guidance, not a rule that fits every building. The actual list, the intervals and the acceptance temperatures come from a competent, site-specific risk assessment that reflects your system and the people who use it [1]. And remember what flushing is: a way of managing a risk that stagnation creates. Where an outlet is genuinely redundant, removing the dead leg beats flushing it indefinitely — and where temperatures or samples point to a real problem, that needs investigation, not just another run of the taps.

FAQ

What actually counts as a “little-used” outlet?

Broadly, any outlet not used often enough for ordinary turnover to keep the water fresh — frequently taken as less than about once a week. The exact line is set by your risk assessment, because a shower in a void hotel room and a hand basin in a busy office sit in very different places on that scale [3].

How long should I run each outlet?

Long enough to clear the standing water and bring the supply through to temperature, which isn’t a fixed number of seconds. A long branch off the riser holds far more water than a tap right next to it, so it takes longer. The practical rule is run the cold until it’s cold and the hot until it’s hot, then check the reading [2].

If a sample comes back positive, do we just flush harder?

No. Flushing is a control that keeps water moving; it isn’t a cure for a colonised system. A positive result, or temperatures that won’t hold, needs proper investigation and remediation — see What to do if you get a positive Legionella test for the steps that follow a positive test.

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, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [5] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html