Legionella Flushing: How Long To Run Taps And Outlets Correctly

Flushing is one of the most useful controls in a water system, and also one of the most misunderstood. Legionella flushing works because it moves stagnant water out of pipework and outlets before bacteria have a chance to multiply in it. The principle is simple. The execution, in most buildings, is where things go wrong.

The question people ask most often is how long to run a tap or shower for it to actually count. The honest answer is that time on its own is not the measure. What matters is whether enough water has passed through to replace what was sitting in the pipe and reach the temperature you would expect at that outlet. A quick two-second run does nothing except waste water and create a record that looks reassuring but proves very little.

This matters because flushing regimes are often the part of a water safety plan that gets delegated to whoever is on site, done inconsistently, and recorded in a way that would not stand up if a case of Legionnaires’ disease were ever linked back to the building. Getting the detail right is not difficult, but it does need to be deliberate.

What Flushing Actually Controls In A Water System

Legionella grows in water that sits still, particularly when it stays in the range that suits the bacteria, roughly 20 to 45 degrees. An outlet that is rarely used becomes a small reservoir of warm, stagnant water feeding off any nutrients in the system. Left alone, that is exactly the environment control measures are meant to prevent.

Flushing breaks that cycle by pulling fresh water through the pipework and out of the outlet. It does two things at once. It clears the standing water that has had time to warm up and stagnate, and it draws through water at the correct temperature from the main supply or the hot water circuit. On the hot side that means near-scalding water reaching the outlet. On the cold side it means genuinely cold water arriving rather than the lukewarm water that has been sitting in a dead leg.

What flushing does not do is fix an underlying problem. If a system runs at the wrong temperatures, has long dead legs, or has redundant pipework left in place after a refit, flushing manages the symptom rather than the cause. It is a holding measure and a legitimate one, but it should never be treated as a substitute for sorting out the design and temperature issues that create stagnation in the first place.

How Long To Run Taps And Outlets For It To Count

There is no single figure that applies everywhere, which frustrates people who want a clean answer. The genuine target is to run each outlet until the water runs through fully and reaches its expected temperature, then hold it there long enough to be confident the standing water has been replaced.

In practice, for most outlets that means running until hot water reaches temperature and cold water runs cold, then allowing a further short period to flush the outlet itself. For a typical basin tap close to a well-used riser, that might be under a minute. For a shower at the end of a long branch that nobody has touched for a fortnight, it can take several minutes before the temperature settles.

The point people miss is that the duration follows the temperature, not the other way round. Setting a blanket instruction such as “run every tap for one minute” produces failures at both ends. Short branches get over-flushed and long ones never clear. A better instruction to whoever is doing the work is to run each outlet until it reaches temperature and stays there, and to record how long that took. If an outlet consistently takes far longer than its neighbours to run hot, that is telling you something about the pipework, not just the flushing.

Showers and spray outlets deserve particular care because they generate aerosols, which is the route by which Legionella actually causes infection. Where a shower is being brought back into use after a period of disuse, running it hard while it is spraying into the air is the last thing you want. These should be flushed with the head removed or bagged, or the flow directed to avoid creating a fine spray, and the head cleaned and descaled separately.

Which Outlets Need Flushing And How Often

Not every outlet needs the same treatment. The regime should be driven by how the building is actually used, which is why it belongs in the risk assessment rather than being copied from a template. The outlets that need attention are the ones that do not get used enough to keep water moving through them naturally.

Flushing infrequently used outlets weekly is the common baseline in guidance, and for most low-use taps and showers that is a sensible starting point. But the interval should reflect the real risk. Some situations warrant more frequent flushing, and some outlets get used enough that a formal flush adds nothing.

• Guest rooms, spare bathrooms and rarely occupied accommodation where taps and showers sit idle for days
• Seasonal or part-occupied buildings, such as schools during holidays or offices with vacant floors
• Cleaners’ sinks, plant room taps and back-of-house outlets that are easily forgotten
• Emergency showers and eyewash stations, which are safety equipment but also a stagnation risk
• Outlets left live after a change of use, where a room is no longer occupied but the water supply was never capped

Where a building has a lot of little-used outlets, the sensible long-term answer is often to reduce the number rather than flush them forever. An outlet that will genuinely never be used again should be removed back to the main pipework, not just capped at the wall, because a capped dead leg still holds water. Flushing infrequently used outlets is a management measure. Removing the reason they need flushing is the fix.

Running Water Long Enough To Reach Temperature

Temperature is what tells you the flush has worked, so it needs to be part of the routine rather than an occasional spot check. On the hot side, water should reach a genuinely hot temperature at the outlet within a reasonable time, and the person flushing should feel it come through hot before they stop. On the cold side, the water should run to a low temperature and stay there, not drift up because a nearby hot pipe is warming the cold supply.

Where temperature monitoring is part of the wider water system flushing regime, it makes sense to combine the two on the same visit at least periodically. Flushing an outlet and then taking a temperature reading at the same point gives you a much clearer picture than doing either in isolation. It is also where problems surface. An outlet that flushes for two minutes and still will not run hot, or a cold tap that never gets below room temperature, is flagging a real issue with the system that flushing alone will not resolve.

One recurring problem is that flushing gets done conscientiously but nobody checks the temperatures being achieved. The water gets run, the box gets ticked, and the fact that a whole wing never reaches control temperature goes unnoticed for months. The flush is only meaningful if someone knows what temperature it produced.

Recording Flushing So It Stands Up To Scrutiny

Flushing records are often the weakest part of an otherwise reasonable water safety regime. A record that just shows a signature against a list of dates tells you almost nothing. If a case of Legionnaires’ disease were ever investigated and linked to a building, the records are the first thing that gets examined, and vague ones offer no protection.

Useful flushing records show, for each outlet or group of outlets, when the flush was done, who did it, and enough detail to show it was done properly. Where temperatures are being taken alongside, those readings belong in the same record. The aim is a document that demonstrates the control was actually carried out, not just scheduled.

A few things separate records that hold up from records that do not:

• Every outlet in scope is identified clearly, so it is obvious what was and was not flushed
• Gaps and missed weeks are visible rather than hidden, and there is evidence they were followed up
• Failed temperatures or problem outlets are flagged and passed to someone who can act on them
• The person carrying out the work is named and the records are reviewed by someone with responsibility, not just filed

The common failure here is records that exist but have never been reviewed. A folder of completed sheets is worth very little if nobody has looked at whether the missed weeks were chased, or whether the same outlet keeps failing to reach temperature. Recording is not the end of the task. Someone has to read what the records are telling them.

Where Flushing Regimes Fall Down In Practice

Most flushing failures are not about the principle. They are about the reality of getting it done every week, in every part of the building, by people who have other jobs to do. The regime that looks perfect on paper is often the one that quietly stops working when the person who understood it leaves.

The recurring problems are consistent across most buildings:

• Outlets that cannot be reached because a room is locked, a floor is under refurbishment, or a tenant controls access
• New outlets added during works that never make it onto the flushing schedule
• A change of occupancy that leaves rooms empty, so outlets that were being used naturally now need flushing that nobody has arranged
• Flushing done too fast to be effective because whoever is doing it is under time pressure
• Records that show a full flush every week for an outlet that was actually behind a locked door the whole time

Tenant and multi-let buildings raise a particular difficulty. The dutyholder may be responsible for the water system, but day-to-day access sits with occupiers who do not see it as their problem. If the flushing regime depends on someone getting into leased space every week and that arrangement was never agreed properly, the regime has a hole in it from the start. This needs sorting in the management plan, not discovered when an assessor asks how the vacant third floor is being flushed.

When Flushing Is Not Enough On Its Own

Flushing sits within a wider set of controls, and there are situations where it should not be relied on by itself. The relevant framework here is the Approved Code of Practice, ACOP L8, together with the technical guidance in HSG274. Both treat flushing as one measure among several, not a standalone answer. The regulations and guidance make clear that the whole system has to be managed on a risk-assessed basis.

Flushing reaches its limits where the underlying conditions favour bacterial growth regardless of how diligently outlets are run. A system running below control temperatures, extensive dead legs, poorly maintained calorifiers, or storage tanks with poor turnover will keep producing risk that weekly flushing cannot manage. In those cases the answer is remedial work on the system itself, sometimes alongside interim measures such as more frequent flushing or temporary treatment while the fix is arranged.

There are also situations that call for more than routine flushing before an outlet goes back into normal use. A building or wing that has been empty for an extended period, a system that has just been through works, or an outlet that has repeatedly failed temperature checks may need a fuller programme of flushing, cleaning, disinfection or sampling depending on what the risk assessment concludes. Understanding how Legionella bacteria behave in stagnant, warm water makes it clear why bringing a long-dormant system back online is not just a matter of turning the taps on.

The judgement about when flushing is sufficient and when it is not should come from a competent assessment of the specific system, not from a generic schedule. A regime that suits a small office will not necessarily protect a large, complex building with vulnerable occupants.

Common Questions About Flushing Little-Used Outlets

Does flushing have to be done weekly?

Weekly is the usual baseline for little-used outlets in guidance, but the correct interval is set by the risk assessment. Higher-risk situations or vulnerable occupants may justify more frequent flushing, and outlets used enough in normal operation may not need a separate flush at all. The frequency should reflect how the building is actually used.

Can we just cap unused outlets instead of flushing them?

Capping at the wall does not remove the risk, because water still sits in the pipe behind the cap and forms a dead leg. To genuinely remove an outlet from the flushing regime it should be cut back to the main pipework so there is no length of stagnant water left feeding it. A capped-but-live outlet still needs managing.

Who is responsible for flushing in a leased or multi-let building?

Responsibility for the water system usually sits with the dutyholder, often the landlord or managing agent, but day-to-day access frequently sits with tenants. Where flushing depends on getting into occupied or vacant leased space, that arrangement needs to be agreed and documented in the water management plan. Assuming a tenant will handle it without a clear agreement is where these regimes tend to fail.

What temperature should the water reach when flushing?

Hot water should reach a genuinely hot temperature at the outlet and cold water should run properly cold and stay there. If an outlet will not reach the expected temperature no matter how long it runs, flushing is not the fix. That points to a problem with the system that needs investigating, such as poor circulation, a long branch, or a control fault.

Is a flushing record enough to demonstrate compliance?

Only if the records show the work was actually carried out properly and someone has reviewed them. A signed list of dates with no detail, and no evidence that missed weeks or failed temperatures were followed up, offers little protection if the regime is ever scrutinised. The record has to demonstrate the control worked, not just that it was scheduled.

If you are a dutyholder, landlord or facilities manager and you are not confident your flushing regime is reaching the right outlets, achieving the right temperatures, or producing records that would stand up to scrutiny, Acorn Safety Services can review your current arrangements, identify the gaps, and set out a practical, risk-based regime that matches how your building is actually used. To talk through the specifics of your system, contact us or get a quote for a Legionella risk assessment.