How do you flush little-used outlets for legionella?
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Key points
- Flush weekly any outlet that is used less than about once a week, so the water in that branch is turned over before it can stagnate.
- Run each outlet until the temperature stabilises, hot reaching 50°C and cold dropping below 20°C, rather than for a fixed guessed time.
- Open taps and showers gently to limit the aerosol, or remove the spray head first, because flushing a colonised outlet is itself a release of spray.
- A flush that is not written down cannot be evidenced. The record is the proof the control happened, so log the date, the outlet, and the temperature reached.
- Flushing is a control measure that belongs in the written scheme of control, with each little-used outlet named and a set frequency.
You flush a little-used outlet by running it long enough to draw fresh water all the way through the branch that feeds it, until the temperature at the outlet settles: hot water rising to at least 50°C, cold water dropping below 20°C. Any outlet used less than about once a week needs this weekly, because a week of standing water is long enough for the branch to drift into the temperature range legionella grows in and start building a biofilm.
The point of flushing is turnover. Legionella needs still water to establish, so moving fresh water through a branch before it stagnates removes the condition the bacteria depend on. Flushing does not sterilise the pipe and it is not a substitute for fixing a system fault, but done on a set schedule it keeps a rarely used outlet from becoming the weak point that seeds the rest of the system.
What counts as a little-used outlet?
The working line is roughly one use a week. If an outlet is drawn off less often than that, treat it as little-used and put it on a flushing regime. Common examples are a guest bathroom, a cleaner's sink, an outside tap, a shower in a rarely occupied room, a first-aid or eyewash point, and any tap left on a branch after the fixture it once served was removed.
Identifying these outlets is the same exercise as finding dead legs, and the two overlap. A little-used outlet is a live dead leg: water does reach it, just rarely. The quickest way to build the list is to walk every branch of the system and ask, for each outlet, how often it is actually used. Anything that cannot honestly be answered "at least weekly" goes on the flushing schedule.
How long to flush: to temperature, not to the clock
The most common mistake is flushing for a fixed number of minutes plucked out of the air. A guessed time either wastes water or, worse, stops before fresh water has reached the outlet. The correct measure is temperature, because temperature tells you the standing water in the branch has actually been replaced.
Run the outlet and watch the temperature at the tap or shower. On a hot outlet, keep it running until it reaches at least 50°C (55°C in a healthcare setting). On a cold outlet, run it until it drops below 20°C. When the temperature stabilises at the right value, the branch has turned over and you can stop. A long branch on a distant outlet takes longer than a short one near the riser, which is exactly why a single fixed time cannot be right for every outlet. Our guide on how long to run taps when flushing works through the timing in more detail, and the temperature chart sets out the target figures.
Flush gently to limit the aerosol
Flushing is the moment you deliberately run water through a branch that may already be colonised, so it is itself a release of spray. Open the outlet slowly rather than fully, so the flow does not atomise into a fine mist. On a shower or spray tap, the safer approach is to remove the spray head or hose before flushing and run the water as a stream, then clean and refit the head afterwards. This matters most on shower heads, which produce the fine aerosol that carries legionella deep into the lungs. Where you can, open a window or run an extractor while you flush.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
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After a shutdown: holiday lets, schools, void properties
A whole building can behave like one large little-used outlet. Holiday lets between guests, schools over the summer holiday, offices after a long closure, and void or newly let properties all leave water standing for days or weeks. When the building comes back into use, every outlet in it has stagnated, not just the rarely used ones.
Before reoccupation, flush the entire system: run every outlet through to temperature, starting nearest the incoming supply and working outward, and give special attention to showers and spray taps. After a long shutdown, or where the risk assessment flags it, sampling or disinfection may be the right call rather than flushing alone. If a shutdown is planned and short, a documented weekly flush throughout the closure keeps the system turned over and avoids a large recommissioning exercise at the end. Where you find water that will not reach temperature, or an outlet that has clearly been standing for a long time, follow the steps in what to do if legionella is detected.
Why the record matters
A flush you did but did not write down is, for the purposes of showing you managed the risk, a flush that did not happen. The written record is the evidence. If an outlet is later linked to a case, or an inspector or insurer asks how you controlled a known little-used outlet, the flushing log is what demonstrates the control was carried out.
Flush to temperature, not to a guessed number of minutes, and write down every flush. An unrecorded flush cannot be evidenced, so on paper it is no control at all.
Log the date, the outlet, who did it, and the temperature reached. A simple log book or template does the job. The flushing regime itself should be named in the written scheme of control: which outlets are on it, how often each is flushed, and to what temperature. That closes the loop between the risk assessment that identified the outlet and the record that proves it is being managed.
Where flushing fits in the guidance
Flushing infrequently used outlets is a standard control measure in HSG274, the HSE technical guidance on controlling legionella in water systems, and it supports the duty under ACOP L8 to identify and control the risk from stored and distributed water. Neither document lets you sign off a little-used outlet on length or age alone: the requirement is to identify each one, set a control, carry it out, and record it. Flushing to temperature on a written schedule is how that requirement is met for outlets you cannot simply remove.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
How long should you flush a tap for legionella?
Flush until the temperature at the outlet stabilises, not for a fixed number of minutes. Run a hot tap until it reaches at least 50°C (55°C in healthcare) and a cold tap until it drops below 20°C. That is the point at which the standing water in the branch has been replaced with fresh water from the main. A long branch to a distant outlet takes longer than a short one near the riser, which is why a single guessed time does not work for every tap.
How often should little-used outlets be flushed?
The general rule is to flush any outlet used less than about once a week on a weekly basis, so the water is turned over before it can stagnate. Some higher-risk situations may need flushing more often, and the frequency for each outlet should be set in the written scheme of control based on the risk assessment. Whatever frequency you choose, carry it out consistently and record each flush, because an unrecorded flush cannot be evidenced.
What is a flushing regime?
A flushing regime is a documented schedule of which little-used outlets are flushed, how often, and to what temperature, together with the record of each flush being carried out. It is a control measure that sits in the written scheme of control and links back to the outlets identified in the legionella risk assessment. A regime is more than the act of running the taps: it names the outlets, sets the frequency, and keeps a log that proves the control happened.
Do you need to flush taps after a property has been empty?
Yes. When a property has stood empty, the water in every outlet has stagnated, so before reoccupation you should flush the whole system, running each outlet through to temperature and paying particular attention to showers and spray taps. Start nearest the incoming supply and work outward. After a long closure, or where the risk assessment flags it, sampling or disinfection may be needed rather than flushing alone, and any outlet that will not reach the right temperature should be investigated before use.
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