Two habits do most of the heavy lifting in day-to-day Legionella control, and neither needs a lab. Keep water moving so it never sits still long enough to grow bacteria — that is flushing. Check on a schedule that your controls are still holding — that is monitoring. Build both into a weekly and monthly rhythm and you have covered most of what a site team is actually responsible for.
“Routine” is not a polite word for trivial. It is the work that happens in the gaps between the annual risk assessment and the occasional sample: the flush logs, the temperature checks, the quick look inside the cold tank. This is the part the site owns directly, and it is usually where control quietly slips.
What you are monitoring, and why you flush
Flushing is deliberate work. You run a tap, shower or other outlet that rarely gets used, on purpose, so the water sitting in that branch is replaced before it goes stale. Stagnant water in roughly the 20–45°C band is where Legionella multiplies [1], and the outlet nobody has touched for a fortnight is the classic place for it to happen.
Monitoring is the evidence trail. The headline check is temperature — confirming hot water still leaves the system hot and arrives hot at the far outlets, and that cold water is still genuinely cold — usually taken at a set of representative or “sentinel” outlets rather than at every tap on site [2]. Around that sit tank inspections, checks for scale and debris, and a habit of noticing what has changed: a newly emptied wing, a refit, an area dropped to part-time use.
The two halves work together but are not the same job. Flushing is a control — it changes the water. Monitoring is verification — it tells you the control is working. Blurring that line is where a lot of programmes go wrong.
A flush is not a five-second tap-on
The most common way to get Legionella flushing wrong is to do it too briefly. Cracking a tap open for a few seconds clears the spout, not the branch behind it. A proper flush runs the outlet until the water has fully turned over and reaches its representative temperature — cold running properly cold, hot running properly hot — which on a long, neglected branch can take noticeably longer than people expect [3].
How you flush matters as much as how long. Showers and spray taps throw off exactly the fine mist that carries Legionella into the lungs [4], so a stale outlet should be flushed in a way that limits aerosol: open it gently, keep the flow modest, and where your risk assessment and the guidance allow, run it with the spray head removed or submerged. The aim is to exchange the water without filling the room with droplets.
Frequency comes from your risk assessment, not from a single national figure. Weekly flushing of little-used outlets is a common baseline, adjusted up or down for how the building is used [3]. A room left void for a month and a tap used every morning are not the same problem and should not sit on the same schedule.
What teams get wrong about routine checks
Plenty of confident assumptions fall apart on site. These are the ones worth unlearning early.
| The assumption | What’s actually true |
|---|---|
| If the water looks clean, the outlet is low risk | Biofilm and scale build up inside fittings and pipework, invisible from the spout. Appearance tells you almost nothing about turnover or temperature |
| Flushing fixes the stagnation problem | Flushing manages a symptom week after week. The lasting fix for a redundant outlet or dead leg is to remove it, not to keep running it forever |
| Monitoring and control are the same thing | Monitoring records the state of things; flushing, temperature and cleaning change it. A spotless logbook over a failing system is just well-documented risk |
| One good reading at a nearby tap covers the floor | Sentinel readings stand in for the wider system. A healthy reading by the door does not excuse a long, cold-running hot branch in the far corner |
| Recording the task is the same as recording control | ”Flushed, 10:14” proves someone opened a tap. It does not show the water reached temperature, or that an out-of-range result was ever acted on |
The mistake that undoes everything else
New teams tend to treat the log itself as the goal. The task gets ticked, the box turns green, everyone moves on. But a tick only proves an action happened. It does not prove the action worked, or that anyone responded when it didn’t.
The fix is to record the result and the decision, not just the event. A hot outlet that came in below the expected temperature is only useful if the reading is written down, flagged, and followed by something real: a re-check, a call to the responsible person, a note that the calorifier needs attention. Programmes that capture exceptions — and what was done about each one — are the ones that genuinely control risk. Programmes that capture only green ticks are decorating it.
If your records live on paper and you find exceptions are easy to lose between visits, that is a known weak point worth designing out; the guide to logbooks covers how to keep records that actually hold up.
Where to start this week
You do not need new equipment to begin. You need a list and a rhythm.
- Walk the building and mark every outlet that rarely gets used: spare WCs, cleaners’ sinks, end-of-corridor taps, unused showers, void rooms.
- Decide, against your risk assessment, how often each one needs flushing and who will do it.
- Set your monitoring points — the sentinel hot and cold outlets that represent the system — and fix a date each month to read them.
- For each task, write down not just the action but the acceptable result and what happens when a reading misses it.
That short list is the backbone of routine Legionella flushing and monitoring. When you are ready to turn it into a full schedule, How to implement a flushing programme for Legionella control walks through building the flushing programme properly, and Temperature control basics for Legionella prevention covers the temperatures your monitoring is checking against.
A word on the numbers
The figures and intervals mentioned here are general guidance, not site rules. What counts as an acceptable temperature, how often you flush, and which outlets you sample all come from a competent, site-specific risk assessment of your building and the people who use it. Where a number appears above, treat it as a prompt to confirm against current HSE guidance and your own assessment, never as a replacement for either. And monitoring, however diligent, is a buildings-safety activity rather than a clinical one: suspected illness belongs with medical professionals, not the logbook.
FAQ
Does flushing actually reduce Legionella, or just move it around?
It genuinely reduces risk. Flushing replaces water that would otherwise stagnate and drift into the growth range, and it stops biofilm-friendly conditions from settling in at little-used outlets [1]. But it manages an ongoing condition rather than curing it, which is exactly why removing redundant pipework beats flushing it indefinitely.
How long should I run an outlet when flushing it?
Long enough for the water in that branch to fully turn over and reach its representative temperature — cold running cold, hot running hot — rather than a fixed count of seconds [3]. A short dead leg clears quickly; a long one takes longer. The outlet’s layout and your risk assessment set the realistic figure.
Is routine monitoring enough, or do I still need to sample?
Temperature monitoring and flushing are your day-to-day controls; sampling is separate verification, used when your risk assessment calls for it, and its frequency depends on the system rather than a fixed calendar [5]. A clear sample never cancels out poor temperatures or stagnation — control comes first, evidence second.
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, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [4] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [5] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm