How do you monitor legionella?
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Key points
- Monitoring is a stack of tasks at different frequencies, not one annual job: weekly flushing, monthly temperatures, quarterly cleaning, six-monthly and annual inspections.
- Your stack is shaped by your system. A combi-fed flat needs almost none of it; a building with a cold water tank and calorifier needs all of it.
- Monthly temperatures are taken at sentinel outlets, the nearest and furthest points on each loop, not at every tap every month.
- Every task ends the same way: a dated, initialled record. The log book is where the stack lives.
- The stack below is the HSG274 Part 2 starting point. Your risk assessment sets the real frequencies and can tighten or loosen each one.
You monitor legionella by running a stack of routine checks, each on its own frequency, and writing down every result. The weekly layer is flushing little-used outlets. The monthly layer is temperatures at sentinel outlets and at the calorifier. The quarterly and six-monthly layers are cleaning and tank inspection. The annual layer is the calorifier inspection and a full review of the whole regime. What changes from building to building is not the logic but the size of the stack: the more stored water and the more outlets you have, the more layers you run.
The task stack at a glance
The table below shows the typical stack from HSG274 Part 2 for a hot and cold water system, and how it shrinks or grows with the type of system you have. These are starting points: the risk assessment sets your actual frequencies.
| Layer | Task | Typical frequency | Combi / mains only | Tank + calorifier | Healthcare / high risk |
|---|---|---|---|---|---|
| Weekly | Flush little-used outlets | Weekly | Only if any exist | Yes | Yes, often more outlets in scope |
| Monthly | Calorifier flow and return temperatures; sentinel outlet temperatures | Monthly | Not applicable | Yes | Yes, hot to 55°C |
| Rolling | Temperatures at all remaining outlets | All covered over about 12 months | Not applicable | Yes | Yes, usually tighter |
| Quarterly | Dismantle, clean and descale shower heads and hoses | Quarterly | As needed | Yes | Yes, or more often |
| Six-monthly | Cold water storage tank inspection | Six-monthly | Not applicable | Yes | Yes |
| Annual | Calorifier internal inspection or base drain-off; full regime review | Annually | Not applicable | Yes | Yes, per the water safety plan |
| Per scheme | Thermostatic mixing valve service and failsafe check | Set by risk assessment | If fitted | If fitted | If fitted, on the HTM 04-01 schedule |
Source: HSE HSG274 Part 2 typical frequencies. Your risk assessment sets the actual intervals, and healthcare premises work to HTM 04-01 on top of this.
The weekly layer: keep water moving
The weekly round exists for one reason: water that stands still for a week can warm into the growth range and start building biofilm. Walk the list of little-used outlets from your risk assessment and run each one gently until fresh water arrives and the temperature stabilises, hot up to 50°C or cold down below 20°C. Note the date, the outlet and the temperature reached. The weekly checks walkthrough takes you through the routine minute by minute, and flushing little-used outlets covers the technique, including why you open spray fittings slowly.
The monthly layer: prove temperature control
Temperature is the primary control, and the monthly round is where you prove it is working. Start at the calorifier: record the flow leaving the vessel, which should be at least 60°C, and the return coming back, which should be at least 50°C. Then visit the sentinel outlets, the nearest and furthest tap on each hot and cold loop, and check each one reaches its target: hot at 50°C or more within one minute, cold below 20°C within two. Sentinel outlets explained shows how to pick them with a worked example, and the temperature chart lists every target figure. The temperature checker will tell you straight away whether a reading passes.
The rest of the outlets join a rolling programme: a share of them each month, so every outlet in the building is measured at least once over a set period, usually a year. The temperature monitoring template is built for exactly this round.
The quarterly and six-monthly layers: cleanliness and condition
Shower heads and hoses are dismantled, cleaned and descaled quarterly, because they produce the fine aerosol that carries legionella deep into the lungs and because scale gives biofilm a foothold. See cleaning shower heads. Cold water storage tanks are inspected six-monthly: lid and screens intact, no debris or corrosion, stored water temperature checked, because a tank that warms past 20°C feeds the whole cold system with water in the growth range. See cold water storage tank inspection.
The annual layer: open the plant and review the regime
Once a year the calorifier gets its internal inspection, or a drain-off from the base to check for sludge and corrosion, because the base of the vessel is the cold spot where legionella survives an otherwise hot system. See calorifier inspection. The same visit is the natural moment for the full regime review: are all the tasks still being done, do the records show it, and has anything about the building changed that should feed back into the risk assessment. The annual checks walks through that review step by step.
The stack is the answer to "how do you monitor legionella": flush weekly, take temperatures monthly, clean quarterly, inspect six-monthly, open the calorifier and review annually, and write down every single one.
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.
Match the stack to your system
A flat with a combi boiler and no stored water sits at the bottom of the table: little to flush, nothing to inspect, and a risk assessment that says so in writing. A house or small office with a cold tank in the loft and a hot water cylinder runs the middle column. A care home, school or hotel with long pipe runs, many outlets and vulnerable occupants runs the full stack, often tightened, and a healthcare premises adds HTM 04-01 on top. Care homes have their own frequency table at how often to check water temperatures in a care home.
If you are not sure which column your building belongs in, that is exactly what the risk assessment answers. The risk assessment builder walks you through the system you have and the checks it needs, and the log book is where you then record each layer as you do it. None of these checks certifies the system safe. They are the ongoing evidence that the controls in your written scheme of control are actually running.
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 do you monitor legionella in a water system?
You monitor legionella by running a stack of routine checks at different frequencies: weekly flushing of little-used outlets, monthly temperature checks at sentinel outlets, quarterly shower head cleaning, six-monthly cold water tank inspection, and an annual calorifier inspection plus a full review. Every check has a target, a result and a written record. Which tasks are in your stack depends on your system: a combi-fed flat has almost no stack at all, while a building with tanks and calorifiers has the full set.
What legionella checks should be done weekly?
The core weekly task is flushing little-used outlets: running any tap, shower or fitting that is used less than about once a week until fresh water reaches it and the temperature stabilises. Some written schemes add small weekly checks, such as confirming stored hot water temperature at the calorifier or a visual check of a tank. The weekly round is usually the quickest part of the regime and the easiest to evidence with a simple log.
What legionella checks are done monthly?
Monthly checks are mostly temperatures. Take the hot water flow and return at the calorifier, then the nearest and furthest outlets on each hot and cold loop: the sentinel outlets. Hot should reach 50°C (55°C in healthcare) within one minute of running; cold should be below 20°C within two minutes. Remaining outlets are worked through on a rolling programme so all are measured over a set period, often a year.
Do I need to monitor legionella in a house with a combi boiler?
A small mains-fed property with a combi boiler and no stored water is at the low end of the risk scale, because the water is heated on demand and rarely sits still. The monitoring stack shrinks to very little: keep an eye on any little-used outlet, flush after the property has stood empty, and descale shower heads periodically. A landlord's risk assessment will usually conclude exactly that, but it should still be written down so the conclusion can be evidenced.
Who can carry out legionella monitoring checks?
Any competent person can carry out the routine checks: a caretaker, site manager or trained member of staff. Competent here means they know what to measure, what the target figure is, how to use the thermometer properly, what to do when a reading is out of range, and how to record the result. The responsible person named under ACOP L8 stays accountable for the whole regime, whoever performs each task.
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