What is the difference between legionella monitoring and testing?
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Key points
- Monitoring is the routine checking that your controls are working: temperatures, flushing, inspections. Testing is laboratory sampling for the bacteria itself.
- ACOP L8's duty is to monitor. Routine laboratory testing is not a general requirement; it answers specific questions the risk assessment raises.
- Inspectors ask for monitoring records first. A clean laboratory result does not patch a gap in the temperature log.
- Monitoring runs continuously and cheaply; testing is episodic and targeted. The two work together, not as substitutes.
- Keep every record, of both kinds, for at least five years.
Monitoring and testing are the two halves of checking a water system, and the distinction between them is exactly the one an inspector applies. Monitoring is everything you do routinely to prove your controls are holding: the monthly temperature readings, the weekly flushes, the tank inspections. Testing is the occasional act of sending water to a laboratory to look for the bacteria itself. Duty holders who blur the two usually end up overpaying for one and under-doing the other.
Side by side
| Monitoring | Testing (laboratory sampling) | |
|---|---|---|
| What it is | Routine checks that controls are working: outlet temperatures, calorifier flow and return, flushing, tank and calorifier inspections | Water samples cultured by a UKAS-accredited laboratory to count legionella in cfu/L |
| How often | On a schedule: weekly, monthly, quarterly, annually | When the risk assessment or a specific regime calls for it |
| Who does it | The responsible person or a competent contractor, on site | You or a contractor samples; the laboratory analyses |
| What it costs | Time, a thermometer, and a log book | Published prices from around £45 per sample |
| What it tells you | Whether the conditions that control legionella are in place, continuously | Whether legionella was present in that outlet, that day, at that count |
| Legal standing | The recognised way of meeting the ACOP L8 duty to check controls | Required only in defined situations, not routinely |
What the guidance says
ACOP L8 requires the risk to be controlled and the controls to be checked, and HSG274 Part 2 supplies the standard monitoring schedule: monthly temperatures at sentinel outlets, monthly calorifier flow and return, weekly flushing of little-used outlets, annual checks across all outlets, and periodic inspections of tanks and calorifiers. Sampling appears in the same guidance as a verification tool for defined situations, not a standing obligation. Our legionella monitoring schedule guide sets the frequencies out in full, and legionella testing covers when sampling joins in.
What an inspector actually asks for
When the HSE or a local authority examines legionella control, the request is consistent: the risk assessment, the written scheme of control, and the monitoring records. The inspector is reconstructing whether the system was managed over time, and only the monitoring log shows that. A sheaf of laboratory reports cannot substitute, because each report is a snapshot of a moment, and a clean moment proves nothing about the months around it. The prosecutions and improvement notices in our enforcement tracker turn on absent or fabricated monitoring records far more often than on any question of sampling.
This is also why the thermometer is the highest-value instrument in water hygiene. A monthly sentinel temperature check takes minutes with the legionella temperature checker to validate the readings, and a line in the log book to record them. Done every month, it is worth more than any single laboratory result.
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.
When monitoring alone is enough, and when testing joins in
For a simple, well-controlled system, hot water at 50°C or more at outlets, cold below 20°C, outlets turned over, tanks sound, monitoring alone is the whole checking regime, and the guidance supports that. Testing joins in where monitoring cannot answer the question: systems that cannot hold temperature and rely on chemical treatment, which must be verified by sampling; cooling towers, spa pools and healthcare premises with set sampling frequencies; investigation of an out-of-range reading or a suspected case; and confirmation that disinfection worked after a positive result. The action levels that turn a count into a response are in legionella CFU levels explained.
The expensive confusion
The pattern that wastes money is a duty holder paying a contractor for quarterly laboratory tests while the monthly temperature checks go undone. It feels rigorous and it is backwards: the quarterly test verifies a moment, while the conditions that actually govern the risk go unwatched for months at a time. Put the effort into the monitoring schedule first, let the risk assessment tell you when sampling is genuinely needed, and record both kinds of check in the same log, kept for at least five years.
Monitoring is the standing watch that keeps the risk controlled day to day; testing is the occasional look closer when the watch raises a question. An inspector reads the watch log first, every time.
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
Is legionella monitoring a legal requirement?
In practice, yes. The duty under the Health and Safety at Work etc. Act 1974 and COSHH, applied through ACOP L8, is to control the risk and to check the controls are working. That checking is monitoring: the routine temperature readings, flushing, and inspections set out in the written scheme of control. Departing from the Approved Code of Practice is possible in law, but only if you can show you met the duty another way, so for most duty holders a monitoring regime is the recognised route to compliance.
What monitoring should I be doing for legionella?
For a typical hot and cold water system, the core regime is monthly hot and cold temperature checks at sentinel outlets, monthly calorifier flow and return temperatures, weekly flushing of little-used outlets, annual temperature checks across all outlets on a rotating basis, and periodic tank and calorifier inspections. The exact schedule comes from your risk assessment and written scheme. Our legionella monitoring schedule guide sets out the usual frequencies.
Do I need testing as well as monitoring?
Most well-controlled systems do not need routine laboratory testing on top of monitoring. Testing joins in when the risk assessment calls for it: where temperature control is not practical and treatment must be verified, in regimes with set sampling frequencies such as cooling towers, spa pools and healthcare, after a positive result to confirm remediation, or when monitoring shows something out of range that needs investigating. Monitoring is the standing watch; testing is the occasional look closer.
How long should legionella records be kept?
Keep monitoring records and laboratory results for at least five years. The records are the evidence that the controls were carried out: an inspector, insurer or court looking back at an incident will ask what was done and when, and only the written record can answer. A monitoring check that was done but not recorded cannot be evidenced, so on paper it did not happen.
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