How often does legionella testing need to be done?
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Key points
- "Testing" covers two different things: routine monitoring (temperatures, flushing, inspections) and laboratory sampling. They run on completely different clocks.
- Monitoring is frequent and fixed: weekly flushing, monthly sentinel temperatures, periodic tank and calorifier inspections.
- Laboratory sampling is risk-based, not routine for most hot and cold water systems — there is no calendar interval in ACOP L8 or HSG274.
- Cooling towers and some healthcare systems are the exception: routine sampling is an expected part of their control regime.
- A lab result is a snapshot of one day; the monitoring log is what shows control over time — and records are kept for at least five years.
There is no fixed interval for legionella testing because "testing" is really two different activities running on two different clocks. Routine monitoring — temperature checks, flushing, inspections — runs weekly and monthly for the life of the system. Laboratory sampling for legionella is risk-based: most ordinary hot and cold water systems never need it on a schedule at all, and it is done when the risk assessment or the monitoring log raises a question that only a laboratory can answer.
The two meanings of "testing"
Most people typing this question are being told two contradictory things — "test monthly" and "you don't need routine tests" — and both are true, about different things:
| Routine monitoring | Laboratory sampling | |
|---|---|---|
| What it is | Thermometer readings, flushing, visual inspections you do yourself | Water samples cultured by an accredited laboratory |
| How often | Fixed frequencies: weekly flushing, monthly sentinel temperatures, periodic tank and calorifier inspections | When the risk assessment or a problem calls for it — no routine interval for most systems |
| What it tells you | Whether the control conditions (hot, cold, moving water) are holding | Whether legionella was actually present in that water on that day |
The monitoring side is the backbone of control and its frequencies come from HSG274: weekly flushing of little-used outlets, monthly checks at sentinel outlets, and the wider schedule of tank, calorifier and shower-head tasks in the monitoring schedule. The temperature checker structures the monthly round, and everything lands in the log book.
So when does laboratory testing happen?
Sampling earns its keep when it answers a question monitoring cannot: validating a new or altered system, checking a system where temperature control is hard to achieve, verifying that disinfection has worked after a positive result or a shutdown, investigating persistent out-of-range readings, or protecting susceptible people in healthcare and augmented care. The full picture of when samples are taken and what the 100 and 1,000 CFU per litre action levels mean is in sampling and action levels and the legionella testing guide. The exception to "no routine sampling" is higher-risk plant: evaporative cooling systems have routine sampling built into their control regime under HSG274 Part 1, and healthcare water safety plans may set a sampling schedule based on local risk.
Monitor on a schedule; sample on a reason. Weekly and monthly checks show your controls are holding every day — a laboratory test answers the specific question those checks cannot.
What this means for landlords and small premises
For a landlord, the practical answer is that routine laboratory testing is not expected. The duty under ACOP L8 is to assess the risk and control it — hot water hot, cold water cold, systems flushed through before re-letting after a void, heads and hoses clean — and to keep the records that show it. The same applies to most small workplaces with simple systems: the risk assessment and the monitoring log are the evidence, not a lab certificate. Anyone selling you a blanket "annual legionella test" for a simple domestic system is selling a snapshot that, on its own, neither controls the risk nor discharges the duty. Our companion answer on how often legionella testing should be done covers the landlord angle in more depth.
Why the records outrank the results
A negative sample tells you about one outlet on one day; it says nothing about next month. The monitoring log, kept for at least five years, is what demonstrates continuing control to an inspector, an insurer or a court. That is why the honest answer to "how often does testing need to be done" is: monitoring constantly, sampling when there is a reason — and everything written down either way.
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.
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 often do you need to test water for legionella?
There is no fixed legal interval for laboratory legionella testing of ordinary hot and cold water systems. Routine control is monitoring — weekly flushing of little-used outlets, monthly sentinel temperature checks, periodic tank and calorifier inspections — while laboratory sampling is risk-based: carried out when the risk assessment calls for it, when control is in doubt, or to verify that disinfection has worked. Higher-risk systems such as cooling towers are the exception, where routine sampling is an expected part of the control regime.
Do landlords have to get legionella tests done?
No law or HSE guidance requires landlords to carry out routine laboratory legionella testing of domestic water systems. The duty is to assess the risk and control it, which in most rented homes means simple measures: keeping hot water hot, cold water cold, flushing before re-letting after a void, and making sure outlets are clean. What a landlord must be able to show is the assessment and the records of the controls, not a laboratory certificate.
Is a legionella test the same as a temperature check?
No, and confusing the two is the commonest gap in water-safety records. A temperature check is routine monitoring you do yourself with a thermometer, weekly or monthly, to confirm the control conditions are holding. A legionella test is a laboratory sample, cultured over several days by an accredited laboratory, which tells you whether the bacteria were actually present in that water on that day. Monitoring is frequent and cheap; testing is occasional and answers a specific question.
When should you actually take a legionella sample?
Sample when the risk assessment identifies a reason: a system where control by temperature is difficult to achieve, a healthcare setting with susceptible occupants, after a positive result to check remediation worked, after long stagnation, or when routine monitoring keeps showing readings out of range and the cause cannot be found. Sampling for its own sake, on a healthy system with good records, adds cost without adding control.
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