How often should water be tested for legionella?
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Key points
- Ordinary hot and cold water systems have no routine lab-testing frequency — sampling happens on triggers, not on a calendar.
- Cooling towers are the exception: routine sampling is part of the control regime under HSG274 Part 1.
- The six triggers: commissioning, doubt about control, post-positive verification, post-disinfection or shutdown, a linked case, or a system flagged by the risk assessment.
- Culture results take roughly ten to fourteen days, and the result describes one day — read it beside the monitoring log, not instead of it.
- Samples go to a UKAS-accredited laboratory and must be taken competently, or a clean result may be a false reassurance.
For most buildings the honest answer is: as often as your risk assessment says — and for a simple, well-controlled hot and cold water system, that is usually not on any fixed schedule at all. Routine legionella control is monitoring (weekly flushing, monthly temperatures, periodic inspections), and laboratory testing is reserved for the systems and the moments where it adds information that monitoring cannot give you.
Frequency by system type
What "how often" looks like depends entirely on what the water system is:
| System | Testing position |
|---|---|
| Domestic-style hot and cold water (rented homes, small offices) | No routine sampling. Temperature monitoring and flushing; test only on a trigger. |
| Larger or complex hot and cold systems (hotels, schools, larger workplaces) | No fixed interval. Sample where the risk assessment flags temperature control as hard to guarantee, or on a trigger. |
| Healthcare and augmented care | Sampling schedule set by the water safety plan under HTM 04-01, reflecting susceptible occupants. |
| Evaporative cooling towers and condensers | Routine sampling is an expected part of the HSG274 Part 1 control regime, with frequency tightened when counts rise or control is being re-established. |
| Spa pools and hot tubs | Regular microbiological testing is part of the HSG282 control regime alongside continuous disinfectant monitoring. |
The pattern is consistent: the more the system relies on chemistry rather than temperature, and the more susceptible the people exposed, the more sampling moves from "on a trigger" to "on a schedule".
The triggers that call for a test
Where there is no standing schedule, these are the events that should put sampling on the agenda. Commissioning a new or substantially altered system, to validate that it starts clean. Doubt about control — for example a sentinel outlet that keeps failing its monthly check with no fault found. Verification after a positive result, to confirm remediation worked. After disinfection or a long shutdown, before normal use resumes. A case of Legionnaires' disease linked to the premises. And any system the risk assessment flags as unable to hold temperature control reliably.
Sample to answer a question: is this new system clean, did that disinfection work, is this branch actually colonised? A sample taken with no question behind it is a cost, not a control.
Getting a result you can trust
A legionella result is only as good as the sample and the laboratory behind it. Samples should be analysed by a UKAS-accredited laboratory using the standard culture method — the testing labs directory lists accredited options — and results take around ten to fourteen days because the method grows the organism before counting it. Sampling technique matters: the right bottles, the right outlets (including outlets that represent the worst case, not just the convenient tap), and prompt transport. What the numbers mean once they arrive — the 100 and 1,000 CFU per litre action levels and the responses each triggers — is set out in sampling and action levels, and what to do if legionella is detected walks through the response.
Testing sits inside the monitoring regime, not above it
Whatever the sampling position, the day-to-day control is still monitoring: weekly flushing of little-used outlets, monthly temperature checks logged with the temperature checker, and the full schedule of inspections in the monitoring schedule, all recorded in the log book for at least five years. A test result joins that record as one entry among many; the regime around it is what keeps the risk controlled between samples.
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 should water be tested for legionella?
For most hot and cold water systems there is no set testing frequency at all: routine control is temperature monitoring and flushing, and laboratory testing is carried out only when the risk assessment identifies a need. Evaporative cooling systems are different and have routine sampling built into their regime under HSG274 Part 1, and healthcare premises may sample on a schedule set by their water safety plan. The frequency, where one exists, always comes from the risk assessment for that specific system.
What triggers a legionella water test?
The usual triggers are: commissioning a new or substantially altered system; doubt that control is working, such as persistent out-of-range temperatures; after a positive result, to verify remediation; after disinfection or a long shutdown, before the system goes back into normal use; a suspected or confirmed case of Legionnaires' disease linked to the premises; and a risk assessment that flags a system where temperature control alone is hard to guarantee.
How long do legionella test results take?
The standard culture method grows the sample on selective media and results typically take around ten to fourteen days from receipt at the laboratory. Faster methods exist, but culture remains the reference method against which action levels are set. Because a result describes the water on the day it was sampled, it should always be read alongside the monitoring log rather than as a standalone verdict on the system.
Who can take a legionella water sample?
Samples should go to a UKAS-accredited laboratory using the standard culture method, and the sampling itself needs to be done competently: correct bottles (usually containing a neutraliser for chlorinated water), the right outlets, and proper transport so the sample reaches the laboratory promptly. A poorly taken or poorly transported sample can produce a result that looks reassuring and is simply wrong.
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