Updated 12 September 2026
Key points
- Temperature monitoring is the primary control for most hot and cold water systems: hot ≥50°C at outlets within one minute, cold <20°C within two minutes, calorifier stored at ≥60°C.
- Laboratory sampling is not routine for most systems. It answers specific questions: verifying treatment where temperature control is not practical, after a positive result or a case, or where a regime such as a cooling tower, spa pool or healthcare premises requires it.
- Samples go to a UKAS-accredited laboratory cultured to BS EN ISO 11731. Results come back in colony-forming units per litre (cfu/L) after around ten days.
- Results are read against the HSG274 action ladder: below 100 cfu/L the system is under control; 100 to 1,000 review and resample; above 1,000 immediate remedial action.
- A dip slide does not test for legionella. It measures general bacteria and cannot substitute for a legionella culture.
- Published laboratory prices start from around £45 per sample, and most laboratories supply the bottles and return packaging.
Duty holders regularly get sold the wrong kind of testing in both directions. Some pay for routine laboratory sampling their system does not need. Others skip sampling they do need because they believe monthly thermometer readings cover everything. Neither extreme matches the guidance in HSG274, which treats monitoring and sampling as different tools for different jobs. This page separates the two, then walks through sampling from the decision to the result.
Temperature monitoring versus laboratory sampling
The single most important distinction on this page. One is a routine control check you do yourself; the other is a targeted laboratory measurement done when there is a reason.
Temperature monitoring is the routine checking that your controls are working. Hot water at sentinel outlets should reach 50°C within one minute of opening the tap (55°C in healthcare), cold water should be below 20°C within two minutes of running, and the calorifier should store water at or above 60°C. Sentinel outlets, the nearest and furthest on each loop, are checked monthly, with every outlet covered at least annually on a rotating basis, and little-used outlets flushed weekly. Each reading only counts if it is written down, and records are kept for at least five years. See legionella temperature control for the detail and the temperature chart for a printable reference.
Laboratory sampling measures legionella directly. A water sample goes to an accredited laboratory, is cultured for around ten days, and comes back as a count in cfu/L. It is a snapshot of one outlet on one day, which is exactly why it is a targeted tool rather than a routine one: a clear result last month says nothing about the reading you would get today if the calorifier return has since dropped into the growth range. Monitoring versus testing sets out the distinction the way an inspector applies it.
Who needs which
For the typical system, the split looks like this. The specific programme for your premises comes from your written risk assessment, not from any table, including this one:
| System type | Temperature monitoring | Laboratory sampling |
|---|---|---|
| Single domestic let (combi boiler) | Initial temperature check at each outlet | Not required |
| Small office | Monthly at sentinel outlets | Not required |
| Care home | Monthly sentinel, annual every outlet | Annual, or as the risk assessment requires |
| School | Monthly sentinel, termly every outlet | Annual, plus post-holiday if risk indicates |
| Hotel | Monthly sentinel, annual every outlet | Annual |
| Hospital | Monthly sentinel, per HTM 04-01 | Quarterly at sentinel outlets |
| Spa pool | Daily water quality checks | Monthly |
| Cooling tower | Weekly water quality, monthly dip slide | Quarterly |
When temperature monitoring is not enough on its own
Some systems cannot hold the target temperatures, most often very long distribution runs, listed buildings where pipework cannot be rerouted or insulated, systems with thermostatic mixing valves on every outlet (as in care homes and schools, where TMV servicing is six-monthly in healthcare and annual elsewhere), and buildings whose cold supply rises above 20°C in summer. In those cases the written scheme specifies an alternative primary control, usually chemical or continuous treatment, and sampling is used to verify the treatment is working. Monitoring continues alongside; it never stops being the record.
Landlords are a special case worth stating plainly: the duty is to assess and control the risk, and for most domestic lets that means temperature checks, flushing before re-letting, and records, not routine laboratory sampling. Legionella testing for landlords walks through what is actually expected, and is legionella testing a legal requirement? gives the legal position.
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.
The sampling process, end to end
The laboratory run is only the middle step. A test that answers a real question starts before the kit arrives and ends after the report comes back.
- Decide whether you need to sample at all. The risk assessment answers this first. Sampling verifies a treatment regime, investigates an out-of-range reading, follows up a positive result or a case, or meets a specific regime such as a cooling tower (sampled at least quarterly under HSG274 Part 1), a spa pool (at least monthly under HSG274 Part 3) or healthcare premises under HTM 04-01.
- Choose a laboratory whose UKAS scope covers legionella in water. Accreditation for other tests does not count; the check takes five minutes on the UKAS register and the section below shows how.
- Plan the sample points before the kit arrives. Each sample comes from a specific, documented outlet chosen from the risk assessment. Composite samples mixed from several outlets are useless because they cannot be traced back to a source.
- Take the sample correctly. Cold water is sampled before any flushing, in a sterile bottle containing sodium thiosulphate to neutralise residual chlorine. Hot water is sampled after the tap has run to temperature, to represent typical user exposure. Every bottle is labelled with the outlet, date and time.
- Get it to the laboratory within 24 to 48 hours, kept cool. Most laboratories supply kits with prepaid courier or return packaging for exactly this reason.
- Read the result against the action levels, act on it, and record it. A report filed unread is the same as no test. The action ladder is in the next section.
How to test for legionella walks through all seven steps in detail, and how to take a water sample covers the bottle-to-lab procedure, including the pre-flush and post-flush distinction and the mistakes that invalidate a sample.
The CFU/L action ladder
The laboratory report gives a colony-forming unit count per litre for each legionella species found, or a “not detected” result. The action thresholds in HSG274 Part 2 turn that number into a decision.
| Result | Status | What you do |
|---|---|---|
| Not detected | Under control | No action beyond routine monitoring. Record the result. |
| Below 100 cfu/L | Under control | System is under control. Continue monitoring. |
| 100 to 1,000 cfu/L | Review | Review the control scheme, resample, and investigate any localised cause. |
| Above 1,000 cfu/L | Act now | Take immediate remedial action and consider whether occupants need to be informed. |
Cooling towers and healthcare premises work to different, lower thresholds under their own regimes. Legionella CFU levels explained goes through what the count means and what to do at each rung of the ladder, and what to do if legionella is detected covers the response step by step.
Culture, dip slides and PCR
Three methods get sold as “legionella testing”. Only one of them is the reference method the guidance is written around.
| Method | What it measures | Typical time | What it can and cannot tell you |
|---|---|---|---|
| Culture (BS EN ISO 11731) | Live, culturable legionella, counted and speciated | Around 10 days | The reference method. The result the HSG274 action ladder is written for. Slow, because the bacteria have to grow. |
| Dip slide | General bacterial growth (total viable count) | 24 to 48 hours incubation | Does not test for legionella. A general hygiene indicator, used for example in cooling tower water-treatment control. It cannot substitute for a legionella culture. |
| qPCR (rapid DNA test) | Legionella DNA | Often within a day | Fast, but cannot distinguish live from dead cells, so it is a screening and triage tool rather than a like-for-like replacement for culture. |
The dip slide confusion is the expensive one: a clear dip slide feels like reassurance about legionella and is nothing of the kind. Dip slides versus laboratory testing is the honest comparison, and legionella test kits reviewed explains what each kind of kit on the market can actually tell you before you buy.
Testing costs
Published laboratory prices start from around £45 per sample for a standard culture analysis. Most laboratories include or supply the sample bottles and return packaging, and the per-sample price usually falls when several samples go together.
The bigger cost mistake is paying for tests you do not need. A blanket “annual legionella test” on a simple, well-controlled domestic system buys a snapshot that neither controls the risk nor, on its own, evidences the duty: the monitoring log is the evidence, not a laboratory certificate. The testing cost breakdown prices the work line by line.
Finding a laboratory
Samples travel by courier, so the nearest accredited laboratory is effectively your letterbox. Choose on UKAS scope, turnaround and price, not distance. These three examples come from our verified directory:
| Laboratory | Turnaround | Price from |
|---|---|---|
| 20/30 Labs Ltd | Not published | Not published |
| ALS Environmental (ALS Laboratories (UK) Limited) | Not published | Not published |
| Alpha Scientific Limited (trading as Alpha Scientific / ADEY) | Not published | Not published |
Examples from the directory, ordered the same way the directory orders them. “Not published” means the figure could not be confirmed from the UKAS register or the laboratory's own site, not that it does not exist.
The full testing laboratory directory compares 12 UKAS-accredited laboratories, each verified against the UKAS register for legionella-in-water scope, and shows you how to check any laboratory's accreditation schedule yourself before you commit. Legionella testing near me explains why geography matters far less than buyers assume.
Every testing guide, in one place
Nine detailed guides sit behind this hub. Each answers one question properly.
- How to test for legionella — the full process in seven steps, from deciding whether you need to test to acting on the result.
- How to take a water sample — the bottle-to-lab procedure, pre-flush versus post-flush, labelling, and the 24 to 48 hour transport rule.
- CFU levels explained — what a cfu/L count means and the HSG274 action ladder rung by rung.
- Dip slides versus laboratory testing — why a dip slide does not test for legionella, and when each method is the right tool.
- Legionella test kits reviewed — the three kinds of kit and what each can and cannot tell you.
- Is legionella testing a legal requirement? — the real legal position, and when sampling is effectively required.
- Legionella testing for landlords — what landlords actually have to do, and why most domestic lets do not need routine sampling.
- Legionella testing near me — why the nearest accredited lab is your letterbox, and how to choose on the facts.
- Monitoring versus testing — the distinction inspectors apply, and why the monitoring log matters more than any lab report.
Legionella testing kits, dip slides, and UKAS-accredited laboratory analysis will be listed here shortly.
What this page is, and is not
This page explains the guidance in ACOP L8 and HSG274 and structures the decisions around monitoring and sampling. It is an indicative starting point for building your own programme: it records and it guides, and it never certifies. Whether you need to sample, where, and how often are questions your written risk assessment answers for your specific premises. No result, log or template on its own makes a system safe or compliant; the controls and the records together are the evidence.