Your Legionella risk assessment sets how often you need to test, based on the type of water system you have. Most hot and cold water systems in offices, schools and rented homes don’t need routine Legionella sampling. Higher-risk systems that create spray or aerosols, such as cooling towers, evaporative condensers and spa pools, need sampling at least quarterly.
Every system still needs routine monitoring – temperature checks, flushing and inspections – and these records are how most dutyholders prove their controls are working between risk assessment reviews.
Jump to:
- Is Legionella testing a legal requirement?
- How often should each type of water system be tested?
- When do hot and cold water systems need Legionella sampling?
- What’s the difference between Legionella testing and routine monitoring?
- Legionella testing FAQs
Is Legionella testing a legal requirement?
Legionella testing isn’t a blanket legal requirement, but controlling the risk from Legionella is. Under the Health and Safety at Work etc. Act 1974, the Control of Substances Hazardous to Health Regulations 2002 (COSHH) and the Management of Health and Safety at Work Regulations 1999, employers and anyone in control of premises must assess the risk from Legionella and then prevent or control it.
ACoP L8 and HSG 274 set out how to meet that duty. Sampling becomes part of it when your risk assessment or HSE guidance calls for it, for example on cooling towers or when temperature control breaks down. Inspectors look for evidence that your control measures are in place and working, and a clear sample result alone won’t show that.
For a full breakdown of the legislation, see our page on Legionella regulations and compliance.
How often should each type of water system be tested?
Cooling towers, evaporative condensers and spa pools need Legionella sampling at least quarterly, while most hot and cold water systems need sampling only when a specific trigger applies. Systems that create spray or aerosols carry a far higher risk than enclosed hot and cold water systems, which is why they need routine sampling.
| Water system | Routine Legionella sampling | Guidance |
|---|---|---|
| Hot and cold water systems (offices, schools, rented homes, most commercial buildings) | Not usually required – only when triggered (see below) | HSG 274 Part 2 |
| Hot and cold water treated with biocides at reduced temperatures | Monthly at first, reduced once control is proven | HSG 274 Part 2 |
| Cooling towers and evaporative condensers | At least quarterly, plus weekly dip slide checks for general bacteria | HSG 274 Part 1 |
| Spa pools | At least quarterly | HSG 274 Part 3 and HSG 282 |
| Healthcare water systems | Set by the Water Safety Group, with routine sampling common in high-risk and augmented care areas | HTM 04-01 |
Cooling towers need more frequent sampling after commissioning, cleaning and disinfection or system changes, and whenever results exceed action levels. Sampling stays at the higher frequency until control is re-established.
When do hot and cold water systems need Legionella sampling?
Hot and cold water systems need Legionella sampling when there’s reason to doubt that control measures are working. HSG 274 Part 2 and good practice identify these triggers:
- Control levels not consistently achieved: if temperatures or disinfectant concentrations repeatedly fall outside target, sample weekly until control is restored.
- Biocide treatment with reduced temperatures: sample monthly at first, then reduce the frequency as confidence in the treatment grows.
- After cleaning and disinfection: sample to confirm the work has been effective.
- Commissioning and recommissioning: sample new buildings, new systems, and systems returned to use after a shutdown or long period of low occupancy.
- Suspected or confirmed cases: sample if anyone linked to the building is suspected of having, or diagnosed with, Legionnaires’ disease.
- Vulnerable occupants: in hospitals, care homes and other settings with immunocompromised users, the risk assessment or Water Safety Group may require routine sampling.
If none of these apply and your controls are holding, your risk assessment may not call for sampling at all. In that case, the priority is consistent monitoring and accurate records.
What’s the difference between Legionella testing and routine monitoring?
Legionella testing shows whether the bacteria are present in a water sample at a given moment. Routine monitoring confirms, week to week and month to month, that the controls preventing their growth are working. Every system needs routine monitoring, and HSG 274 Part 2 sets the core checks:
| Check | Frequency |
|---|---|
| Hot water at sentinel outlets (50°C within one minute; 55°C in healthcare) | Monthly |
| Cold water at sentinel outlets (below 20°C within two minutes) | Monthly |
| Calorifier flow and return temperatures | Monthly |
| Flushing little-used outlets | Weekly |
| Cleaning and descaling showerheads and hoses | Quarterly, or as the risk assessment requires |
| Cold water storage tank inspection | Annually |
| Representative outlet temperatures | Annually, on a rotating basis |
The temperature targets behind these checks are explained in our guide to what temperature kills Legionella.
Acorn’s water hygiene monitoring covers these checks to a pre-planned schedule. Our engineers log every reading in ServiceTracker, our digital record system, and you can view each timestamped record through our online portal.
Legionella testing FAQs
These answers cover the Legionella testing questions landlords and dutyholders ask most often, from certificates and positive results to who can take samples.
How often should landlords test for Legionella?
Landlords don’t usually need routine Legionella testing. Their legal duty is to assess the risk and keep it under control. For most rented homes with simple hot and cold water systems, that means a risk assessment and simple control measures, with sampling only if the assessment finds a higher risk.
Do landlords need a Legionella certificate?
No. HSE doesn’t require landlords to obtain or produce a Legionella test certificate, and it doesn’t recognise one. Landlords meet their duty by assessing the risk and putting proportionate controls in place, such as keeping hot water hot, flushing little-used outlets and cleaning showerheads.
What happens if a Legionella test is positive?
The response depends on the count and how many samples are positive. Between 100 and 1,000 colony-forming units per litre (cfu/litre), resample if a minority are positive, and review controls if similar results return. If most samples are positive, review controls and the risk assessment immediately. Above 1,000 cfu/litre, resample, review immediately and consider disinfection.
Who can take Legionella samples?
A trained sampler – usually a water hygiene contractor – should take Legionella samples, following BS 7592 as HSE guidance recommends. Samples should go to a UKAS-accredited laboratory that takes part in a water microbiology proficiency testing scheme, so the results are reliable enough to act on.
Need help setting the right Legionella testing schedule?
Acorn’s water hygiene team builds Legionella testing and monitoring schedules around your risk assessment, so you sample when guidance calls for it and your records hold up at inspection. Acorn has been in business since 1990 and supports dutyholders across Manchester, Leeds, Liverpool, Sheffield and the wider UK. As LCA members, we work to ACoP L8, HSG 274 and HTM 04-01, and we record every check and sample in ServiceTracker.
Managing water systems across several sites? Get a testing schedule built around each site’s risk assessment – call a qualified engineer on 0161 483 7112.