How Often Should Legionella Testing Be Done? UK Frequency Guide

"How often should legionella testing be done?" has a confusing answer because "testing" can mean two very different things. If you mean checking water temperatures, the answer is usually monthly. If you mean sending water to a lab to be tested for legionella bacteria, the answer for most ordinary buildings is: not routinely at all. This guide gives the frequency for each type of check, broken down by building type.

The quick answer

For a standard building — a rented home, a small office, most care settings with ordinary plumbing — the recurring checks are:

• Temperature monitoring: monthly (sentinel outlets and hot water storage)
• Flushing of unused outlets: weekly
• Showerhead cleaning/descaling: quarterly
• Risk assessment review: at least every two years (and after any change)
• Laboratory sampling for legionella: not routine — only in specific circumstances

The monthly temperature check is the "testing" most duty holders are actually asking about. Laboratory sampling — the thing most people picture when they hear "legionella test" — is the exception, not the rule.

Temperature monitoring vs laboratory sampling

This is the distinction that clears up the confusion. They are not the same activity, and they have completely different frequencies.

	Temperature monitoring	Laboratory sampling
What it is	Checking hot and cold water temperatures	Sending water samples to a lab for legionella culture
How often	Monthly (for sentinels and storage)	Not routine for most systems
Who needs it	All duty holders	Cooling towers, healthcare, after an incident
Cost	Minimal — a probe thermometer	Per-sample lab fees

HSE is explicit that lab sampling "is not usually required for domestic hot and cold water systems, and is only needed in very specific circumstances." So when a managing agent or contractor talks about "annual legionella testing," they almost always mean the risk assessment review or the ongoing temperature regime — not a lab test. For the full legal breakdown of which type of testing applies to you, see our UK legionella testing requirements guide.

Frequency of temperature checks, in detail

The monthly figure is a simplification. Different parts of the system are on different schedules, drawn from HSG274 Part 2:

Check	Frequency
Hot water sentinel outlets	Monthly
Hot water storage (calorifier) temperature	Monthly
Cold water sentinel outlets	Monthly
Cold water storage tank temperature	At least every 6 months
Cold water storage tank inspection	Annually
Showerhead cleaning/descaling	Quarterly
Flushing of outlets unused 7+ days	Weekly

For the thresholds and how to record these checks, see our temperature monitoring guide.

Frequency by building type

The recurring temperature and control checks are broadly the same across building types — what changes is the level of scrutiny, the review cycle, and whether lab sampling enters the picture.

Residential lets and HMOs

• Temperature monitoring: monthly is good practice; many landlords with very simple systems rely on the risk assessment plus tenant advice rather than monthly readings, depending on what their assessment concludes.
• Risk assessment review: at least every two years. In Scotland, the guidance states this two-year maximum explicitly — see our Scotland landlords guide.
• Lab sampling: not required for standard domestic systems.

Care homes and healthcare settings

• Temperature monitoring: monthly, and taken seriously — occupants are more vulnerable, so the healthcare distribution threshold is 55°C rather than 50°C.
• Risk assessment review: annual review is appropriate for higher-risk buildings.
• Lab sampling: may be required, particularly in augmented-care units, on a frequency set by the risk assessment.

Offices, schools, and small commercial

• Temperature monitoring: monthly for sentinel outlets and storage.
• Flushing: important here because outlets in unused rooms, holiday-closed schools, and seasonal facilities stagnate quickly.
• Lab sampling: not routinely required for standard systems.

Cooling towers and evaporative condensers

• This is the exception. These systems carry the highest risk, must be notified to the local authority, and require routine monitoring including periodic legionella sampling under HSG274 Part 1. If your building has a cooling tower, the "no routine sampling" rule does not apply to you.

When lab sampling IS needed

Regardless of building type, laboratory sampling for legionella becomes appropriate when:

• You operate a cooling tower or evaporative condenser
• You run a healthcare or augmented-care facility where the risk assessment requires it
• There has been a legionella incident or a positive result and you need to confirm remediation worked
• Temperature control cannot be maintained and you need to know whether bacteria are actually present
• A building has been closed or underused for a long period and you want to confirm it is safe before re-occupancy

If you do sample, use a UKAS-accredited laboratory — results from an unaccredited lab carry no evidential weight.

Practical takeaway

1. "Testing" usually means temperature monitoring — monthly for most systems.
2. Lab sampling is the exception — cooling towers, healthcare, and after incidents, not routine domestic checks.
3. Review the risk assessment every two years at minimum, sooner if anything changes.
4. Frequency scales with risk — same core checks, more scrutiny in vulnerable settings.
5. Keep records of everything — the dates and readings are your evidence of compliance.

Whatever your building type, consistent records of the routine checks are what an inspector wants to see — far more than a one-off lab result.

This guidance covers legionella monitoring frequency under ACoP L8 and HSG274 for the UK. This is general compliance guidance, not legal or professional advice — for site-specific frequencies, follow the conclusions of your own risk assessment and consult a competent person as defined by ACoP L8.

Sources

• HSE — Hot and cold water systems
• HSE — Hot and cold water systems: things to consider
• HSE — Legionella and landlords' responsibilities
• HSE — HSG274: Legionnaires' disease technical guidance