Testing for legionella means taking a water sample and having a laboratory culture it. It is not the normal way of showing a system is under control, and buying it instead of monitoring is the most common mistake in this subject. HSE is direct about it: testing should not be confused with temperature monitoring, which is a reliable method for confirming the water system is under control.
When sampling is actually needed
In defined circumstances rather than as routine. Where control measures are not reliably achieving what the written scheme requires. Where the system is treated with biocides rather than temperature. In healthcare and other settings with highly susceptible people. In cooling towers and evaporative condensers, where sampling is routine. After a case of the disease is linked to the system, and after remedial work such as a disinfection. HSE states that sampling is not usually required for domestic hot and cold water systems.
What temperature monitoring does instead
It measures the thing the control actually depends on. Hot water leaving the calorifier and returning, hot water at sentinel outlets, cold water at sentinel outlets and at the tank, and a rotating sample of the rest. Readings that stay in range are evidence the system is controlled, taken every month rather than once a year, and they cost a fraction of a sampling programme.
How a sample is taken and what it means
Samples are taken by a trained person to a defined method, kept cool and delivered to a UKAS accredited laboratory within a set time, and cultured over several days. A negative result means legionella was not detected in that sample at that outlet on that day, which is weaker than it sounds. A positive result triggers the response the written scheme should already have set out, not a decision made under pressure.
What testing costs depend on
The number of sample points, whether an engineer attends specially or samples during an existing monitoring visit, the turnaround the laboratory is asked for, and whether the price includes interpretation and a report. A quote per sample with no attendance or reporting included is not comparable with one that has them.
Questions people ask about legionella testing
Is legionella testing a legal requirement?
No. The requirement is to assess and control the risk. Sampling is needed in specific circumstances rather than as routine, and HSE states it is not usually required for domestic hot and cold water systems.
How often should legionella testing be done?
Where sampling is warranted, the frequency comes from the written scheme of control and the system type. Cooling towers are sampled routinely; domestic hot and cold water systems usually are not sampled at all.
What is the difference between testing and monitoring?
Testing cultures a water sample for the bacteria. Monitoring measures temperatures and checks that the control measures are being carried out. Monitoring is the routine evidence that the system is controlled.
Do I need a legionella test certificate?
HSE states that health and safety law does not require landlords to obtain or produce a legionella test certificate. What is required is that the risk is assessed and controlled.