What is the difference between legionella monitoring and a risk assessment?

A legionella risk assessment tells you what could go wrong in your water system and what controls it needs. Monitoring is the routine checking that those controls are actually working: the temperature readings, the weekly flushing, the tank and calorifier inspections, each carried out at a set frequency and written down. The assessment is the diagnosis and prescription; monitoring is taking the medicine and keeping the chart.

The two are constantly confused because both involve "checking legionella". They differ in purpose, frequency, and output, and confusing them leads to two characteristic failures, which this page names. The mechanics of each are covered in legionella risk assessment and the legionella monitoring schedule; here they are side by side.

The two compared

How the loop is meant to run

The compliance cycle is a loop, and the two activities are opposite sides of it. The assessment walks the system, finds the dead legs, the little-used outlets, the tank with the ill-fitting lid, and the tepid blended water, and sets a control for each. The written scheme of control turns those controls into scheduled tasks with named owners and frequencies. Monitoring then executes the scheme: run the outlets on the flushing list weekly, read the sentinel temperatures monthly, inspect the calorifier and the tank on their cycle. The records accumulate. And the results feed back: when monitoring shows a persistent failure, an outlet that never reaches temperature, a tank that keeps warming, that is a trigger to revisit the assessment, because the system is telling you the control is not working.

That feedback is the point of the whole arrangement. Monitoring is not box-ticking for its own sake; it is the early-warning system that catches drift before it becomes an exposure. Check your readings against the target figures with the temperature checker and the temperature chart.

The two failure modes

Monitoring without an assessment is checking blind. Some buildings have a caretaker who faithfully takes temperatures every month at the same two taps, with no idea whether those are the right taps, whether the flushing list is complete, or whether the system even has stored water. The records look reassuring and prove almost nothing, because nobody ever identified what the risks were. The readings are real; the coverage is luck.

An assessment without monitoring is a document in a drawer. The mirror image is the professional assessment, properly done, whose recommendations were never scheduled. No flushing happens, no temperatures are taken, and two years later nothing has changed except the date. In enforcement terms this is often the worse position, because the duty holder was told in writing what the risks were and did not act. An unimplemented assessment is not evidence of compliance; it is evidence of notice.

Getting the rhythm right

For most premises the rhythm is simple to state. Assess, then schedule: weekly flushing of little-used outlets, monthly temperature checks at the sentinel outlets and the calorifier, periodic tank, calorifier, and TMV inspections, all recorded as they happen. Review the assessment about every two years, and immediately when the building changes use, the system is altered, occupants change, or monitoring reveals a persistent failure, as set out in how often to review a risk assessment. The risk assessment builder produces the assessment and the scheme in the same structure, so the monitoring tasks fall out of the findings automatically, and neither side of the loop gets lost. Neither activity certifies the system; together they are what controlling it actually looks like.