---
title: "Beyond the survey: continuous Legionella risk assessment"
source_url: https://legionella.io/articles/beyond-the-survey-continuous-legionella-risk-assessment/
canonical_url: https://legionella.io/articles/beyond-the-survey-continuous-legionella-risk-assessment/
pillar: "Legionella Risk Assessment"
summary: "Your last survey came back clean, yet control still drifts. Read monitoring signals as live risk inputs, trace each fault to its cause and act early."
primary_keyword: "continuous risk assessment"
date_published: 2025-12-02
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Beyond the survey: continuous Legionella risk assessment

A risk assessment dated eighteen months ago tells you the system was under control on the day someone walked it. It says nothing about the cold tank that has crept warm since the plant room ventilation was reworked in March, or the third-floor shower that has not run since the team moved desks. Control is a daily condition. A survey is a photograph.

Continuous risk assessment closes that gap, and it does not mean commissioning more surveys. It means treating the evidence you already gather — temperature logs, flushing records, complaints, sample results, a note that a wing has reopened — as live inputs that can change your risk picture today rather than at the next scheduled review. Done properly it turns into a fault-finding habit: every odd reading becomes a chance to catch where control slipped before it becomes an exposure.

## Start with the signal, not the spreadsheet

The instinct when something looks off is to open the logbook and check whether a box was ticked. Reverse it. Begin with the live signal — the out-of-range temperature, the missed flush, the complaint, the unexpected sample result — and trace backwards to whatever allowed it.

Work the most likely causes first, because in UK building stock they cluster predictably. Tepid water sits in long cold runs and oversized tanks; it stagnates in low-use outlets and forgotten dead legs; a tired thermostatic mixing valve lets hot and cold cross-feed; and the asset register quietly falls out of step with a building that has been altered, mothballed and reopened a dozen times. Most signals you chase will land on one of those.

Then ask the question that turns troubleshooting into assessment: is this a one-off defect, or a symptom of a control that has quietly stopped working? A single missed flush on one outlet is a defect. The same outlet missed for three months, while two nearby outlets drift warm, is a management weakness. Surfacing that difference is the whole point of keeping the assessment live between reviews.

## A fault-finding flow for the signals you actually see

Use the table below the way you would use a diagnostic chart in a workshop: match the symptom, take the likely cause as a starting hypothesis, run the one check that confirms or kills it, then act — and note whether the action is a repair or a change to the assessment itself.

| Warning sign | Most likely cause | The check that confirms it | What it changes |
| --- | --- | --- | --- |
| A blended outlet's cold side reads above the cold target your scheme sets | Heat gain along a long cold run, or cross-flow through a worn TMV | Read the cold temperature at the nearest unblended tap and back at the tank | If upstream cold is fine, it is local heat gain or a TMV fault — log as a defect; if several outlets read warm, re-rate the cold distribution as a control gap |
| The same low-use outlet keeps missing its weekly flush | The outlet is effectively redundant, or no one owns the task with the access to do it | Cross-check the asset register against the real use pattern; find the named task owner | A genuine one-off: reschedule. A pattern: either remove the dead leg or re-assess the use pattern and reassign the task |
| An occupant reports cloudy, smelly or "off" water | Stagnation or biofilm in a low-use branch, or a fault at the storage tank | Inspect the tank and that branch; check when the outlet was last used and last cleaned | Localised: clean and flush. Several reports: inspect the tank and re-rate the affected zone |
| A sample returns a detectable count | A control that has been drifting — temperature or stagnation — rather than a lab quirk | Pull the temperature and flushing history for that outlet around the sample date | Identifies which control failed; remediate and treat the result as a review trigger, not a one-line logbook entry |
| New TMVs or showers fitted, a wing reopened, occupancy changed | The asset register and schematic no longer match the building | Walk the change against the schematic and mark up what is new | Update the register, re-rate the new or reactivated outlets, and record a formal review trigger |

Hot and cold temperatures sit behind several of these rows. As a general expectation, hot water should leave the calorifier hot and still arrive hot at distant outlets, and cold should stay genuinely cold rather than tepid [5]; the exact figures and tolerances are set by your assessment, not by a rule of thumb.

## When a defect becomes a review trigger

L8 gives the risk assessment and the review of control measures Approved Code of Practice status, which makes them a standing duty rather than an optional survey [1]. The duty does not stop at "review every so often". You are expected to review whenever there is reason to believe the assessment is no longer valid — a change to the water system or how it is used, a change to the people exposed, new information about the risk, monitoring results showing a control is no longer effective, or a case associated with the system [3].

Continuous risk assessment is how you notice those triggers in week two rather than at the calendar review. The HSG274 written scheme of control is the place each decision lives, so the discipline that makes the habit work is small and unglamorous: record the decision, not just the task [2]. "This outlet is flushed weekly because use is intermittent; a missed flush escalates to the responsible person; three misses in a quarter triggers a use-pattern review" is worth more than a tick, because it tells the next person what good looks like and when to act.

Escalation is where the fault-finding mindset earns its keep. When the same weakness appears in more than one place, stop treating outlets one at a time and raise it to a formal review with the responsible person — you are now looking at a system failure, not a defect. A detectable sample result, or a suspected case of illness, is a different order of problem: that goes to the duty holder immediately, sampling and its frequency follow the system and the assessment rather than a fixed schedule [4], and a confirmed case linked to a workplace water system can carry separate reporting obligations.

## Keeping the assessment alive between reviews

None of this works if the assessment lives in a drawer and the monitoring lives in a different system that nobody reconciles. The practical move is to wire the two together so that an out-of-range reading or a missed task lands in front of the person who can re-rate the risk, which is really a question of how Legionella sits inside your wider safety routine — covered in [Integrating Legionella risk assessment into health and safety management](https://legionella.io/articles/integrating-legionella-risk-assessment-into-health-and-safety-management/). Over time, a risk assessment that keeps absorbing live evidence is also the natural foundation for a fuller water safety plan, the direction set out in [From risk assessment to Water Safety Plan: next steps](https://legionella.io/articles/from-risk-assessment-to-water-safety-plan-next-steps/).

## A note on limits

Continuous risk assessment supplements the formal written assessment carried out by a competent person; it does not replace it, and nothing here sets your numbers for you. Which temperatures count as out of range, how often you flush, the count that prompts action and the point at which you escalate all belong in your own assessment, decided for your system and your users. Treat this as general guidance rather than legal advice — and remember that a suspected case of Legionnaires' disease is a clinical and public-health matter, diagnosed by clinicians and investigated by the relevant authorities, not something to read off a logbook.

## FAQ

### If we assess risk continuously, do we still need the periodic survey?
Yes. The formal written risk assessment by a competent person remains the duty; continuous assessment keeps it current between those reviews and feeds them with real evidence. Think of the survey as the anchor and the day-to-day signals as the thing that tells you when to re-cast it early [1].

### What actually counts as a trigger to revisit the assessment, rather than just fix the fault?
A change to the system or its use, a change to who is exposed, new information about the risk, or monitoring results showing a control is no longer working [3]. A single repaired defect usually is not a trigger; the same defect recurring, or showing up across several outlets, is — that points at the control, not the component.

### Who owns the fault-finding day to day, us or our contractor?
The responsible person owns it. A contractor can supply readings, samples and remedial work, but interpreting the signals, deciding whether one is a defect or a drift, and triggering a review are duty-holder calls. Outsourcing the tasks does not outsource the judgement.

## Related reading

- [Integrating Legionella risk assessment into health and safety management](https://legionella.io/articles/integrating-legionella-risk-assessment-into-health-and-safety-management/)
- [Designing plumbing systems for optimal temperature control](https://legionella.io/articles/designing-plumbing-systems-for-optimal-temperature-control/)
- [From risk assessment to Water Safety Plan: next steps](https://legionella.io/articles/from-risk-assessment-to-water-safety-plan-next-steps/)
- [The cost of non-compliance: legal and business impacts](https://legionella.io/articles/the-cost-of-non-compliance-legal-and-business-impacts/)

## Sources

[1] HSE, "Legionnaires' disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)". https://www.hse.gov.uk/pubns/books/l8.htm
[2] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] HSE, "Legionnaires' disease - what you must do". https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm
[4] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm
[5] HSE, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm