---
title: "Case study: temperature failures leading to Legionella growth"
source_url: https://legionella.io/articles/case-study-temperature-failures-leading-to-legionella-growth/
canonical_url: https://legionella.io/articles/case-study-temperature-failures-leading-to-legionella-growth/
pillar: "Water Temperature Control"
summary: "A UK case study tracing how small hot and cold water temperature slips combine into Legionella growth, and the routine checks that catch the drift early."
primary_keyword: "Legionella temperature case study"
date_published: 2025-08-05
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."
---

# Case study: temperature failures leading to Legionella growth

Temperature failures rarely arrive as a single dramatic event. There is no burst pipe, no flashing alarm. Hot water simply leaves the plant a degree or two cooler than it should, a loft tank warms through a hot July, a refurbished wing sits unused for a month, and one quiet morning a routine sample comes back positive.

The walkthrough below is a composite. It is built from the failure patterns that recur across UK buildings, not from any single real site, and nobody in it falls ill. Read it the way an investigator would: not "what went wrong" as a verdict, but where control drifted, who could have seen it, and what would have stopped it. That is the most useful way to handle a Legionella temperature case study, as a fault-finding exercise rather than a compliance label to file away.

## The building, before anything went wrong

Picture a four-storey, 60-bed care home with a gas-fired calorifier feeding a long distribution loop out to a far wing, cold water drawn from a storage tank in the loft, and thermostatic mixing valves (TMVs) fitted at resident basins and baths to guard against scalding. Monthly sentinel temperatures were recorded on a paper logbook. An annual sample was taken. On paper, it was a textbook arrangement.

The residents were older and, in many cases, frailer than the general population, so the stakes were never abstract. The point of the story is that none of the individual decisions that follow looked reckless at the time. Each was defensible in isolation. The growth came from how they stacked up.

## How the drift actually happened

It started with a sensible-sounding decision. An energy review recommended lowering the calorifier storage set point to trim the gas bill, and the home dropped it from around 60 degrees C into the low 50s. No one revisited the water risk assessment, because turning a dial down felt like a maintenance tweak rather than a change to a control measure.

HSE's hot and cold water guidance sets out the long-standing benchmark this system depended on: store hot water at around 60 degrees C and distribute it so it reaches roughly 50 degrees C (55 degrees C in healthcare premises) at the outlet within about a minute, while keeping cold water below around 20 degrees C [1]. Lower the stored temperature and you eat into the margin the rest of the system relies on to stay hot all the way to the tap.

That margin mattered most at the far wing. The horizontal run out to it was long, so by the time water arrived it had cooled into the mid-40s. Each TMV then blended it down further for scald protection, which is exactly its job, but it left a short tepid tail of pipework downstream of every valve that never saw hot water again. Mid-40s feeding into 40-degree blended legs is squarely in the band where Legionella multiplies.

The cold side drifted in parallel. The loft tank was poorly insulated, and across a warm summer the stored cold water crept above 20 degrees C, losing the low-temperature protection it should have given. Seasonal swings like this are predictable and worth planning for in advance ([Seasonal changes: managing water temperature in summer and winter](https://legionella.io/articles/seasonal-changes-managing-water-temperature-in-summer-and-winter/) covers managing them through the year).

Then came the refurbishment. One wing was taken out of use for redecoration, and its outlets sat unflushed for weeks. The flushing of low-use outlets was supposed to continue on the paper schedule, but with the wing closed off and staff stretched, the entries became patchy and then stopped.

The most telling part is what the records did show. The monthly sentinel readings were taken and written down faithfully, and they were quietly sliding downward month on month. Each single figure looked close enough to wave through. Nobody plotted the line. The trend was sitting in the logbook the whole time, unread.

The discovery came from the annual sample, taken on the far wing, which returned a count above the action level. That triggered an investigation, a disinfection, and a hard look at every reading taken in the previous year.

## What the records should have caught

This is where reading the case as fault-finding pays off, because almost every warning was already on file.

The downward slide in sentinel temperatures was the loudest signal, and it was missed because the home checked each reading against a pass/fail threshold instead of watching the trend. A reading of 51 degrees C is "in range"; three readings of 54, then 52, then 51 over three months are a system losing heat, and only the second view tells you to act. Sentinel outlets exist precisely to be that early-warning sample of the wider system ([Sentinel outlets: what they are and how to monitor them](https://legionella.io/articles/sentinel-outlets-what-they-are-and-how-to-monitor-them/) explains how to choose and monitor them).

The set-point change was the root event, and it slipped through because it was treated as plant maintenance rather than a modification to a control measure. Monitoring frequency and action levels are not arbitrary; they are set by the risk assessment, and that assessment was built around the old storage temperature [2]. Change the input and the whole scheme needed re-examining.

The stagnant wing and the warming loft tank were both visible to anyone walking the building with the right question in mind. Neither needed a lab to detect. If you want a structured way to trace a cool outlet back to its cause, [Troubleshooting temperature problems in plumbing systems](https://legionella.io/articles/troubleshooting-temperature-problems-in-plumbing-systems/) sets out a diagnostic order.

## The decisions that would have changed the ending

Three habits, none of them expensive, would have broken the chain.

- **Treat any heating, plant or energy change as a risk-assessment trigger.** A new set point, a different boiler, a fresh efficiency scheme: each is a prompt to ask whether the control measures still hold, before the change goes live.
- **Prove the temperature at the point of use, not just at the plant.** A healthy calorifier reading tells you little about a blended leg three floors away. The numbers have to be hit where people actually draw water.
- **Read the trend, not the reading.** A logbook full of "in range" entries that are gently falling is a system telling you it is in trouble. Plotting the line, even by hand, turns recorded data into early warning.

## Reading this across to your own building

This is an illustrative composite, not a record of a real home, and the temperatures quoted are the commonly cited HSE benchmarks rather than fixed legal limits. Your own action levels, monitoring intervals and target temperatures come from a competent, site-specific risk assessment and the written scheme built from it [3]. Sampling has its place for investigation and verification, but a clear result describes one outlet at one moment and never substitutes for keeping water hot, cold and moving [4]. If anything here mirrors your own site, the response is a review with a competent person, not a quiet adjustment to match the story.

The single most useful thing you can do this week is pull your last twelve months of sentinel temperatures for one or two outlets and plot them in order. If the line is sloping the wrong way, you have found your own drift before a sample does.

## Common questions

### Was the lowered storage temperature alone the cause?

Not on its own. The reduced set point removed the safety margin, but the growth needed the long cool distribution legs, the tepid tails downstream of the TMVs, the warming loft tank and the stagnant wing to line up with it. Temperature failures usually compound rather than acting singly, which is why a single "fix" rarely closes the case.

### All our sentinel readings are within range, so are we safe?

In range at the sentinels is reassuring but not conclusive. Sentinels are chosen to represent the extremes of the system, yet a tepid leg beyond a TMV, a little-used outlet or a warm cold-water tank can sit outside what those points capture. Watch whether the readings are stable or drifting, and check the outlets your assessment flags as awkward, not just the easy ones.

### Should an energy-efficiency upgrade trigger a fresh risk assessment?

If it changes water temperatures, flow or how long water dwells in the system, then yes, it should prompt a review before it goes ahead. Lowering a storage temperature, changing a heat source or re-zoning pipework all alter the conditions the existing assessment assumed, so the assessment needs to catch up rather than be discovered after a positive sample.

## Related reading

- [In-house vs professional Legionella risk assessments](https://legionella.io/articles/in-house-vs-professional-legionella-risk-assessments/)
- [Sentinel outlets: what they are and how to monitor them](https://legionella.io/articles/sentinel-outlets-what-they-are-and-how-to-monitor-them/)
- [Troubleshooting temperature problems in plumbing systems](https://legionella.io/articles/troubleshooting-temperature-problems-in-plumbing-systems/)
- [Seasonal changes: managing water temperature in summer and winter](https://legionella.io/articles/seasonal-changes-managing-water-temperature-in-summer-and-winter/)

## Sources

[1] HSE, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[2] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] 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
[4] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm