---
title: "The danger zone: Legionella risks in lukewarm water"
source_url: https://legionella.io/articles/the-danger-zone-legionella-risks-in-lukewarm-water/
canonical_url: https://legionella.io/articles/the-danger-zone-legionella-risks-in-lukewarm-water/
pillar: "Water Temperature Control"
summary: "Tepid water is where Legionella thrives. See how a UK site drifted into the danger zone, the signals its team missed, and how to read your own temperatures."
primary_keyword: "lukewarm water risk"
date_published: 2025-11-23
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."
---

# The danger zone: Legionella risks in lukewarm water

Cold water that has crept up to 24°C, or hot water that limps to a tap at 41°C, rarely sets off alarms. It should. Tepid is the one band where Legionella does its best work, and a building drifts into it quietly, one degree at a time, in the spots nobody checks between scheduled rounds.

The trap is treating a single warm reading as a near-miss to note and move past. A reading describes one outlet at one minute. What matters is whether the whole system can hold cold water cold and hot water hot, week after week, when the weather turns and the building fills up. That is the question a lukewarm reading is really asking.

## Why the middle is the dangerous part

Legionella sits dormant in genuinely cold water and is killed off in properly hot water. The trouble lives in between. UK guidance reflected in the ACoP L8 framework treats roughly 20–45°C as the range where the bacteria multiply most readily [1], which is why the benchmark numbers exist to keep water out of that band: cold below 20°C where practicable, hot stored around 60°C, and hot water reaching about 50°C at the outlet within a minute (55°C in healthcare premises) [2].

Hit those numbers and the danger zone has nowhere to live. Miss them by a little, in enough places, and you have built a warm-water reservoir feeding outlets that turn water into a breathable mist — showers, spray taps — pointed straight at someone's breathing zone [3]. Lukewarm is not "nearly fine". It is the exact condition the whole control scheme is built to prevent.

## A building that drifted into the danger zone

What follows is an illustrative composite, not a real named incident, but every step in it is ordinary.

A mid-sized council office, recently part-refurbished, ran the usual monthly sentinel checks. Over one summer, three things happened that each looked minor on its own.

First, the cold water storage tank sat in a roof void that baked under a new flat roof. Sentinel cold readings, normally 14–16°C, came back at 21°C, then 23°C. The contractor logged them, wrote "warm weather", and moved on.

Second, an energy-efficiency drive had nudged the calorifier setpoint down to save gas. Stored hot water still tested fine near the plant, but the far wing — a long horizontal run to a rarely-used kitchenette and an accessible shower — was now arriving at 44°C. Hot to the hand, not hot enough to control.

Third, the refurbishment had added thermostatic mixing valves at several basins to guard against scalding. Sensible in isolation. But the blended water sat in short tepid sections downstream of valves that were barely touched between cleaners' visits.

No single line in the logbook screamed failure. Read together, three months on, they told one story: the building had quietly settled into the danger zone in precisely the places least likely to be flushed.

## What the readings were really saying

The responsible person who eventually joined the dots did not find one dramatic fault. She found a system that had lost its margins on three fronts at once, and a record-keeping habit that logged numbers without ever judging them.

The cold-tank reading was not "warm weather". It was a sign the tank's location no longer kept water below 20°C reliably — a design weakness the season merely exposed. The 44°C far outlet was not a quirk; it was the predictable result of lowering the source temperature without re-checking what reached the end of the longest run. And the tepid TMV sections were a known trade-off that had never been paired with the more frequent flushing that low-use blended outlets need.

The decisions that fixed it came next, and none was exotic. The tank was insulated and a longer-term relocation went on the risk assessment's action list. The calorifier setpoint went back up, with a plain message to the energy team that storage temperature is a control measure, not a discretionary saving. The low-use blended outlets got a named flushing task with an escalation route. Most usefully, the monthly form was redesigned so a reading outside range could not be signed off without a recorded action beside it.

## Carrying the lesson to your own site

The transferable point is not "watch the cold tank" or "leave the calorifier alone", useful as both are. It is this: a lukewarm reading is data about the system, not just about that one tap. Ask what it implies for everything upstream and downstream before you write "monitoring weather" and turn the page.

Three habits do most of the work. Treat any cold outlet creeping toward 20°C, or any hot outlet falling short of its target, as a prompt to check the source and the run, not just the fitting — stratification inside the cylinder can leave the lower draw-off tepid even when the top tests hot, which is worth understanding on its own terms (see [Temperature stratification in hot water cylinders](https://legionella.io/articles/temperature-stratification-in-hot-water-cylinders/)). Re-run the risk assessment after any change that touches temperature — a refurbishment, a new TMV, an energy setpoint, a layout tweak — because the choices that create tepid water tend to arrive disguised as improvements; the design side is covered in [Designing plumbing systems for optimal temperature control](https://legionella.io/articles/designing-plumbing-systems-for-optimal-temperature-control/). And record the decision, not just the number, so a slow drift into the danger zone cannot hide inside three months of tidy logs.

## Where this guidance stops and your assessment starts

The temperatures above are the common UK benchmarks, not fixed legal limits for every building. The figures that apply to your site, how often you check them, and what counts as an acceptable result all come from a competent, site-specific risk assessment carried out under L8 and HSG274 [1][4]. Sampling can support that picture, but testing frequency follows the system and the assessment rather than a calendar [5]. Read this as general guidance, not legal, clinical or engineering advice for your premises.

## FAQ

### Is one lukewarm reading actually a problem, or just an off day?

One reading is a moment, not a verdict — but a tepid result is never simply "nothing". Treat it as a question about the system: is the source drifting, is the run too long, is the outlet barely used? An off day that repeats is a trend, and the only honest way to tell the two apart is to act on the first one and keep watching.

### Why does cold water end up lukewarm in the first place?

Usually heat gain it should never be picking up: storage tanks or pipes routed through warm plant rooms, roof voids or ceiling spaces, cold runs sharing tight space with hot pipes, or long stretches that warm while water stands still in a heated building. Summer makes it visible; the path for the heat was there all along.

### Does fitting TMVs increase lukewarm water risk?

Thermostatic mixing valves are there to prevent scalding and are often essential, but they do blend water down into the growth range and leave short sections downstream sitting tepid. The valve is not the problem; an unflushed, low-use blended outlet is. Pair every TMV with cleaning and flushing matched to how little it actually gets used.

## Related reading

- [Integrating Legionella risk assessment into health and safety management](https://legionella.io/articles/integrating-legionella-risk-assessment-into-health-and-safety-management/)
- [Legionella flushing after system repairs: restart checklist](https://legionella.io/articles/legionella-flushing-after-system-repairs-restart-checklist/)
- [Temperature stratification in hot water cylinders](https://legionella.io/articles/temperature-stratification-in-hot-water-cylinders/)
- [Designing plumbing systems for optimal temperature control](https://legionella.io/articles/designing-plumbing-systems-for-optimal-temperature-control/)

## 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, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[3] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html
[4] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[5] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm