---
title: "Temperature control basics for Legionella prevention"
source_url: https://legionella.io/articles/temperature-control-basics-for-legionella-prevention/
canonical_url: https://legionella.io/articles/temperature-control-basics-for-legionella-prevention/
pillar: "Water Temperature Control"
summary: "Hot water hot, cold water cold sounds simple, but one reading proves nothing. How UK facilities teams show the system holds Legionella temperature control."
primary_keyword: "Legionella temperature control"
date_published: 2025-05-07
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."
---

# Temperature control basics for Legionella prevention

"Keep the hot water hot and the cold water cold." It is the oldest line in Legionella control, and it is correct. The catch is that a building does not run on a slogan; it runs on whether those temperatures actually reach every tap, shower and far corner of the pipework, and then stay there. A good reading at the calorifier proves the calorifier is hot. It tells you almost nothing about the shower at the end of a thirty-metre run on the top floor.

So the real skill in temperature control is not taking a reading. It is reading the whole system: knowing where heat leaks away, where cold creeps up, and whether your records show control holding over weeks rather than on the one morning someone happened to check.

## The temperatures that matter, and why

Legionella multiplies in warm, still water. As a general guide reflected in HSE guidance, the active growth band sits at roughly 20-45°C: below about 20°C the bacteria stay dormant, and above about 60°C they are progressively killed off [1]. Everything in temperature control is an effort to keep your water out of that middle band wherever it can sit still.

That turns into a small set of benchmark figures UK guidance has used for years. HSE's hot and cold water guidance describes cold water kept below 20°C where practicable, hot water stored at 60°C or above, and hot water reaching at least 50°C at the outlet within a minute of running — 55°C in healthcare premises, where the scalding trade-off is managed differently [1]. Treat those as the common targets, not gospel for every building. The figures that apply to yours, and how tightly you hold them, come from the risk assessment, not from a forum post.

## Where temperature quietly gets lost

The reason a single reading misleads is that temperature is lost, and sometimes gained, all the way through a system. The number you set at the plant is rarely the number that arrives at the tap.

Hot water sheds heat along every metre of pipe, so a long horizontal run to a distant wing can deliver lukewarm water even when storage is sitting bang on 60°C. A hot-water return loop that has lost its push lets the far end cool into the growth band between draw-offs. Cold water has the opposite habit: a storage tank in a warm roof void, or a cold pipe clipped tight against a hot one, drifts upward through 20°C with no one touching a thing.

Then there are the fittings that blur the picture. Thermostatic mixing valves (TMVs) are fitted to stop people scalding, and they do their job — but downstream of the valve the water is deliberately blended to a safe, lukewarm temperature, which is exactly the temperature Legionella favours. That short blended section needs its own thought, not a shrug. Dead legs and little-used outlets tell the same story: a capped spur or a spare-room basin no one runs holds a pocket of room-temperature water for weeks.

## What the thermometer won't tell you

A clean reading can hide more than it reveals. The assumptions that catch people out are almost always about mistaking one part of the system for the whole of it.

| What gets assumed | What's actually going on |
| --- | --- |
| The calorifier reads 60°C, so the system is safe | Storage temperature only describes storage. Heat is lost along the run; the outlet at the far end is what exposes people |
| Turning the hot water down a few degrees is a free energy saving | Drop storage or circulation below guidance and outlets can slide into the growth band. That is a risk-assessment decision, not a quiet turn of a dial |
| A TMV makes an outlet safer | It prevents scalding, but the blended water beyond it sits in the warm zone and needs its own control and flushing |
| Cold is cold — only the hot side needs checking | Tanks in warm roof spaces and cold pipes run beside hot ones drift above 20°C unnoticed |
| One monthly check at the plant room covers the building | The plant is the easy bit. Sentinel and rotating outlet checks are what show the whole system holds temperature |

## The one habit that changes everything

New duty holders and new contractors tend to treat the thermometer like a breathalyser: one number, pass or fail, move on. A reading is a single data point, in one place, at one moment. Control is a pattern.

This is why guidance leans on sentinel outlets — the nearest and furthest taps on a circuit, chosen to represent the extremes — checked routinely, with the remaining outlets rotated through over a longer cycle [2][3]. One hot sentinel and one cold sentinel, read every month and logged, tell you far more than a dozen scattered spot-checks ever will. The value is in the trend: an outlet that has crept down two degrees a month for three months is failing slowly, and only a record shows it.

And write down the decision, not just the digits. "47°C" on its own is a number. "47°C at the far shower, short of the 50°C-plus we expect within a minute; flagged to the responsible person; flushing increased and re-checked next week" is control. Routine checks and flushing sit right alongside this — see [Routine monitoring and flushing](https://legionella.io/articles/routine-monitoring-and-flushing-the-basics/) for how to run that rhythm.

## A note on the numbers

The temperatures above are the long-standing HSE benchmarks and a sound default, but they are not a setting you copy from another building. A care home managing frail, scald-prone residents, a hospital working to HTM rules, and a small serviced office each balance scalding, energy use and Legionella control differently, and their monitoring intervals differ too. The figures that bind you, the outlets you treat as sentinels, and what counts as an unacceptable reading are decisions for a competent, site-specific risk assessment and the written scheme that records them [2][4]. None of this is medical or legal advice — it is the groundwork that lets a competent person make the call for your building.

## Where to start this week

Pick one hot circuit and one cold circuit and find their true extremes: the outlet nearest the source and the one furthest away. Run them, time how long the hot takes to reach temperature, and note what the cold sits at after running. Do it again next week, and the week after, writing each reading down with the date and whatever action you took.

Three or four weeks of that on your real sentinels will tell you more about whether the building holds temperature control than any one-off survey, and it leaves you with the start of a record you can stand behind. If the numbers begin to wander, that is your risk assessment talking — time to revisit it rather than nudge a thermostat. [Legionella risk assessment basics](https://legionella.io/articles/legionella-risk-assessment-basics-what-it-is-and-why-you-need-it/) covers that next move.

## FAQ

### What hot and cold water temperatures should I actually aim for?
As a common UK benchmark, HSE guidance describes cold water below 20°C, hot water stored at 60°C or above, and hot water reaching at least 50°C at the outlet within a minute — 55°C in healthcare settings [1]. Treat those as the default targets and confirm the exact figures for your building through its risk assessment.

### Is it safe to lower the hot water temperature to save energy?
Not without checking first. Reducing storage or circulation temperature can let outlets drift into the range where Legionella multiplies, so any energy-saving change should trigger a review of the risk assessment and the control scheme rather than a quiet adjustment [2].

### Why bother checking outlets when the plant room temperatures are fine?
Because people are exposed at the outlet, not at the calorifier. Heat is lost along the pipework, cold can be gained, and TMVs blend water down — so sentinel and rotating outlet checks are what prove the right temperature actually reaches the point of use [1][3].

## Related reading

- [Legionella risk assessment basics: what it is and why you need it](https://legionella.io/articles/legionella-risk-assessment-basics-what-it-is-and-why-you-need-it/)
- [Routine monitoring and flushing: the basics](https://legionella.io/articles/routine-monitoring-and-flushing-the-basics/)
- [Hot water temperature guidelines to prevent Legionella](https://legionella.io/articles/hot-water-temperature-guidelines-to-prevent-legionella/)

## Sources

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