---
title: "Poor temperature control: a recipe for Legionella"
source_url: https://legionella.io/articles/poor-temperature-control-a-recipe-for-legionella/
canonical_url: https://legionella.io/articles/poor-temperature-control-a-recipe-for-legionella/
pillar: "Common Failures & Enforcement"
summary: "A gauge reading 60C proves nothing on its own. The temperature myths that let Legionella regrow in UK water systems, and how to judge the whole system."
primary_keyword: "poor temperature control"
date_published: 2025-05-31
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."
---

# Poor temperature control: a recipe for Legionella

Temperature is the control most duty holders are sure they have nailed. The cylinder is set to 60, the cold tank feels cold to the hand, and the monthly readings come back in range. Yet poor temperature control sits behind a large share of Legionella incidents and enforcement findings — not because nobody set a thermostat, but because a number at one point on one day got mistaken for control of the whole system.

The bacteria do not read your gauge. They respond to the water they actually sit in: its temperature, at the outlet, over weeks. The distance between "we hit the number" and "the system holds the number everywhere that matters" is where almost every temperature failure lives.

## Why temperature is the control people misread

HSE guidance frames temperature as the traditional defence: keep hot water hot and cold water cold, so water spends as little time as possible in the band where Legionella multiplies — broadly the 20-45°C range [1][2]. The familiar benchmarks follow from that. Cold water kept below about 20°C where practicable; hot water stored at around 60°C; hot water delivered to reach roughly 50°C at the outlet within a minute, and 55°C in healthcare premises [3].

Those figures are the easy part — they are arithmetic. Holding them across a whole building, through a UK summer and half-empty floors and a dozen blended outlets, is operational, and operational things drift. That drift is what gets missed, because people check what is easy to check rather than what decides the risk.

## Four temperature myths that quietly let Legionella back in

### Myth: "The cylinder reads 60°C, so our hot water is safe"

Stored temperature and delivered temperature are not the same thing. A calorifier holding 60°C tells you the water leaving the plant room is hot. It says nothing about the water reaching a basin on the top floor of the far wing. On a long run with weak or absent circulation, hot water bleeds heat into the building and can arrive at the tap well below the 50°C an outlet should reach within a minute [3]. The plant-room gauge is reassuring precisely because it is the one place the temperature is almost guaranteed to be right. Control is decided at the outlets people use, not at the source — which is why the hot side is judged at the taps, not the tank (see [Hot water temperature guidelines to prevent Legionella](https://legionella.io/articles/hot-water-temperature-guidelines-to-prevent-legionella/) for the delivered-temperature detail).

### Myth: "Cold water is cold by default"

The cold side fails quietly because nobody is watching it. A cold main is only cold until the building warms it. Pipework run through a hot plant room, bundled alongside hot flow and return, or threaded across a sun-baked ceiling void picks up heat — and an oversized tank that turns over slowly in a warm loft does the same. Any of these can push cold water above 20°C and into the growth band, especially across July and August. Because the tap still says "cold", the drift goes unnoticed until someone puts a thermometer on it. The cold side deserves the same scrutiny as the hot ([Cold water temperature guidelines to prevent Legionella](https://legionella.io/articles/cold-water-temperature-guidelines-to-prevent-legionella/) covers what to look for).

### Myth: "A reading in range proves the system is under control"

A monthly sentinel reading is a snapshot: one outlet, one moment, one set of conditions. It is evidence, not a verdict. Temperatures drift between visits — a failed circulating pump, a plant-room door propped open in the heat, a mixing valve slowly slipping out of adjustment. An in-range figure at the nearest tap can sit perfectly happily alongside a low-use shower two corridors away that has stood stagnant for three weeks. The honest question is not "was this outlet in range today?" but "can the system hold its temperatures across every point that matters, week after week?" A trend answers that. A single number cannot. HSE's enforcement logic runs the same way: foreseeable risks have to be assessed, controlled, monitored and reviewed — not measured once and filed [4].

### Myth: "The mixing valves take care of the outlet temperature"

Thermostatic mixing valves (TMVs) exist to stop people being scalded, by blending hot and cold to a safe-touch temperature right at the outlet. That is a real safety duty, not an optional extra. But it means the water at, and just upstream of, a TMV-served outlet sits in the warm band by design. The valve does not remove Legionella risk; it relocates it. The hot supply feeding the valve still has to be genuinely hot, the cold genuinely cold, the blended tail kept short, and the valve itself maintained. Measuring downstream of a TMV tells you about the blend, not about whether the hot and cold supplies behind it are doing their job — so the readings that actually prove control are usually taken before the valve, not at the spout.

## Why these myths stick

All four share a root: temperature control gets judged by the easiest thing to measure rather than the thing that matters. A gauge in the plant room, a tap labelled cold, a tick in a logbook, a valve fitted years ago — each is visible and reassuring, and each describes one point rather than the system. Hitting 60°C at the cylinder is straightforward; holding 50°C at every outlet through August on a part-occupied floor is not. Most temperature control failures are not exotic — they are this gap, left unattended.

## How to judge whether your system can actually hold control

Treat every in-range reading as one data point in a trend, not a pass mark.

- **Read the line, not the last dot.** Plot the awkward points over months. A run that creeps from 52°C to 48°C to 45°C at a far tap is a system losing its grip, even while each reading "passed".
- **Measure where it is hard, not only where it is easy.** Sentinel taps near and far, hot and cold; supply temperatures taken ahead of TMVs; the outlets nobody enjoys reaching.
- **Give the cold side equal weight.** A warm cold tap is a control failure as real as a tepid hot one, and far more often overlooked.
- **Manage the warm pockets you designed in.** Keep TMV tails short, keep the valves serviced, and keep low-use outlets flushed regardless of what the system could theoretically deliver.
- **Record the decision, not just the number.** A reading means little without the rule behind it: what result is acceptable, who acts when it is missed, and what a repeat exceedance triggers. Records that capture that hold up under scrutiny in a way a column of figures never does — see [Incomplete records: the pitfalls of bad documentation](https://legionella.io/articles/incomplete-records-the-pitfalls-of-bad-documentation/).

A concrete start this week: pull the last six months of temperature monitoring for the three or four hardest points in your building — the furthest hot outlet, the least-used cold tap, anything served through a mixing valve — and look at the line each one traces rather than the most recent reading. If you cannot see a trend because the monitored points keep changing or readings are missing, closing that gap is the first job, ahead of any new sampling.

## A note on the numbers

The temperatures above are long-standing HSE guidance figures, not a switch that makes water automatically safe or unsafe. The exact targets, which points you monitor, how often, and what action a failed reading sets off are decided by a competent, site-specific risk assessment for your building and the people who use it [1][2]. Scald risk has to be weighed against Legionella risk at the same time — which is precisely why TMVs exist, and why turning every outlet up to scalding is not a control strategy. And a clear sample or an in-range reading verifies conditions at one point and time; it does not stand in for day-to-day control, and testing frequency should follow the system and the assessment rather than a calendar habit [5].

## FAQ

### What hot and cold water temperatures am I aiming for?
As general HSE guidance, cold water is kept below about 20°C where practicable, hot water stored at around 60°C, and hot water delivered to reach roughly 50°C at the outlet within a minute — 55°C in healthcare premises [3]. Treat those as the benchmark your risk assessment then adapts to your actual system and users [1].

### My monthly temperatures are in range — could I still have a problem?
Yes. A reading confirms one outlet at one moment; it does not show whether the system holds temperature between visits or at the points you never measure. A stagnant low-use shower, a cold tap warmed by neighbouring pipework, or a pump that failed last week can all coexist with a tidy set of in-range sentinel figures. Watch the trend and cover the awkward outlets, not just the convenient ones [4].

### Why does the tap run cooler than the temperature leaving the cylinder?
Hot water loses heat as it travels, and the loss grows with distance, with poor or absent circulation, and with dead legs where water stands still. Blending through a TMV lowers it further by design. If a far outlet runs well below the delivered-temperature benchmark, check the circulation, the run length and any redundant pipework before assuming the cylinder is at fault [3].

## Related reading

- [Hot water temperature guidelines to prevent Legionella](https://legionella.io/articles/hot-water-temperature-guidelines-to-prevent-legionella/)
- [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/)
- [Automated flushing systems to prevent Legionella](https://legionella.io/articles/automated-flushing-systems-to-prevent-legionella/)

## 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, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[4] HSE, "Legionnaires' disease - what you must do". https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm
[5] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm