---
title: "HSG274 guidance explained: practical Legionella control"
source_url: https://legionella.io/articles/hsg274-guidance-explained-practical-legionella-control/
canonical_url: https://legionella.io/articles/hsg274-guidance-explained-practical-legionella-control/
pillar: "UK Legionella Law & Compliance"
summary: "HSG274 is the practical detail beneath L8. See how UK duty holders turn it into defensible control: the schematic that proves it, and where good teams slip up."
primary_keyword: "HSG274"
date_published: 2025-05-25
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."
---

# HSG274 guidance explained: practical Legionella control

HSG274 is the document people name-drop in tenders and rarely read past the contents page. It is the Health and Safety Executive's technical guidance on controlling Legionella — the detailed "how" that sits beneath the legal "what" of the Approved Code of Practice, the ACoP usually called L8 [1][2]. Owning a copy proves nothing. What separates a controlled building from an exposed one is whether the evidence on site matches what the guidance expects to see.

So the useful way to read it is not as a rulebook to obey line by line, but as a menu of recognised good practice that your risk assessment selects from: the assessment decides which recommendations apply to your system, and HSG274 tells you what "good" looks like once you have.

This piece assumes you already know how Legionella grows and reaches people — if that part is hazy, [on how it spreads](https://legionella.io/articles/how-legionella-spreads-through-water-systems/) is the better starting point. What follows is about applying the guidance: the decisions, the evidence, and the places competent teams still come unstuck.

## How HSG274 sits under L8

L8 is an Approved Code of Practice, and that status carries weight. A court can treat a failure to follow it as evidence of a breach unless you can show you achieved compliance some other way [1]. HSG274 has no such special standing — it is guidance — but it is where the practical detail lives, and following it is the most straightforward way to demonstrate you met the Code. The [explainer on ACoP L8](https://legionella.io/articles/acop-l8-understanding-the-uk-legionella-code-of-practice/) covers that legal standing in more depth.

The technical guidance is published in three parts, split by system type: evaporative cooling systems such as cooling towers; hot and cold water systems; and other risk systems like spa pools, fountains and humidifiers [2]. Most commercial duty holders live mainly in the hot-and-cold part. But a single rooftop cooling tower or a reception water feature pulls a second part into scope, and changes your monitoring and notification duties with it.

The chain it expects is consistent across all three parts: know your system, assess foreseeable risk, write a scheme of control, carry it out, monitor it, record it, and review when something changes [2]. HSE frames the core duty the same way — assess the sources of risk, put controls in place, keep records, and meet the duties that follow [3].

## The schematic is the real test

If you want one quick check of whether a building is genuinely controlled or merely documented, ask to see the water-system schematic and then walk it against the building.

A schematic that earns its place shows the whole journey of the water, and the responsible person should be able to sketch most of it from memory. Start at the incoming mains and the stop valve. Draw the cold water storage tank: note its capacity, where it sits — a warm roof void is a flag — and the temperature you would expect to read there. From the tank, trace the cold distribution out to every outlet. Separately, draw the calorifier or water heater, the stored hot temperature, and the flow-and-return loop that is meant to keep hot water hot all the way to the furthest tap.

Then label the things that actually drive risk. Mark every outlet and classify it by how often it is used — daily, weekly, rarely. Mark the sentinel outlets, the nearest and furthest points on each loop, because that is where temperature monitoring tells you the most. Mark every thermostatic mixing valve, because the blended water downstream of a TMV sits squarely in the growth range. And mark the dead legs and blind ends: the capped spur left from a removed sink, the spare connection kept "just in case". Those are where stagnation hides.

The figures most schemes work to are familiar — hot water stored hot, commonly around 60°C and still hot at the tap, with cold water kept below roughly 20°C [4]. Treat those as the working expectation your risk assessment confirms for your plant, not as numbers to defend regardless of what the system is doing.

When the drawing carries all of that and matches the building, you can have a sensible conversation about control. When the schematic is a decade old, or shows outlets ripped out in a refit, that gap is itself the finding — a control scheme written against a fictional building cannot be working. This is why the schematic belongs at the heart of the risk assessment rather than in an appendix; see [on the key components of a Legionella risk assessment](https://legionella.io/articles/key-components-of-a-legionella-risk-assessment/).

## Where competent teams still come unstuck

A few recurring traps, none of them about not trying hard enough.

**Confusing monitoring with control.** A monthly temperature reading is evidence that control is working — it is not the control itself. The control is the design and operation that keeps water hot, cold and moving. Teams that diligently chase readings while leaving a redundant pipe full of tepid water are measuring the wrong thing very carefully.

**Treating sampling as the headline.** Legionella sampling has a real place in verification and in investigation after a problem, but HSE is explicit that how often you test should follow the system and the risk assessment, not a fixed calendar habit [5]. A clean sample from one outlet on one day is thin evidence on its own, and it never substitutes for temperature, cleanliness and flushing.

**Outsourcing accountability with the work.** You can contract out the monitoring; you cannot contract out the duty. The duty holder still has to be competent enough to appoint the right provider, brief them properly, read their reports critically and challenge what looks wrong. The Legionella Control Association's code of conduct is a practical reference point when you are choosing or auditing a service provider [6]. A logbook full of green ticks nobody has questioned is proof of filing, not proof of control. Who answers for what is worth nailing down early, and [on duty holder and responsible person roles](https://legionella.io/articles/duty-holder-and-responsible-person-roles-in-legionella-management/) sets it out.

**Recording the task but not the decision.** This is the quiet one. "Flushed outlet 14" tells a future investigator nothing. "Outlet 14 flushed weekly because the room is let intermittently; two missed weeks escalate to the responsible person; a recurring pattern triggers a review of whether the outlet stays" tells them you are managing a risk rather than performing a ritual. Write down why each control exists and what happens when it fails.

## A note on using this guidance

HSG274 is general good practice, not a verdict on your building. Nothing above replaces a competent, site-specific risk assessment: the figures, the monitoring intervals and the right remedial action all depend on your system, who uses it and how. Treat anything numeric here as a starting expectation to confirm, not a fixed rule, and have a competent person apply it to your actual plant. This is compliance guidance, not legal or engineering advice for your specific site.

## FAQ

### Is HSG274 a legal requirement?
Not in itself. The legal duties flow from health and safety law and the L8 Approved Code of Practice; HSG274 is technical guidance on how to meet them. Following it is the most defensible way to show you have controlled the risk, and a court can weigh a departure from recognised guidance against you [1][2].

### Do I have to follow every recommendation in it?
No. It describes recognised good practice across many system types, and your risk assessment decides which parts apply to your building. A small office with no cooling tower and no spa will never touch large sections of it. What matters is a reasoned, recorded decision about what applies — not blanket application of everything.

### What is the difference between L8 and HSG274?
L8 is the Approved Code of Practice: the outcome you must achieve, with special legal standing. HSG274 is the supporting technical guidance: the practical method, published in three parts covering hot and cold water systems, evaporative cooling systems, and other risk systems [1][2]. Use L8 for the duty and HSG274 for the detail.

## Your next move

Do not start by reading HSG274 cover to cover. Pull out your current schematic and walk one water system against it — cold tank to furthest tap — marking every outlet, dead leg, TMV and sentinel point you can find. Where the drawing and the building disagree, you have found the first thing to fix, and the rest of the control scheme follows from getting that picture right.

## Related reading

- [How Legionella spreads through water systems](https://legionella.io/articles/how-legionella-spreads-through-water-systems/)
- [Key components of a Legionella risk assessment](https://legionella.io/articles/key-components-of-a-legionella-risk-assessment/)
- [ACoP L8: understanding the UK Legionella Code of Practice](https://legionella.io/articles/acop-l8-understanding-the-uk-legionella-code-of-practice/)
- [Duty Holder and Responsible Person roles in Legionella management](https://legionella.io/articles/duty-holder-and-responsible-person-roles-in-legionella-management/)

## 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, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[5] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm
[6] Legionella Control Association, "Code of Conduct for Service Providers". https://www.legionellacontrol.org.uk/