---
title: "HSG274 Part 3 explained: other risk systems beyond hot and cold water"
source_url: https://legionella.io/articles/hsg274-part-3-explained-other-risk-systems-beyond-hot-and-cold-water/
canonical_url: https://legionella.io/articles/hsg274-part-3-explained-other-risk-systems-beyond-hot-and-cold-water/
pillar: "UK Legionella Law & Compliance"
summary: "HSG274 Part 3 is HSE's catch-all for water systems beyond taps, showers and cooling towers. What it covers, the key risks, and how to tell if you have one."
primary_keyword: "HSG274 Part 3"
date_published: 2026-04-07
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."
---

# HSG274 Part 3 explained: other risk systems beyond hot and cold water

HSG274 is the Health and Safety Executive's technical guidance on controlling Legionella, and it comes in three parts. Part 1 is for evaporative cooling — cooling towers and evaporative condensers. Part 2 is for hot and cold water systems, the taps, showers, calorifiers and storage tanks nearly every building runs. Part 3 is the one most people have never opened: "other risk systems" [2].

Think of Part 3 as the catch-all. It is the part you reach for when your building has something that makes a breathable spray of water but is neither a cooling tower nor ordinary plumbing. A spa pool. A reception fountain. A humidifier feeding an archive store. A misting fan over a pub garden. Each is a genuine Legionella source, and none of them fits neatly into Parts 1 or 2.

If your premises only has hot and cold water services, you will spend almost all your time in Part 2 and may never need Part 3 at all. The trap is assuming that is true without checking.

## How HSG274 is divided, and why a third part exists

All three parts sit beneath the same legal ceiling: the L8 Approved Code of Practice and the duty under health and safety law to assess and control the risk from Legionella [1]. The parts do not change the duty. They just describe recognised good practice for different kinds of system.

The reason there is a separate part for "other risk systems" is simple. Legionella does not care what a piece of equipment is called. It needs water held roughly in the 20–45°C growth range, somewhere to multiply, and a way to throw fine droplets into the air where someone can breathe them in [5]. Plenty of equipment ticks all three boxes without being a tap or a tower, and that equipment needs guidance of its own. Part 3 is where it lives. For the system most buildings actually run, [on HSG274 Part 2](https://legionella.io/articles/hsg274-part-2-explained-controlling-legionella-in-hot-and-cold-water-systems/) is the companion piece; this one is about everything Part 2 does not cover.

## When you actually reach for Part 3

The honest answer is: only after your risk assessment has found a system that belongs there. Most ordinary offices, shops and homes do not have one. But the list of premises that quietly do is longer than people expect — leisure centres with spas, hotels with atrium water features, museums and data rooms with humidity control, garages with recirculating wheel washes, garden centres with produce misters.

The recognition problem is the whole game. A Part 3 system is usually bolted on after the building was designed, owned by a different contractor, and filed mentally under "decoration" or "comfort", not "water system", so it slips past the assessment that covers the plumbing. The fix is a habit, not a document: when you walk a site, ask of anything wet, "does this make a mist?" If it does, it belongs in the assessment, and Part 3 is where you check how to control it.

## A map of Part 3's other risk systems

Part 3 addresses a range of systems. The table below covers the ones a UK duty holder is most likely to meet, with the reason each is a risk and the headline control. The exact regime — what disinfectant, what frequency, what temperature — is always set by a competent, site-specific risk assessment, never lifted from a table.

| Other risk system | Why it can grow and spread Legionella | Headline control (your assessment sets the detail) |
|---|---|---|
| Spa pools and hot tubs | Warm, aerated, recirculated water shared by bathers — close to an incubator | Continuous disinfection, dilution and frequent testing; detailed guidance is in HSG282 [4] |
| Decorative fountains and water features | Warmish water pumped in a loop and thrown into the air near people | Bring under the risk assessment, keep water moving and treated, clean and disinfect on schedule |
| Humidifiers, air washers and spray humidification | Make breathable mist from a standing reservoir of water | Feed from a clean supply, drain and clean the reservoir, disinfect on a planned schedule |
| Vehicle, plant and wheel washes | Recirculated, often warm wash water sprayed into the air | Manage the recirculation tank, control over-spray, clean and disinfect the system |
| Misting and fog systems (cooling, humidity, display) | Built to turn water into fine aerosol, often from a line that can stagnate | Keep the supply fresh, clean nozzles and reservoir, disinfect regularly |
| Emergency showers and eyewash stations | Used rarely, so water sits stagnant in the standing line for months | Flush often enough to replace the standing water, and record each flush |

That is not the whole of Part 3 — it also touches things like sprinkler and fire systems, water softeners and other plant — but it shows the shape of what the part is for. Three of the rows have their own detailed write-ups already: spa pools in [on safe operation under HSG282](https://legionella.io/articles/safe-operation-of-spa-pools-and-hot-tubs-under-hsg282/), water features in [on decorative fountains](https://legionella.io/articles/decorative-fountains-and-water-features-hidden-legionella-risks/), and humidifiers and misting in [Misting systems and humidifiers: Legionella in unexpected places](https://legionella.io/articles/misting-systems-and-humidifiers-legionella-in-unexpected-places/).

A note on spa pools, because it confuses people. Spa pools are an "other risk system", but they were significant enough to earn their own dedicated document — HSG282 — which goes well beyond what Part 3 says [4]. If you run a commercial spa or a business-use hot tub, HSG282 is your primary reference; Part 3 simply points you there.

## The thing beginners get wrong about Part 3

The single most common mistake is reading "not in Part 2" as "not my problem". A building can have a spotless hot and cold water scheme — temperatures logged, dead legs removed, calorifier serviced — and an unmanaged humidifier or fountain in the next room doing exactly what the controlled system is designed to prevent.

Part 3 being technical guidance rather than law does not soften the duty either. The legal obligation to identify and control foreseeable Legionella risk applies to every system on site, whatever part of HSG274 happens to describe it [1][3]. Picking the right part is about finding the right control advice, not about deciding whether the risk counts.

So the order of operations matters. First you find the system. Then you decide which part of the guidance applies. Only then do you worry about the specific controls. Skipping the first step is how aerosol sources stay invisible for years.

## Before you apply any of this

Part 3 describes recognised good practice for a broad family of less common systems. It is not a verdict on your particular fountain, humidifier or wash bay, and it does not replace a competent, site-specific risk assessment — which decides whether a system is in scope, which controls apply, and how often you check them. Where this article gives a temperature band or a control, treat it as a prompt to confirm against the current guidance and your own assessment, not a fixed rule. This is general compliance guidance, not legal, medical or engineering design advice for your premises.

## FAQ

### Does HSG274 Part 3 apply if my building only has taps and showers?
Usually not. Hot and cold water services sit in Part 2, and if that is all you have, that is the part you work to. Part 3 becomes relevant only when the building also runs an "other risk system" — a spa, fountain, humidifier, wash bay, misting unit and so on [2]. The job is to be sure, through your risk assessment, that you genuinely have none rather than to assume it.

### Is following HSG274 Part 3 a legal requirement?
The guidance itself is not the law; the duty is. Health and safety law and the L8 Approved Code of Practice require you to assess and control foreseeable Legionella risk from any system, and Part 3 sets out how to do that for the less common ones [1][2]. Following it is the most defensible way to show you controlled the risk, and an unexplained departure from it can be weighed against you.

### Do spa pools come under Part 3 or HSG282?
Both, in effect — but HSG282 is the one to use. A spa pool is an "other risk system" that Part 3 recognises, yet the detailed operating, monitoring and disinfection guidance lives in the dedicated HSG282 document [4]. For any commercial or business-use spa or hot tub, treat HSG282 as your primary reference and Part 3 as the signpost to it.

## Your starting point

Before you open Part 3 properly, do the cheap thing first: walk the site and list everything that makes a spray, mist or fine droplet, room by room. Note who owns each item and whether it appears in your current Legionella risk assessment. Anything wet that is missing from that assessment is your real finding for the day — and the kind of overlooked asset a live digital register keeps in front of you instead of letting it drift off the schedule. Once the list is complete, match each item to the right part of HSG274 and confirm the controls with a competent person.

## Related reading

- [HSG274 Part 2 explained: controlling Legionella in hot and cold water systems](https://legionella.io/articles/hsg274-part-2-explained-controlling-legionella-in-hot-and-cold-water-systems/)
- [Safe operation of spa pools and hot tubs under HSG282](https://legionella.io/articles/safe-operation-of-spa-pools-and-hot-tubs-under-hsg282/)
- [Decorative fountains and water features: hidden Legionella risks](https://legionella.io/articles/decorative-fountains-and-water-features-hidden-legionella-risks/)
- [Misting systems and humidifiers: Legionella in unexpected places](https://legionella.io/articles/misting-systems-and-humidifiers-legionella-in-unexpected-places/)

## 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, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[4] HSE, "Control of legionella and other infectious agents in spa-pool systems (HSG282)". https://www.hse.gov.uk/pubns/books/hsg282.htm
[5] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html