If you manage a building with hot and cold water systems (and let's be honest, that's almost every building), you've probably seen "HSG274" mentioned somewhere in your compliance paperwork and quietly hoped someone else understood what it meant. You're not alone. It's one of the most-searched terms in UK legionella compliance, and one of the least-explained.
HSG274 is the Health and Safety Executive's technical guidance on controlling Legionella bacteria in water systems, and it works alongside ACOP L8. Think of it this way: ACOP L8 is the rulebook, HSG274 is the instruction manual. L8 sets out what you're legally required to do; HSG274 explains exactly how to do it, depending on the kind of water system you're responsible for.
The good news? You almost certainly don't need all of it. HSG274 is split into three parts, and most buildings only need to properly get to grips with one of them. As a specialist water hygiene company working across ACOP L8 and HSG274 compliance every day, we're going to walk you through exactly what each part covers, which one applies to you, and what happens if you get it wrong.
HSG274 stands for Health and Safety Guidance 274, and it's published by the HSE as the practical companion to ACOP L8 (the Approved Code of Practice for controlling legionella). Where ACOP L8 tells you what your legal duties are, HSG274 gets into the how: the specific monitoring tasks, temperature ranges, inspection frequencies, and control measures needed for different types of water systems.
It isn't a single document. It's a series of three parts, each covering a different category of system:
You need to work through whichever parts are relevant to the systems you actually have on site. For a lot of commercial buildings, that's just Part 2. For sites with cooling towers or spa facilities, it's more.
If you haven't already read our guide on ACOP L8 compliance, it's worth starting there for the legal framework HSG274 sits underneath; we won't repeat all of that ground here.
Part 1 covers cooling towers and evaporative condensers, and if you operate one, this is the part you need to know in detail. These systems are considered the highest-risk category for Legionella because they generate fine water droplets, aerosols, that can travel a genuinely surprising distance and be inhaled by anyone nearby, whether they work in the building or not.
A few things worth knowing if Part 1 applies to you:
Part 2 is the section that applies to the greatest number of sites by a wide margin. It covers the hot and cold water systems found in virtually every commercial building: storage tanks, calorifiers, distribution pipework, taps and showers.
Part 2 sets out the monitoring tasks and frequencies you need to follow, and two areas come up again and again as genuine risk points:
This is also where routine temperature checks come in: cold water should stay below 20°C and hot water should reach 60°C at the calorifier (with checks confirming it stays above 50°C at the outlet after a set flush time), because Legionella thrives in the 20 to 45°C range in between. If your temperature monitoring isn't currently structured around these thresholds, our temperature testing & monitoring services are built specifically around Part 2 compliance.
Part 3 picks up everything that doesn't fit neatly into cooling towers or hot and cold water systems, but still carries a meaningful Legionella risk: spa pools, hot tubs, humidifiers, and decorative water features.
Spa pools deserve a specific mention here. They typically operate at 30 to 40°C, which sits right in the middle of Legionella's preferred growth range, and the air jets used to create bubbles generate exactly the kind of fine aerosol that makes inhalation a real risk. If you operate any of these systems, commonly the case in leisure facilities, hotels, and care settings, Part 3 isn't optional reading.
Rather than working through all three parts to find your answer, here's a quick way to check. If your building has cooling towers or evaporative condensers, you need Part 1. If you have hot and cold water systems, taps, showers, tanks, which covers most buildings, you need Part 2. If you have spa pools, hot tubs, humidifiers or decorative water features, you need Part 3. And if your site has a combination of these, for example, a hotel with both a cooling tower and a spa, all the relevant parts apply together.
If you're not sure what falls into which category, that's genuinely common, and it's exactly the kind of question our on-site surveys are designed to answer properly, rather than guessing from a checklist.
HSG274 isn't just a box-ticking exercise; it has real, practical consequences attached to it.
If water sampling returns a legionella count above 50 colony-forming units per litre (CFU/L), that result is treated as actionable. Depending on the level found, the response escalates from re-sampling and enhanced monitoring, through cleaning and disinfection, up to isolating or shutting down the affected part of the system entirely until it's brought back under control. Once remedial work is complete, the guidance expects a full review of your control measures, not just a repeat test, to understand why the failure happened in the first place.
Beyond the immediate health risk, failing to follow HSG274 can also be used as evidence against an organisation if an outbreak occurs and the matter ends up in front of a court. Guidance documents don't carry the same legal weight as regulations, but ignoring them when something goes wrong rarely goes well for the duty holder.
We're a specialist UK water hygiene company built around exactly this kind of technical compliance. Whichever parts of HSG274 apply to your site, our team can help you get there and stay there:
If you're not confident your current control scheme reflects the latest HSG274 guidance, particularly the 2024 update to Part 1, contact us and we'll talk you through exactly where you stand.
HSG274 itself is guidance, not law, but it supports your legal duties under the Health and Safety at Work Act 1974 and COSHH regulations. Failing to follow it can be used as evidence of non-compliance if something goes wrong, so in practice, treating it as mandatory is the only sensible approach.
ACOP L8 sets out your legal duties for controlling legionella. HSG274 provides the detailed, practical guidance for meeting those duties across different types of water systems. You need both: L8 for the legal framework, HSG274 for the operational detail.
Part 2 sets out specific monitoring frequencies for temperature checks, outlet flushing and system inspections, with little-used outlets and showerheads needing particular attention. The exact schedule depends on your system and risk assessment findings; our team can confirm the right frequency for your site.
Yes. Part 1 (cooling towers and evaporative condensers) was updated and republished as a second edition in 2024. Parts 2 and 3 remain as originally published.
Only the parts that match the systems you actually operate. Most buildings only need Part 2. Part 1 applies if you have cooling towers or evaporative condensers, and Part 3 applies if you have spa pools, hot tubs, humidifiers or similar systems.
Stay safe. Stay compliant. Stay protected.
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