HSG274 is the HSE's technical guidance for controlling legionella. While ACOP L8 tells you what you must do, HSG274 tells you how to do it. If you're responsible for water systems, understanding HSG274 is essential for practical compliance.
Which water systems are you responsible for?
This helps us point you to the right part of HSG274.
What is HSG274?
HSG274 is a three-part technical guidance series published by the Health and Safety Executive. It provides the detailed, practical guidance for implementing the requirements of ACOP L8.
Think of ACOP L8 as the "what" and HSG274 as the "how." L8 establishes your legal duties. HSG274 tells you exactly how to meet them for different types of water systems.
The three parts of HSG274
| Part | Title | Covers |
|---|---|---|
| Part 1 | Evaporative cooling systems | Cooling towers, evaporative condensers |
| Part 2 | Hot and cold water systems | Domestic-type water systems in all premises |
| Part 3 | Other risk systems | Spa pools, hot tubs, humidifiers, water features |
Most duty holders need Part 2 — it applies to virtually every building with running water. Parts 1 and 3 apply to specific higher-risk systems.
HSG274 Part 1: Cooling towers
Cooling towers and evaporative condensers are the highest-risk systems for legionella. They create large volumes of aerosol and have caused multiple major outbreaks.
Who needs Part 1?
You need HSG274 Part 1 if you have:
- Cooling towers
- Evaporative condensers
- Evaporative cooling systems
- Wet cooling systems in industrial processes
Cooling towers must be notified to your local authority within one month of commissioning. Failure to notify is a criminal offence. If you have a cooling tower that hasn't been notified, do so immediately.
Key requirements for cooling towers
Notification:
- Register with your local authority
- Update notification if systems change
- Notify when systems are decommissioned
Risk assessment:
- Comprehensive assessment by competent person
- Review at least annually
- Update after any significant changes
Written scheme:
- Detailed control scheme specific to the system
- Defined responsibilities and frequencies
- Emergency procedures for high bacteria counts
Monitoring:
- Daily and weekly operational checks
- Regular microbiological sampling (typically quarterly)
- Continuous monitoring of key parameters
Water treatment:
- Chemical treatment programme
- Regular dosing and monitoring
- Treatment verification through testing
Record keeping:
- Comprehensive logs of all activities
- Test results and trend analysis
- Maintenance and remedial actions
Temperature and treatment targets
Cooling towers operate in the temperature range ideal for legionella growth (20-45°C), so temperature control alone cannot prevent bacterial growth. Instead, control relies on:
- Chemical biocides to kill bacteria
- Dispersants to prevent biofilm
- Regular cleaning to remove nutrients
- System design to minimize stagnation
Cooling Towers vs Hot/Cold Water Systems
Cooling Towers (Part 1)
- •Must notify local authority
- •Cannot rely on temperature control
- •Requires chemical treatment
- •Quarterly microbiological testing
- •Daily operational checks required
- •High-risk - potential for major outbreaks
Hot/Cold Water (Part 2)
- •No notification required
- •Temperature is primary control
- •Chemical treatment often unnecessary
- •Testing only if problems suspected
- •Monthly monitoring typical
- •Lower risk if properly managed
Bottom line: Cooling towers require specialist management and should be overseen by water treatment professionals. Most organisations outsource cooling tower management entirely.
HSG274 Part 2: Hot and cold water systems
Part 2 is the most widely applicable section of HSG274. It covers the domestic-type hot and cold water systems found in virtually every building.
Who needs Part 2?
You need HSG274 Part 2 if your premises has:
- Hot water cylinders or calorifiers
- Cold water storage tanks
- Showers or spray taps
- Any hot and cold water distribution system
This includes offices, shops, factories, care homes, hotels, rental properties, schools, and almost any other type of premises.
The temperature principle
Temperature control is the primary defence against legionella in hot and cold water systems:
HSG274 Part 2 Temperature Requirements
| System Component | Target Temperature | Monitoring Frequency |
|---|---|---|
| Hot water storage (calorifier) | 60°C or above | Monthly |
| Hot water distribution (flow) | 50°C or above within 1 minute | Monthly at sentinel points |
| Hot water return (if fitted) | 50°C or above | Monthly |
| Cold water storage tank | Below 20°C | Monthly |
| Cold water at outlets | Below 20°C within 2 minutes | Monthly at sentinel points |
Sentinel points are the nearest and furthest outlets from the storage/generation point, plus other representative locations.
Legionella thrives between 20-45°C. Below 20°C, it survives but doesn't multiply significantly. Above 50°C, it starts to die. At 60°C, it's killed rapidly. Temperature control exploits these characteristics.
Monitoring requirements
HSG274 Part 2 specifies monitoring tasks and frequencies:
Monthly checks:
- Temperature at sentinel hot water outlets
- Temperature at sentinel cold water outlets
- Calorifier/hot water storage temperature
- Cold water tank temperature (where accessible)
Quarterly checks:
- TMV (thermostatic mixing valve) operation
- Showerhead condition and cleaning
- Little-used outlet flushing verification
Annual checks:
- Cold water tank inspection and cleaning
- Calorifier inspection (where possible)
- Full system review
As required:
- Flushing of little-used outlets (weekly if unused)
- Descaling of showerheads and spray taps
- Remedial action for out-of-spec temperatures
What to do when temperatures are wrong
If monitoring reveals temperatures outside the target range:
Hot water below 50°C at outlets:
- Check calorifier temperature — is it at 60°C?
- Check for long pipe runs causing heat loss
- Check for faulty TMVs reducing temperature
- Consider increasing storage temperature
- Consider insulation improvements
Cold water above 20°C:
- Check tank location — is it in a warm space?
- Check for hot pipes running near cold pipes
- Check tank insulation
- Check for low usage causing warming
- Consider relocating tank or improving ventilation
Temperatures outside range don't necessarily mean legionella is present — but they indicate conditions where it could multiply. Investigate and correct the cause rather than just recording non-compliant readings.
Little-used outlets
Stagnant water is a significant risk factor. HSG274 requires:
- Identify all little-used outlets (used less than once a week)
- Flush them for several minutes at least weekly
- Consider removal if outlets are never needed
- Record all flushing activities
Little-used outlets include:
- Guest rooms in hotels/B&Bs between guests
- Vacant offices or meeting rooms
- Seasonal facilities
- Disabled toilets with low usage
- Outside taps rarely used
Showerheads and spray devices
Showerheads are a key risk point because they create aerosols directly inhaled by users. HSG274 requires:
- Quarterly cleaning and descaling
- Replacement if heavily scaled or damaged
- Regular use or flushing if showers are little-used
- Documented cleaning records
System design considerations
HSG274 Part 2 also covers design issues to address:
- Dead legs — remove or flush regularly
- Tank sizing — avoid oversized tanks with long residence times
- Pipe insulation — keep hot pipes hot and cold pipes cold
- System complexity — simpler systems are easier to control
HSG274 Part 3: Other risk systems
Part 3 covers systems that don't fit neatly into Parts 1 or 2 but still pose legionella risks.
Spa pools and hot tubs
Spa pools operate at temperatures ideal for legionella (30-40°C) and create significant aerosol through air jets. They require:
- Water treatment — disinfection is essential (typically chlorine/bromine)
- Filtration — continuous filtration and regular backwashing
- Testing — at least daily pH and disinfectant testing
- Cleaning — regular filter cleaning and periodic deep clean
- Monitoring — weekly microbiological testing recommended for public spas
Domestic hot tubs also require proper maintenance. Even private hot tubs have caused legionella cases. Follow manufacturer guidance and maintain disinfectant levels.
Humidifiers
Humidifiers that use water reservoirs and create aerosols can be a source of legionella:
- Spray humidifiers — highest risk, require water treatment
- Evaporative humidifiers — lower risk but still require maintenance
- Steam humidifiers — lowest risk as water is heated to steam
Control measures include regular cleaning, water treatment, and consideration of steam systems for new installations.
Water features and fountains
Decorative water features with sprays or aerosols pose risks:
- Operate at temperatures that support growth
- Create aerosols in public areas
- Often have stagnant water periods
Controls include water treatment, regular cleaning, and proper maintenance schedules.
Emergency showers and eyewash stations
These safety devices are rarely used but create aerosols when activated:
- Flush weekly to prevent stagnation
- Include in monitoring programme
- Document all flushing
Implementing HSG274 in practice
Step 1: Know your systems
Before applying HSG274, identify all water systems:
- What type of hot water generation? (calorifier, combi boiler, point-of-use)
- Any cold water storage tanks?
- Where are all the outlets?
- Any specialist systems (cooling, spa, etc.)?
Step 2: Match to the right part
- Standard hot/cold systems → Part 2
- Cooling towers → Part 1
- Spa pools, fountains, humidifiers → Part 3
Step 3: Create your monitoring schedule
Based on HSG274, create a practical schedule:
| Task | Frequency | Who |
|---|---|---|
| Sentinel hot water temperatures | Monthly | Responsible person |
| Sentinel cold water temperatures | Monthly | Responsible person |
| Calorifier temperature | Monthly | Responsible person |
| Little-used outlet flushing | Weekly | Designated staff |
| Showerhead cleaning | Quarterly | Maintenance staff |
| TMV checks | Quarterly | Competent person |
| Cold water tank inspection | Annual | Water hygiene specialist |
Step 4: Set up record keeping
HSG274 expects comprehensive records:
- All temperature readings
- Flushing logs
- Cleaning records
- Maintenance activities
- Remedial actions taken
- Risk assessment reviews
Keep records for at least 5 years.
Step 5: Train your people
Anyone conducting monitoring or maintenance needs to understand:
- Why they're doing it
- How to do it correctly
- What results to expect
- When to escalate concerns
Frequently asked questions
ACOP L8 is the Approved Code of Practice setting out your legal duties — it tells you what you must do. HSG274 is technical guidance explaining how to do it for different types of systems. L8 has special legal status; HSG274 provides the practical detail. You need both: L8 for understanding your obligations, HSG274 for implementing the controls.
Most premises only need Part 2 (hot and cold water systems). You only need Part 1 if you have cooling towers or evaporative condensers. You only need Part 3 if you have spa pools, hot tubs, humidifiers, or decorative water features. Check what systems you have and focus on the relevant parts.
HSG274 Part 2 recommends monthly temperature monitoring at sentinel points (nearest and furthest outlets from storage). This applies to both hot and cold water. Some higher-risk premises like care homes may need weekly monitoring. Record all readings and investigate any that fall outside the target ranges.
Cold water above 20°C indicates conditions where legionella could multiply. Investigate the cause: Is the tank in a warm location? Are hot pipes running near cold pipes? Is usage low, allowing water to warm up? Address the root cause rather than just recording the reading. If you can't achieve below 20°C, you may need additional controls or more frequent flushing.
For most hot and cold water systems, routine testing is not required if temperature controls are effective. HSG274 emphasises that controlling temperature and preventing stagnation is more important than testing. However, testing may be appropriate for cooling towers (usually required), after remedial work, to verify controls are effective, or if there's a suspected problem.
Yes, for basic hot and cold water monitoring, you can conduct checks in-house if you're trained and competent. You need a suitable thermometer, understanding of what you're checking, and a system for recording results. For complex systems, cooling towers, or if you're unsure, use a water hygiene specialist.
HSG274 expects records of: temperature monitoring readings, flushing activities, cleaning and maintenance, remedial actions taken, risk assessment and reviews, training provided. Keep records for at least 5 years. Make them accessible for inspection by HSE or local authority if required.
HSG274 is available free to download from the HSE website: Part 1 (cooling towers), Part 2 (hot and cold water systems), and Part 3 (other systems). You can also purchase printed copies. The guidance is periodically updated, so ensure you're using the current version.
Next steps
Understand your legal duties under ACOP L8:
ACOP L8 Compliance Checklist →
Learn about legionella risk assessments:
L8 Risk Assessment: What's Required →
Create your written scheme of control:
Legionella Written Scheme Guide →
Need help implementing HSG274 requirements? A water hygiene specialist can assess your systems, set up compliant monitoring programmes, and train your staff on proper procedures.
Related articles:
- ACOP L8 Compliance Checklist
- L8 Risk Assessment guide
- Legionella Written Scheme guide
- Temperature Monitoring guide
Useful resources:
- HSG274 Part 1 — Cooling towers
- HSG274 Part 2 — Hot and cold water systems
- HSG274 Part 3 — Other risk systems
- ACOP L8 — Legal requirements
Tools:
Disclaimer: This guide summarises HSG274 for general understanding. It does not replace the official HSE guidance. For complex systems, cooling towers, or high-risk premises, always consult the full HSG274 documents and consider professional water hygiene support.