If you're responsible for managing a building and its water systems, or installing water heating systems for commercial offices, care homes, leisure centres or any business for that matter.
There is a fair chance you've heard of legionella bacteria and the ACOP L8 guidance. But what does preventing legionella bacteria proliferation really mean in practice?
How does your choice of water heating system impact legionella bacteria growth, feedstock and prevention?
In this blog, we will review hot and cold water systems and their impact on Legionnaires' disease risk management.
What Is Legionella and Why Should You Care About Its Potential Health and Safety Risks?
Legionella is a type of bacteria that can cause Legionnaires’ disease, a potentially fatal form of pneumonia. It thrives in warm water systems. Ideally water temperatures for legionella proliferation are between 20°C and 45°C.
The legionella bacteria without appropriate control measures can become airborne through water droplets, like those from showers or cooling towers.
This is where ACOP L8 guidance comes in.
What Is ACOP L8 and Why Should Duty Holders Understand It?
ACOP L8 is an Approved Code of Practice L8, published by the UK’s Health and Safety Executive (HSE). It provides practical guidance on how to comply with legal duties under the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health (COSHH) Regulations 2002.
See the full document from the health and safety executive below:
Legionnaires' disease. The control of legionella bacteria in water systems - HSE
The guidance is aimed at responsible persons or duty holders such as employers, landlords, and facilities managers. These people are responsible for managing water systems, plumbing systems, sanitisation strategies, cleaning regimes, risk assessments and overall legionella control.
Therefore the guidance provides as follows:
- Legionella risk assessments.
- Implementation of control schemes and control measures
- Appointment of competent persons (the “responsible person” "duty holder")
- Monitor and review control measures
- Keep records of all actions taken
- Health and safety responsibilities
- Water temperature guidance
Why is Water Heating Important in Legionella Control and Risk Assessment?
One of the most effective ways to control Legionella is through temperature control. According to HSE guidance, legionella risk management can be supported by:
- Hot water should be stored at 60°C or higher.
- Hot water should be distributed at 50°C or higher (or 55°C in healthcare settings).
- Cold water should be kept below 20°C.
- Therefore, the type of water heating system you use can significantly impact your ability to meet these standards.
Comparing Water Heating Methods
Let’s take a look at three of the most common types of water heating systems in terms of Legionella control, energy efficiency, and practicality.
1. Traditional Storage Water Heaters or Cylinders
How they work: These systems heat water and store it in the cylinder until it’s needed.
Pros:
- Simple to maintain high storage temperatures (60°C+).
- Familiar and widely used in commercial settings.
- Can be integrated with Legionella pasteurisation cycles.
Cons:
- Heat loss from stored water (unless well-insulated).
- Risk of stratification where cooler water sits at the bottom of the tank.
- Requires regular flushing and temperature checks.
Legionella risk assessment: Moderate to low if the water system is well-maintained, regularly monitored and water temperature management is in place.
2. Heat Pump Hot Water Systems
These systems use electricity to move heat from the air or ground into the water tank. They work in a similar fashion to a fridge in reverse.
Pros:
- Highly energy-efficient (especially air-source and ground-source models).
- Lower carbon footprint.
- Can be paired with solar PV for greener energy.
Cons:
- May struggle to consistently reach 60°C without a backup heater.
- Slower recovery times.
- More complex to install and maintain.
- Require storage vessels for hot water
Legionella risk assessment: Higher if not properly configured. Many systems include an electric boost to reach pasteurisation temperatures.
3. Continuous Flow (Tankless) Water Heaters
These instant types of water heater heat water on demand as it flows through the unit. They often include no storage tank.
Pros:
- No storage cylinder = no standing body of water = lower Legionella risk.
- Compact and space-saving.
- Energy-efficient
- Provides precise temperature control over hot water and therefore can eliminate legionella bacteria.
Cons:
- May not be scalable for very high-demand environments.
- May need a storage vessel in high demand environments.
Legionella risk assessment: Low, especially if water is heated above 60°C for secondary returns and if there is no need for storage on the system.
Best Practices for Legionella Control in Water Heating
Regardless of the system used for water heating, below are some simplified tips from the ACOP L8 guidance.
- Regularly monitor water temperatures at outlets.
- Flush unused outlets weekly.
- Descale and disinfect showerheads and taps.
- Keep records of all maintenance and checks.
- Conduct legionella training on the correct awareness levels and procedures.
- Avoid water stagnation points with minimal storage and dead legs.
Summary
Understanding ACOP L8 is about protecting people. The choice of water heating system can play a crucial role in managing Legionella risk. Whether that is a traditional cylinder, heat pumps or continuous flow system, the key is temperature control, regular maintenance, and good record-keeping.
If you're unsure where to start, consider bringing in a Legionella risk assessor to help you build a compliant and effective control scheme or conduct staff training.
Case Study: Continuous Flow Water Heaters in a Leisure Centre
An example of continuous flow water heaters being used for Legionella control comes from a large leisure centre in the UK.
The Challenge
Leisure centres are high-risk environments for Legionella due to:
- High water usage.
- Complex pipework systems.
- Periods of stagnation, especially overnight or during closures.
In this case, the centre had no storage tanks, but the pipework alone could hold up to 10,000 litres of static water when not in use, creating a potential breeding ground for Legionella.
The Solution
The facility installed gas-fired continuous flow water heaters that:
- Heated cold mains water instantly to above 60°C, eliminating the need for stored hot water.
- Maintained a secondary return temperature of 42°C during operational hours for user safety.
- Delivered intermittent thermal shocks at 60°C during off-hours to disinfect the system.
This approach ensured that:
- Water was never stored at temperatures conducive to Legionella growth.
- The system could be safely used by the public during the day.
- Legionella risk was mitigated without compromising energy efficiency or user comfort.
The Outcome
- Legionella control was achieved without the need for traditional hot water storage.
- The system was cost-effective, space-saving, and responsive to demand.
- The facility met HSE temperature guidelines while reducing energy waste.
This case highlights how continuous flow systems, when paired with smart temperature management, can be a robust solution for high-demand, high-risk environments like leisure centres.
To find our more about legionella best practice within water heating system design, contact the Rinnai design team today.
Read more below:
Legionella management with continuous-flow hot-water systems - Modern Building Services
