This guide from Comfort Services Group explains how expansion vessels are used in water systems and what expansion vessel flushing is. It also highlights the legionella risks they can pose and provides advice on managing these risks.
The guidance follows health and safety regulations from the Health and Safety Executive, including ACOP L8 and HSG274 Part 2.
What are Expansion Vessels?
An expansion vessel (or expansion tank) is an important part of a pressurised domestic water system. Its job? To stop pressure from building up and causing damage.
In most systems, expansion vessels contain half water and half air. As pressure increases, the air compresses to absorb the excess, protecting components like the cylinder and pipework. When working properly, the vessel partially fills and empties depending on water usage.
When water in the cylinder heats up and expands, it needs somewhere to go. The expansion vessel takes care of this with a bladder that inflates or deflates depending on the water’s temperature and use.
These vessels are common in pressurised water systems like mains or pumped setups. You’ll often find them in schools, care homes, hotels, and dental practices.
What is Expansion Vessel Flushing?
Expansion vessel flushing is essential to ensure that your expansion vessel operates safely and efficiently. It removes debris, sediments, and contaminants from the water content to prevent pressure build-up and reduce the risk of Legionella bacteria growth. Legionella thrives when the expansion vessel itself creates a source of nutrients and stagnant conditions. This process aligns with the approved code of practice for maintaining safe water systems.
Comfort Services Group uses advanced equipment to perform thorough flushing, improving performance and extending the lifespan of your system.
Types of Expansion Vessels
There are three types of expansion vessels:
Diaphragm
Diaphragm expansion vessels use a rubber membrane to separate water from pressurised air, helping to manage pressure changes. They stop systems from overloading and keep everything running smoothly. Common in heating and cooling systems, they maintain steady pressure without fuss.
Bladder
Bladder expansion vessels have a rubber bladder inside to keep water and air separate. They maintain system pressure and stop water hammers from wreaking havoc. These vessels are used in heating, cooling, and water supply systems. Simple, effective, and they get the job done.
Flow Through
Flow-through expansion vessels keep water moving through the tank, stopping it from sitting still and cutting down on corrosion. They’re a smart choice for systems where good water quality is a must.
Identifying Expansion Vessels on Heating Systems
The first step in checking for legionella risk is to see if your heating or hot water system has an expansion vessel.
If you have a combi boiler (which provides hot water on demand and heating on a timer), the expansion vessel is usually inside the boiler, hidden behind the cover. Don’t take the cover off or try to inspect it yourself—leave that to a professional.
In other heating systems, the expansion vessel is separate from the boiler and looks like a stand-alone unit. It’s often a cylindrical shape and may be red to show it’s for hot water.
The size of the vessel varies. For homes, it’s usually 8 or 12 litres, but larger systems—like those in offices or care homes—might use 18 or 25 litres.
If you’re not sure whether your system has an expansion vessel, contact a qualified engineer or a water treatment service provider like Comfort Services Group for an inspection.
Expansion Vessels and Legionella Risk
The Health and Safety Executive has identified expansion vessels as a potential Legionella risk. This is because they often have low water flow or stagnant water, especially when system pressures and temperatures stay steady. Essentially, they can behave like dead legs where bacteria can thrive.
If your water system in the UK has an expansion vessel, it’s your legal duty to manage the risk of Legionella bacteria and other waterborne pathogens. Start with a Legionella risk assessment to work out what actions are needed.
ACOP L8 & HSG 274 Regulations
Under the Health and Safety at Work Act 1974, duty holders, including employers and those managing premises, must assess potential risks such as Legionella and other water-borne bacteria. They are required to establish control measures to protect employees and anyone else who may be affected.
Reduce the Risk from Legionella in Expansion Vessels
Whether you have an existing expansion vessel or are installing a new one, there are steps you can take to reduce the risk of Legionella growth. Expansion vessels should be:
- Installed in cool areas on cold-flowing pipes
- Placed as close as possible to the incoming water supply
- Mounted vertically to prevent debris from being trapped
- Fitted with an isolation valve and drain valve to allow for flushing and sampling
- Designed to minimise the volume of water retained inside
- Built to encourage water flow within the vessel.
Expert Water Safety and Engineering Management Solutions – Comfort Services Group
Contact Comfort Services Group for expert legionella risk management and water engineering solutions. We can help you maintain water quality, meet safety standards, and comply with Legionella regulations.
With 30 years of experience, our team of trained engineers and technical specialists provides professional and cost-effective water treatment and environmental commissioning services.
FAQs
Why does Legionella grow inside expansion vessels?
Legionella can grow in an expansion vessel under these conditions:
- Water stagnates or flows poorly.
- Scale, rust, and sediment provide nutrients.
- Water temperatures are between 20°C and 50°C.
Where can legionella and other bacteria build up inside an expansion vessel?
One of the biggest risks to water safety is the growth of legionella and other bacteria within a water system. If left unchecked, bacteria can multiply to dangerous levels, posing serious health risks. Legionella bacteria thrive under specific conditions, including:
- Warm water temperatures between 20°C and 45°C.
- Low-flow or stagnant water.
- The presence of nutrients like scale, rust, or sediment.
What is included in an expansion vessel flushing service?
An expansion vessel flushing service clears out stagnant water, sediment, and debris that encourage bacteria growth. The vessel is drained and flushed thoroughly, ensuring it stays clean and functional. This simple yet essential maintenance prevents contamination and boosts system performance. Think of it as giving your system a refreshing reset—it’s basic upkeep with big benefits.
How often do I need to flush our expansion vessels?
Expansion vessels should be flushed every six months to comply with legionella guidelines (HSG 274).
How long does an expansion vessel flush take?
Flushing an expansion vessel depends on its size and the pipe length. It usually takes 5 to 20 minutes.
We are proud to share our experience and knowledge and take pride in offering this to all clients when required. If I, or one of the team can help then please schedule a meeting via the office or get in touch via one of our social media platforms.
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