Expansion vessels are small tanks fitted to many hot and cold water systems to absorb pressure changes as water heats and cools. They are easy to miss, often tucked away in plant rooms or under stairs, but they can hold water that barely moves and sits at lukewarm temperatures. That makes them worth including in any legionella risk assessment.
What an expansion vessel does
When water is heated it expands. In sealed or pressurised systems, an expansion vessel gives that extra water somewhere to go, protecting the system from excess pressure. Inside, a rubber membrane or bladder separates the water from a cushion of air or gas.
Why they can be a legionella risk
- Stagnant water. In many vessels, the water inside is not regularly replaced, so it can sit still for long periods.
- Lukewarm temperatures. Vessels are often not insulated and may sit in warm plant rooms, bringing water into the range where bacteria grow.
- Rubber membranes. Some rubber materials can support bacterial growth and biofilm. Our guide to legionella sampling vs risk assessment explains why a risk assessment looks beyond sample results.
- Out of sight. Because they are hidden and rarely looked at, problems can go unnoticed.
What good practice typically looks like
- Know where they are. A legionella risk assessment should identify and record every expansion vessel on the system.
- Consider the type. Some vessels are designed so that water flows through them rather than sitting in a single connection. Your plumber or water treatment specialist can advise whether that suits your system.
- Install them sensibly. Where possible, vessels are typically fitted on the cold supply, kept cool and connected with short pipe runs to avoid creating a dead leg.
- Flush and maintain them. Where a vessel can be drained or flushed, include this in your maintenance routine, and follow the manufacturer’s instructions for checking pressures and membranes.
- Record what is done. Note any checks or work in your legionella logbook.
- Review after changes. New vessels, boiler replacements or system changes should prompt a fresh look.
These are common practice approaches rather than fixed legal rules, and the right arrangement depends on your system.
Where you will often find them
Hot water systems with unvented cylinders, booster pump sets, heating systems connected to the mains, and larger commercial plant rooms. If your building has an unvented hot water cylinder, it is likely there is an expansion vessel nearby. Our post on hot water cylinders and calorifiers covers that part of the system.
Your duties
Section 19 of the Safety, Health and Welfare at Work Act 2005 requires employers to identify hazards and assess risks. The HPSC National Guidelines for the Control of Legionellosis in Ireland (2009) set out how water systems should be assessed and managed.
Get it checked
A legionella risk assessment from Safer Solutions maps your system, including the easy-to-miss parts. Call 086 275 9728, email info@safersolutions.ie or contact us.