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An expansion vessel is a vital component of a pressurised closed water system. When water is heated it expands. A closed water system – where it is essential that pipes remain full of water – is, by definition, sealed, so expansion would cause a problem if the extra volume of water had nowhere to go. An expansion vessel is an essential element of a closed water system because it provides a safe space for the water to flow into as it heats up. In doing so, it protects the system – and any appliances plumbed into the system – from a build-up of pressure which could cause burst pipes and damage. 

An expansion vessel looks like a small water tank – one side of the tank is connected to the water system and holds the water, and the other side is pre-charged with compressed air. A flexible, moving membrane (diaphragm) separates the two. As the water heats up and expands, the extra volume flows into the vessel and pushes against the membrane, allowing space for the water to be safely stored. As the water cools and contracts, the compressed air pushes against the membrane, returning the water on the other side into the system. The expansion vessel partially fills and empties depending on the demand and use of hot water, maintaining the pressure in the system at safe levels throughout.   


How to calculate the size of expansion vessel required

For an expansion vessel to be effective, it is crucial that it is the right size to accommodate the amount of water needed to maintain an acceptable level of pressure in the system. The size of the expansion vessel required depends on the maximum system temperature, system volume, and the static head above the vessel. The operating temperature range and the system volume dictate the vessel size, and the static head dictates the pre-charged pressure. 

Expansion vessel sizes are given in litres, and there are two methods for working out the size required (based on the assumption that the system is 82°C/71°C flow and return and system pressure is up to 1.5 bar):

  • 10% of the system volume – rounding up to the nearest vessel size available. Take all elements of the system into consideration when calculating the system volume – including the volume of water in each radiator as well as the boiler itself. 
  • 1 litre capacity minimum per kW boiler output – again rounding up to the nearest vessel size available.

It is important, however, to use any resulting figures as an approximate guide – a number of additional factors must be considered, such as where in the system the vessel is installed and the type of system it is being installed in. This is why it is essential that any work carried out to install or maintain an expansion vessel is done by a qualified engineer.


Expansion vessels and legionella

If you are responsible for a building’s water system, it is your legal duty to be aware of the potential risks from legionella and to take appropriate action – as indicated by a thorough risk assessment – to minimise those risks. 

The Health and Safety Executive (HSE) has identified expansion vessels as a potential legionella risk, particularly in systems that are operating at a steady temperature and pressure and may have long periods without exchanging any significant amount of water. Stagnation and low water flow, temperatures that do not increase or decrease, and nutrients such as scale, rust and sediment combine to offer the perfect conditions for microbial growth. In addition, the membrane in the vessel is often made of synthetic rubber, and this rubber can support the growth of legionella.

There are a number of steps that can be taken to minimise microbial growth. The first is correct installation of the expansion vessel. Key considerations include installing the vessel in a cool area with cold flowing pipes and as close to the incoming water supply as possible, mounting it vertically on pipework to minimise any trapping of debris, and fitting it with an isolation and drain valve to allow for flushing and sampling. 

Once installed, the expansion vessel must be maintained at regular intervals, as determined by the risk assessment. The HSE advises that expansion vessels should be, where practical, flushed through and purged to drain, and membranes should be changed according to the manufacturer’s guidelines or as indicated by the risk assessment.