What Are Expansion Vessels?

Expansion vessels are crucial for maintaining a constant and adequate water pressure level within a drinking water or heating system. These vessels look just like small water tanks and are also known as expansion tanks. Within these vessels is a rubber diaphragm, the function of which is to keep the compressed air and the water from the system apart.

Expansion vessels can be either potable or nonpotable. Potable expansion vessels are used in drinking water systems and must meet stringent standards around cleanliness. Non-potable vessels are used within heating systems where water is not suitable for drinking.

When water is heated, it expands. The expansion vessel is designed to deal with this and gives the extra water volume somewhere to go. Without this, pressure would build and build, and the system pipes could potentially burst. Any appliances that are plumbed into the system could also be damaged.

The diaphragm within the vessel is expanded by the additional volume of water, allowing space for it to be managed safely. When the water cools, the diaphragm retracts and allows the water to return into the system. This helps to maintain consistent and adequate pressure levels, and ensures maximum efficiency.

Old Versus New Systems

Older vessels ended to use open-vent systems which relied on gravity to enable water to return from an overflow tank. Newer vessels are 'sealed' and use a flexible internal diaphragm. This is both more hygienic and more energy-efficient.

Newer combi boilers will come with a vessel that is appropriately sized, and usually located within the casing of the appliance.

What Should The Pressure Inside Expansion Vessels Be?

The pressure level within expansion vessels is usually pre-set by the manufacturer. In domestic systems, it is usually around 0.82 bar (approximately 12 psi). In larger systems/taller buildings, it can be as high as 1.38 bar (20 psi).

Vessel Sizes

If the water volume of the system is unknown, there are some general guidelines around how to calculate the size of the expansion vessel that is required. These are based on the boiler's power. As a general rule 25kW boilers will need a 24-litre vessel, 60kW boilers require a 50-litre vessel, and 120kW boilers will need a 100litre vessel at a minimum.

Expansion Vessels - Manufacturers

Here at Heating and Plumbing World, we offer expansion vessels from several well-known manufacturers, including Altecnic, Reflex, and Stuart Turner.

Altecnic is one of the UK's leading manufacturers of hydronic solutions for both domestic and commercial applications. They offer several expansion vessels with capacities ranging from 8 litres right up to 800 litres. These come with a factory pre-charge of 1.5bar which uses Nitrogen and increases the lifespan of the vessel. They have a non-replaceable diaphragm, are suitable for temperatures of up to 120°C, and are made of either Stainless Steel or Carbon Steel.

Flamco are part of Aalberts Industries NV. They offer both potable and nonpotable vessels for commercial or domestic use, with capacities ranging from just 2 litres all the way to a substantial 8000 litres. These high-quality steel vessels offer a maximum working pressure of six or ten bar, are made in accordance with EN13831 standards, can cope with temperatures up to 120°C, and are finished with a red epoxy powder coating.

Reflex (full name Reflex Winkelmann GmbH) is one of the world's leading providers of heating and hot water supply systems and technologies. They make a range of expansion vessels for use in heating systems and drinking water systems. They offer potable vessels (vertical and horizontal), anti-legionella potable vessels, and heating and cooling water vessels. These vessels are made of steel and have capacities ranging from 2 litres to 5000 litres. The maximum pressure ranges from 3 bar to 25 bar.

Stuart Turner are a leading British manufacturer of water boosting solutions. These include a range of WRAS approved expansion vessels with capacities of up to 1000 litres, operating pressures of 3 or 6 bar, and maximum operating temperatures of 70°C. They have threaded connections, factory pre-set pressure levels, and are finished in a grey powder-coated finish.

The cost of expansion vessels very much depends on the type and size of the vessel that is required. Very small capacity vessels (such as those used in small domestic heating systems) can cost less than a hundred pounds, whilst the very largest can cost many thousands. Additionally, potable vessels tend to be slightly more expensive than non-potable models.

Expansion vessels offer an extremely effective way of dealing with pressure in hot water and drinking water systems. If you need further advice, please don't hesitate to contact us.