What are expansion vessels?

Expansion vessels are key components of any unvented hot water system  and should be present in all pressurised hot water systems. Closed water system components can be damaged by increases in pressure and expansion vessels help to maintain pressure levels. They are usually located in or near the water heater and are filled with air or a gas such as nitrogen. They will often be available as part of an unvented kit.

How do expansion vessels work?

Expansion vessels contain air, or an inert gas, and water from the system. These are separated within the vessel by a membrane – either in the form of a rubber sheet (or diaphragm) or a bladder. As cold, pressurised mains water is heated in an unvented hot water system the water expands and this pressure builds. Water in the expansion vessel therefore also expands and the air within the vessel compresses to absorb this increased volume and, hence, the pressure. When the water cools – and the pressure reduces - the water level within the expansion vessel drops again.


How an expansion vessel works

What are flow through expansion vessels?

Flow through vessels have been specifically developed to reduce the risk of stagnation and contamination from slow moving water. They do this by keeping water moving through the expansion vessel using a Flowjet valve.

We can provide standalone flow through expansion vessels and a flow jet valve that diverts part of the water flow in the system into the vessel, preventing water stagnation and guarding against the breeding of harmful bacteria – most notably legionella (which causes the potentially fatal Legionnaires disease).

Flow through expansion vessel with Flowjet valve reduces water stagnation

The Flowjet valve also allows for expansion vessels to be drained, bypassed or isolated during removal or replacement.

The flow through expansion vessel from Hamworthy has an internal bladder manufactured in Butyl rubber to limit the risk of bacteria growth, including Legionella, which makes it particularly suited to healthcare premises.

What are the benefits of flow through expansion vessels for unvented kits?

When a hot water system is running as usual, the expansion vessel will regularly partially empty and then refill, exchanging water with the system. However, expansion vessels are known to be points that run the risk of bacterial growth as they can suffer from low flow rates or stagnant water. This is particularly true if the system is unused, or the pressure and temperature remain steady over a period of time. In that case the normal fill and replace cycle won’t be naturally changing the water and bacteria can flourish.

A flow through expansion vessel reduces the risk of legionella growth in an unvented system, reduces the amount of pipework in the system and gives more flexibility for maintenance and replacement.

At Hamworthy we are frequently seeing more flow through expansion vessels specified on NHS projects.

In order to meet the needs of our customers we can provide standard unvented kits with standard expansion vessels, or a standalone flow through expansion vessel with flowjet valve.

Please talk to us to understand the requirements of your next hot water project.