What Is an Expansion Vessel For?

Water heaters are wonderful devices that allow us to enjoy a stream of hot water when we open the tap. However, they can be complicated, particularly if you’re dealing with an unvented cylinder (also called a closed water heating system) and require quite a few components in order to work properly. One of those is the expansion vessel.

If you’ve noticed that there’s a smaller tank attached to your water cylinder, this is the expansion vessel. It may be mounted to the top of the cylinder, or it could be attached in some other location. With some models, it is actually within the casing of the water heater. It all depends on the make and model in question.

You’ll find quite a few different styles of expansion vessels have been used over time, but the most modern (and most common) style uses a flexible membrane within the vessel itself. The expansion vessel actually contains two sections. A rubber diaphragm separates the two. One side of the vessel is connected to the hot water system and holds water. The other side holds air.

The purpose of an expansion vessel is to help prevent overpressure within an unvented cylinder. The pressure of the air within the expansion vessel should be set at the same level as the incoming water pressure, typically 3 bar. As water is heated, it expands. Without the expansion vessel the pressure within the unvented water cylinder would rise forcing the safety valves to open. When an expansion vessel is fitted the pressurised air inside the vessel compresses and absorbs additional volume of water and prevents an increase in pressure. Expansion vessels need to be recharged once every twelve months so that they retain their charge. If they lose their charge, they fill with water and offer no protection. This then results in damaged valves, damaged cylinders or burst expansion vessels.

For the most part, expansion vessels operate without any need for your intervention. You may need to drain and recharge your expansion vessel periodically depending on its age and the style of vessel in question. However, they can suffer from some problems, particularly as they age. For instance, hard water and even water treatment chemicals can cause the rubber membrane within the vessel to degrade and break down. Eventually, it will fail, requiring replacement. 

If you think your expansion vessel needs to be repaired or replaced, contact a engineer experienced with unvented cylinders and their operation. EasyFlow can send an engineer to you if you’re located in Greater Manchester, Warrington, Liverpool, Chester or Leeds.