An expansion vessel is a device fitted to a heating system or unvented hot water cylinder to prevent the pressure inside it increasing which would cause safety valves to open. It works by containing water and air or inert gases such as nitrogen under pressure* on the same side of a diaphragm.
The pressure within an expansion vessel is a very important factor in its operation and it should be set to the same level as the incoming water pressure. This is because the pressure of the water in an unvented cylinder increases when it is heated and as this happens the air or gas in the expansion vessel will be compressed, absorbing any additional pressure before it can rise.
Expansion vessels are a vital part of the safe and efficient operation of many domestic heating systems. They help to keep the water in your system at a constant temperature and minimise corrosion which can reduce your maintenance costs.
There are a number of different types of expansion vessels. They can be compression tanks or bladder/diaphragm tanks (which also have a gauge glass on the side to show the air in the tank).
Compression Tanks
The most common type of expansion vessel is a compression tank – usually made from steel, and these are mounted horizontally above the boiler. They’re easy to install, and come with a gauge glass on the side to give you an idea of the amount of air in the tank.
Bladder/Diaphragm Tanks
There’s also a type of bladder/diaphragm tank which is usually more expensive than a compression tank but has a ring base which sits on the floor and makes it easier to position. The tank is also much smaller so it’s more compact and fits neatly on top of the boiler.
These tanks have an internal butyl rubber bladder attached to the system water connection which allows the vessel to absorb the increase in system pressure as it’s filled with hot water. The bladder/diaphragm is also semi-permeable to gases so the air pressure in the bladder gradually decreases as the water is filled into the expansion vessel.
When the vessel’s air pressure is lower, it can’t cope with the expansion of the water so it will have to work harder to overcome the pressure in the cylinder. This can lead to excess pressure and the vessel will eventually rupture and lose its ability to keep your system at a constant temperature.
It’s a good idea to drain the tank and recharge it at least once a year so that it doesn’t lose its charge. Once this is done it will operate at its best and offer protection for the remainder of its lifespan.
Re-Pressurise Your Expansion Vessel
Normally the expansion vessel’s air chamber is pumped up to about 0.75 bar as it’s manufactured and this should be kept at this pressure when the vessel is re-pressurised. However, if you can’t get the air chamber up to this level, it’s probably time to call for an engineer to check your expansion vessel.