What does an Expansion vessel do?

When water is heated up it expands in size by approximately 4%. This expansion has to go somewhere. On a combi boiler, it has an expansion vessel to take up this expansion of water.

When the expansion vessel is faulty it will cause a rise in pressure when your boiler is heating up your radiators.

What is an expansion vessel?

An expansion vessel is a steel vessel within the boiler. Inside the vessel is a rubber diaphragm with your central heating system water on 1 side and air (sometimes nitrogen) on the other side.

How does the expansion vessel work?

The expansion vessel works in the following way. When the water is heated through your boiler when your heating is on it will start to expand within your radiators, pipework and boiler. That expansion has to go somewhere, so it expands into the space that is occupied by the air in the expansion vessel. The air compresses in the vessel. As the water cools down the air pushes back into its normal space.

How do I know if my expansion vessel is faulty?

The first signs are when your heating is on, the pressure will rise on the gauge on the front of your boiler.

When your boiler gets to 3 bar in pressure on the gauge this will activate the pressure relief valve. The water will be evacuated through the pressure relief valve that is preset to operate at 3 bar of pressure for safety reasons.

When the boiler cools down the pressure will go back down to zero bar pressure.

It’s at this point your boiler will go to fault mode for low pressure and you need to fill the system back up with water.

The outlet for the pressure relief valve is commonly on the outer wall of your boiler. You should be able to see water coming out if activated or a wet patch going down the wall from the outlet. If this is happening on a daily basis you really do need to get this fault rectified.

Adding fresh oxygenated water all the time will cause a lot of rust corrosion to the system which is not advisable for the long term health of your central heating system.

There are other causes for your boiler losing pressure with no sign of a leak.

This could include a cracked and corroded primary heat exchanger and ground floor leak that is not viewable without taking floorboards up or possibly the pipes have been laid in a screed of concrete without suitable protection causing pinholes due to corrosion.

If you suspect you have a leak it is best to get it investigated as soon as possible.

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