In which case do I need a membrane expansion vessel ?

  • what does an expansion tank do?
  • what types do exist?
  • how do I recognize a defective expansion vessel?

The expansion vessel is known by many names and also has quite a distinctive appearance, but what exactly it is installed for and why something in the system must expand most do not know. In principle, as is so often the case with a heating system, the primary goal is to save energy and protect your system. How this works exactly and what you should pay attention to, we explain in today’s article.


What does an expansion vessel do?

Due to the temperature fluctuations in the water, the volume also changes. Since a heating system is a closed circuit, there is a certain pressure that is raised or lowered by the change in volume. This is not a high risk at first, if you have a working safety valve. The safety valve prevents the pressure from rising too high by draining water and thus protects against too high pressure. However, this also means that when the pressure rises (above 2.5bar), water is always drained and the heater must therefore be refilled again and again. The expansion tank helps with this problem. A round container, which is divided inside by a membrane into two chambers, is filled on the one hand with nitrogen, while the other chamber remains openly connected with the system. If the temperature in the system increases, the water expands and pushes the membrane towards the nitrogen. If the temperature decreases, the volume of the water decreases and the nitrogen can expand again. This ensures that the pressure remains constant. If there is a constant pressure, the safety valve is prevented from draining water and thus causing damage due to excessive pressure.

Different types of expansion vessels

Expansion vessels are available for many applications: for heating, drinking water or even for solar systems. The task always remains the same. However, there are 2 types of expansion vessels: open and closed systems. Closed systems are the ones we have already explained. They have a membrane so that the nitrogen is always separated from the water. Open systems do not have a membrane. As a result, the gas has direct contact with the water, which in the long run favors corrosion. Therefore, open systems are quite rare nowadays. They are located at the highest point of the system and always have a safety line.

How to recognize a defective expansion vessel?

Signs of a defective expansion vessel include a frequently dripping safety valve, uneven pressure, and gurgling and cold radiators. However, it should be noted that these side effects can also have other reasons, which is why there are methods to check whether an expansion tank is actually defective or not.

Methods for checking

Knock test:

You can determine if an expansion tank is defective or not by the sound it makes. If there is a hollow sound from about halfway along the expansion tank, everything should be in working order and there should still be enough gas. If, on the other hand, a muffled sound occurs along the entire length, the membrane has ruptured and water has flowed into the vessel.

Valve test:

In this method, you open the valve on the front of the expansion tank with a pin, it is the same valve that is used in bicycle tires. If only air escapes, there is still gas in the vessel. If water escapes, the membrane is already ruptured. You should also pay attention to the color of the water. If the water is brown, corrosion has occurred in the system.

The DIN 4807-2 specifies an annual inspection of the expansion vessel.


The expansion vessel is known by many names. So do not be confused if you find an expansion vessel, diaphragm expansion vessel or pressure equalization vessel, because it is the same device. The function is both simple and important for your heating system, as it prevents damage and energy waste. During the annual inspection of your system, you should also have the expansion vessel checked. If you or a professional finds a defect, do not hesitate to replace the expansion tank. An expansion tank has a lifetime of 5-10 years.

Autor: Jan Bittner