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In water systems that increase or decrease the volume of water, such as hot water systems, central heating systems, etc. - it is imperative to use a expansion vessel. For this purpose, both open and closed expansion vessels are used. The use of enclosed expansion vessel has several advantages over open expansion vessels. Keep in mind - the expansion tank shuld always be installed on the return pipe.

Advantages of useing enclosed expansion vessels:

  1. When the water evaporates in the central heating system, the vapor stays in the system and condensates back to the water when the system cools down. The water can go to 100C (the system will never run out).

  2. Under the influence of temperature, air or steam is released from the water, which can produces hydraulic shock. A properly positioned expansion tank is a suppressor for hydraulic shocks, reducing system breakdowns and noise levels.

  3. Excessive air contamination in water causes corrosion of metal parts in the system, which reduces the life of the system. To avoid mixing air with water circulating in the system, the expansion vessel is equipped with a membrane (bag), where the membrane separates system water from air. While protecting the expansion tank from corrosion as the water is located inside the membrane (inside the bag) and the inner walls of the tank are only in direct contact with compressed air or compressed gas.

The expansion tank made of ordinary steel has a space between the membrane and the expansion tank wall filled with Nitrogen (by manufacturer) to prevent corrosion.   , Nitrogen has no odor, taste and colorless gas. Nitrogen dissolves in water to a minimum and is not toxic. In the air he is 78% by volume and from there he is also being produced.

NB! Normal air filling reduces the lifespan of the expansion tank, as the air contains 5% water (by weight) and 21% oxygen (by volume). It is a prerequisite for the rusting of the expansion tank in a warm room.

Pressure in the expansion vessel

For a closed system, the pressure in the expansion tank must be 0.1 bar higher than the static pressure of the system. If the pressure in the tank is significantly higher than the aforementioned, the efective capacity of the expansion vessel is substantially reduced for the expansion due to the heating of the water. Therefore, the pressure in the system may rise too high and the system will break. If the pressure in the tank is lower than the static pressure of the system, the expansion vessel in the system does not have a sufficient water reserve and the system will suck through the air intakes of the automatic air vent valves, the circuit stops in the upper radiators and the premises do not heat up.

The size of the expansion vessel

Expansion vessel shuld be 7...10% of system water volume.