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Calculation and Selection of Expansion Tanks

The height from the connection point of the expansion tank to the upper point of the heating system
Maximum pressure for the heating system at the connection point of the expansion tank
Temperature range T1 - T2 of the heating system
Volume of water in the heating system
If the volume is unknown

Calculation of Expansion Tank

Calculation of an expansion tank is performed to determine its volume, the minimum diameter of the connecting pipeline, the initial pressure of the gas space, and the initial operating pressure in the heating system.

The method of calculating expansion tanks is complex and routine, but in general, the following relationships can be established between the volume of the tank and the parameters affecting it:

  • The larger the capacity of the heating system, the larger the volume of the expansion tank.
  • The higher the maximum water temperature in the heating system, the larger the tank volume.
  • The higher the maximum permissible pressure in the heating system, the smaller the volume of the tank.
  • The smaller the height from the installation point of the expansion tank to the upper point of the heating system, the smaller the volume of the tank.

Since expansion tanks in the heating system are necessary not only to compensate for the changing volume of water but also to replenish minor hot-water leaks, a certain amount of water, called the operating volume, is provided in the expansion tank. In the calculation algorithm given above, the operational volume of water is laid down in the amount of 3% of the capacity of the heating system.

Selection of Expansion Tanks

Selection of an expansion tank should be performed taking into account its temperature characteristics and strength. The pressure and temperature at the point of connection of the tank should not exceed the maximum permissible values.

The volume of the expansion tank must be greater than or equal to the volume obtained as a result of the calculation. There are no negative consequences from overestimating the volume, beyond the calculated one.

If the installation of expansion tanks is planned indoors, it should be taken into account that tanks with a diameter of more than 750 mm and a height of more than 1.5 m may not pass through the door, and mechanization means will be needed to move them. In this case, it is better to prefer not one, but several membrane tanks of smaller capacity.


1. When using glycol mixtures as hot-water, it is recommended to choose an expansion tank with a volume that is 50% larger than the calculated one.

2. The first sign of an incorrectly calculated expansion tank or its incomplete adjustment is when the safety valve often trips.

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