Knowing the following data:

  1. The pre-charge pressure
  2. The set pressure for the safety valve
  3. The difference in level
  4. The volume of the fluid in the system
  5. The maximum working temperature of the system
  6. The possible percentage of antifreeze glycol in the system fluid

it’s possible to calculate the required volume of the expansion vessel.


  • The calculation is valid in the hypothesis that the initial pressure (hydrostatic pressure) on the expansion vessel is not higher than the pre-charge pressure. On this point, consider that a water column of 10 meters exerts a 1 bar pressure.
  • The difference in height (meters) is the difference between the position in which the safety valve is installed and the position of the vessel. Enter a positive difference if the valve is located higher than the vessel, negative otherwise.
  • The set pressure for the safety valve, summed to that one due to the possible difference in height, must be lower than the maximum working pressure of the vessel.
  • The calculation (compliance to EN 12828, VDI 4708) provides only an indication of the required volume of the vessel and must be checked by a qualified and authorized technician.