Procombi 8L Expansion Vessel 20138102. This is a small chamber that is used to protect closed water heating systems from pressure surges.
Procombi 85HE Expansion Vessel 20138102 is filled with air that acts as a cushion against pressure surges within the system.
This also refereed to as an boiler Expansion Tank, and it replaces old part number 2204 and R2204.