Ariston Expansion Vessel, 6 Litre Expansion vessels, also known as expansion tanks contain water and air from the central heating system. The purpose of this part is to maintain the right level of pressure in the system. The expansion vessel is essential as it prevents pressure build-ups and water/gas leaks. See suitability below: Ariston EXCALIBUR 80 Ariston EXCALIBUR 100 Ariston INTESA TP 23 MFFI Ariston INTESA TP 30 MFFI Ariston MICROGENUS 23 MFFI Ariston MICROGENUS 27 MFFI Ariston MICROSYSTEM 15 RFFI De serie MICROSYSTEM 10 RFFI MICROSYSTEM 15 RFFI MICROSYSTEM 21 RFFI MICROSYSTEM 28 RFFI