Ariston 406801 KIT A 2L Expansion Vessel & Non-Return Valve, WhiteThe expansion vessel prevents any build up of pressure within the domestic hot water system, while the non return valve stops any backflow of hot water into the cold water main. Ideal to use with our Andris R or Andris Lux Eco water heaters. _Set of 2L expansion vessel and non return valve _To protect closed water heating systems from excessive pressure _For optional use with Ariston Andris range water heaters _Model number: 406801 _Fully UKCA