Worcester Bosch Expansion Vessel Assembly 12i to 24i 8716117023

The Worcester Bosch 8716117023 is an expansion vessel assembly for the Greenstar i range of gas boilers. It is a sealed cylinder divided internally by a rubber diaphragm, with one side holding system water and the other holding a pre-charged gas cushion. As the water in a sealed heating system heats up and expands, the diaphragm flexes to absorb the extra volume, keeping system pressure within a stable working band instead of stressing pipework, seals and the pressure relief valve.

• System pressure climbs sharply as the boiler warms up and falls back again once it cools, with the gauge swinging across a wide range.
• The pressure relief valve discharges water, often with the gauge sitting close to 3 bar during a heating cycle.
• Water is present at the vessel's charging valve, or the gas charge cannot be held.
• The vessel feels heavy or sounds full of water when tapped, indicating the diaphragm has failed and the unit is waterlogged.
• The boiler repeatedly locks out on low pressure and the system needs topping up more often than usual.

• Check the GC number on the appliance data badge against the compatibility list to confirm the vessel suits your model.
• Compare the number stamped on the existing vessel with 8716117023 and the alternative numbers listed above.
• Inspect the sealing washer at the vessel connection and replace it if it is compressed, hardened or damaged.
• This part is fitted to a gas appliance; installation must be carried out by a Gas Safe registered engineer.