Viessmann Membrane Expansion Vessel 12L 7822351

The Viessmann 7822351 is a 12-litre membrane expansion vessel used in the sealed heating circuit of a Viessmann gas boiler. The vessel absorbs the volume increase of the system water as it heats, keeping circuit pressure within the working range. It is listed by the manufacturer under the supplied product name MEMBRANE EXPANSION VESSEL 12L.

• System pressure climbs sharply once the boiler is up to temperature, then falls again when it cools
• Pressure relief valve discharges water during heating cycles
• Water or moisture appears at the Schrader valve on the vessel when the charge is checked
• Boiler locks out or shows a low-pressure fault shortly after filling the system
• The vessel feels heavy or waterlogged and does not hold its air charge

• Confirm the boiler model and serial number against the fitment list for this part
• Check the existing vessel's capacity and mounting arrangement match the 12-litre unit
• Verify the air charge side of the system and the vessel connection are intact before replacement
• Check the system pressure gauge and pressure relief valve operation as part of the same job