Vaillant Expansion Vessel 12L 0020181039

The Vaillant 0020181039 is a 12 litre expansion vessel used on Vaillant VIH and Unistor unvented hot water cylinders. It provides a pressurised air cushion that absorbs the volume increase as the stored water heats, keeping system pressure within safe limits. The vessel is a serviceable component of the cylinder's pressure control arrangement.

• Water discharging from the cylinder's expansion relief or temperature and pressure relief valve during heat-up
• System pressure rising sharply when the cylinder is heated and not settling back as it cools
• The vessel feels heavy or sounds full of water when tapped, indicating a failed internal diaphragm
• Pressure needs topping up repeatedly, or the gauge reads high with no draw-off
• Visible corrosion, weeping or damp staining around the vessel connection

• Confirm the cylinder model from the data plate, as several VIH and Unistor variants use different vessel arrangements
• Check the capacity and connection details of the vessel being removed against the replacement
• Verify the part number 0020181039 against the manufacturer's parts list for your cylinder
• Check the cylinder's charge pressure and relief valve operation before refitting, as a failed relief valve can mimic vessel failure