How to Replace a Boiler Expansion Vessel (UK Guide)

Diagnose vessel failure

Depress the Schrader valve on the vessel — if water squirts out, the diaphragm has burst and it must be replaced. If only air escapes but pressure still climbs when heating, the pre-charge is low: recharge to 0.75–1.0 bar cold.

Sizing

Most UK combis ship with an 8 L vessel — undersized for systems over 100 L. Fit an external 12 L or 18 L in-line vessel on larger properties.

• 8 L — up to 100 L system volume.
• 12 L — 100–150 L (typical 3-bed house with 10 rads).
• 18 L — 150–200 L, larger unvented + heating combined.

Refit procedure

Isolate flow/return, drain to zero on the gauge, unclip the vessel, transfer the flexible hose. Charge to 0.75 bar with a foot pump before refilling the system to 1.2 bar.