BoilerFaultCodesTechnical description
Expansion Vessel 12 Litre (Primary)
What causes this fault?
This fault occurs when the boiler's internal expansion vessel has failed or lost its air charge, leaving no room for water to expand as it heats up. The resulting surge in pressure often forces the safety valve to leak, which eventually leaves the system with too little water to operate safely. High pressure readings followed by the boiler losing pressure entirely are tell-tale signs of this component failing.
This issue often becomes apparent during the first cold spell of autumn when the heating is turned on for long periods, causing the water to expand more than it does in the summer.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.