Boiler displays E18, burner stops, and the system pressure may be too high.

Safety first: Hot water under pressure can cause scalding. Allow the system to cool before venting water. Do not exceed the maximum pressure indicated on the boiler rating plate.

Step-by-step checks:

• 1. Read pressure gauge: If the gauge shows above 2.5–3.0 bar when cold, the system is over-pressurised.
• 2. Reduce pressure: Bleed water from a radiator or drain point into a container until the pressure drops to about 1.2 bar.
• 3. Check filling loop: Ensure both valves on the filling loop are fully closed and that the hose is removed if required. A leaking filling loop can slowly overfill the system.
• 4. Expansion vessel check (basic): With the system pressure at zero and isolated, briefly press the Schrader valve on the expansion vessel. If water comes out, the diaphragm has failed and the vessel must be replaced.
• 5. Reset and monitor: After correcting pressure, reset the boiler and monitor pressure rise during heating. Rapid rise suggests expansion vessel issues.

When to stop: Expansion vessel replacement and pressure sensor diagnosis should be done by a heating engineer.