Boiler locks out with E9; overheating or dry-fire condition

Safety: Turn off the boiler and allow it to cool completely. Do not repeatedly reset an overheating boiler.

• Check system pressure: Verify that the pressure gauge shows adequate pressure (around 1–1.5 bar cold). Top up via the filling loop if low and then bleed radiators.
• Open valves: Ensure all isolation valves under the Junkers boiler and on the heating circuit are fully open.
• Bleed air: Bleed air from radiators and any manual vents near the boiler to restore circulation.
• Reset STB: Some models have a manual reset button on the safety thermostat inside the boiler (technician only). Press only once after resolving circulation issues.
• Technician checks:
  • Verify pump operation and replace the circulation pump if seized or not delivering flow.
  • Inspect the primary heat exchanger for scale or sludge; flush or replace if necessary.
  • Test the safety temperature limiter and replace if it trips at incorrect temperature.