Boiler displays A05 and stops, often after overheating or poor circulation

Safety: Allow the boiler to cool completely before resetting. Do not repeatedly reset an overheating boiler; risk of damage or scalding.

• Check system pressure: Ensure the pressure gauge reads within the recommended range (typically 1.0–1.5 bar when cold). Top up via the filling loop if low, following the manual.
• Bleed radiators: Air in the system can cause poor circulation. Bleed radiators starting from the lowest floor, then recheck pressure.
• Verify valves: Ensure all isolation valves under the boiler and on the heating circuit are open.
• Reset overheat thermostat: Some De Dietrich models have a manual reset button on the safety thermostat inside the boiler. Press only once after cooling. If it trips again, stop and call a technician.
• Technician checks: A professional should check the circulation pump, clean the primary heat exchanger, and verify the overheat thermostat operation.