Heater shows E9 and shuts down; water may be very hot just before lockout.

Safety: Turn off power and gas. Do not touch hot surfaces. Allow the unit to cool fully before servicing.

• Check system pressure and air: Verify that the system is properly filled and bled. Open air vents or bleed valves to remove trapped air from the heat exchanger and piping.
• Inspect circulation pump: Confirm that the internal or external pump runs when there is a heat demand. Listen for operation and feel for vibration. If the pump is seized or silent, it may need replacement.
• Clean heat exchanger: If overheating recurs, the heat exchanger may be scaled. Follow Buderus procedures for descaling or flushing the heat exchanger with an approved cleaning solution.
• Reset high-limit (if manual): Some models have a manual reset high-limit thermostat. Locate it on the heat exchanger and press the reset button once the unit has cooled.
• Check high-limit sensor: Test the high-limit sensor or thermostat for continuity and replace if defective.
• Restore and test: After correcting flow and scaling issues, restore power and gas, reset the fault, and monitor operation to ensure temperatures remain within normal range.