Heater displays H81; unit may run at reduced output or shut down.

Safety: Turn off power and allow system to cool before touching piping.

• Check valves: Ensure all isolation valves on the heater flow and return lines are fully open.
• Inspect pump operation: Confirm that the circulation pump runs when there is demand. Listen for unusual noises and feel for vibration.
• Clean strainers: If strainers are installed, isolate and remove them, then clean out debris.
• Bleed air: Remove air from the system using bleed valves. Air pockets can reduce flow and cause high return temperatures.
• Verify system design: Ensure that the hydraulic layout and pump sizing meet Buderus recommendations for the heater's output.
• Restore and test: After improving flow, restore power and monitor temperatures to confirm that H81 does not recur.