F5 error, unit shuts down during operation, often associated with overheating

Safety: Turn off power and close isolation valves. Allow the heater to cool before touching any hydraulic components.

• Bleed air from system: Open the automatic air vent on the top of the heat exchanger or pump housing (if fitted). Use the manual bleed screws on nearby radiators or service valves to remove trapped air.
• Check circulation pump: Listen for pump operation when there is a hot water demand. If silent, check the pump connector at the main PCB. With power off, remove the pump motor and check for seized impeller. Free gently if stuck; replace if noisy or burnt.
• Inspect plate heat exchanger: If hot water temperature fluctuates and F5 occurs, the DHW plate heat exchanger may be scaled. Isolate and remove the plate exchanger from the Viessmann hydraulic block and flush with descaling solution according to manufacturer instructions.
• Verify flow sensor: Ensure the DHW flow sensor or turbine (often in the cold inlet of the hydraulic group) is clean and spins freely. Replace if the sensor does not register flow in diagnostics.
• Reset fault: Restore power, open valves, and purge air. Reset the F5 error and test under sustained hot water draw.