F15 error, unit indicates a condensate or drainage problem.

Safety: Switch off power. Condensate is mildly acidic; wear gloves and eye protection when handling.

• Inspect condensate trap: Locate the plastic condensate trap at the base of the heat exchanger. Remove and flush it with clean water to remove sludge and debris.
• Check discharge pipe: Follow the condensate pipe to its termination. Ensure it is not frozen, kinked, or blocked. Thaw frozen sections gently with warm towels; do not use open flames.
• Verify slope: Ensure the condensate pipe slopes continuously downward with no sags that can collect water.
• Check level sensor: If your model uses a condensate level sensor, inspect and clean it according to the manual. Replace if corroded or not switching.
• Reset and test: Reassemble the trap, restore power, and run the heater. Confirm that condensate flows freely during operation.