Boiler displays error 18, often associated with condensate or flue gas recirculation issues.

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

• Check condensate pipe externally: Look for freezing or kinks in the plastic condensate pipe, especially outside. Thaw gently with warm water if frozen.
• Clean condensate trap: Remove the condensate trap from the bottom of the Biasi boiler, empty it, and rinse out any sludge or debris. Refit ensuring seals are correctly seated.
• Verify pipe fall: Ensure the condensate pipe runs downhill continuously to the drain with no upward loops that could trap water.
• Engineer flue check: A technician should confirm that the flue terminal is correctly positioned and that there is no recirculation of flue gases into the air intake.