F13 error, unit may start then quickly shut down, sometimes with gurgling noises

Safety first: Turn off power. Condensate is mildly acidic; avoid skin and eye contact.

Steps:

• Check condensate pipe: Follow the plastic condensate pipe from the heater to the drain. Look for kinks, sags, or external blockages.
• Inspect trap: Remove the condensate trap/siphon under the heat exchanger. Empty and rinse it thoroughly to remove sludge.
• Defrost if frozen: In cold weather, the external section of the condensate pipe may freeze. Thaw gently with warm (not boiling) water and insulate the pipe.
• Reassemble and test: Refit the trap, ensuring seals are correctly seated. Restore power and run the heater, checking for leaks.

Warning: Do not discharge condensate into unsuitable drains. Follow local regulations.