Boiler locks out with F5, no ignition, fan may run continuously

Safety: Flue and combustion air issues must be handled carefully. Never modify the flue system yourself.

• Reset once: Attempt a single reset. If F5 returns, do not keep resetting.
• Check condensate system: Inspect the Viessmann condensate trap and pipe for blockages or freezing (in cold weather). Thaw gently and clear any debris.
• Visual flue check: Ensure the flue is properly supported and not crushed or disconnected.
• Inspect pressure tubes: With power off, remove and blow through the small tubes to the air pressure switch to ensure they are clear (do not blow into the switch itself excessively).
• Technician tasks: A professional should test the air pressure switch contacts, verify correct differential pressure, and confirm flue design complies with Viessmann installation instructions. Replace the air pressure switch if faulty.