Heater displays F05 and shuts down; blower may not start.

Safety first: Turn off power and gas. A flue overheat condition can indicate dangerous venting problems and potential carbon monoxide risk.

• Inspect vent termination: From outside, check that the exhaust outlet is not blocked by debris, vegetation, snow, or covers. Clear any obstructions.
• Check for recirculation: Ensure the exhaust outlet is not too close to the air intake or building openings. Hot exhaust being drawn back into the heater can trigger F05.
• Examine flue sensor: With power off, remove the access panel and locate the flue temperature sensor on the exhaust collector. Inspect wiring for damage or loose connections. If the sensor is sooted or corroded, it may need replacement.
• Vent configuration: Verify that vent pipe diameter and length match Aquacomfort installation specifications. Excessive elbows or undersized venting can cause overheating; this should be evaluated by a professional.
• Do not bypass safety: Never bypass or jumper the flue sensor. If F05 persists after clearing obvious blockages, contact a qualified technician.