F16 displayed, unit reports flue gas recirculation or combustion air fault

Safety: Turn off the heater and do not operate it if flue gases may be entering the room. Risk of carbon monoxide exposure.

• Inspect flue terminal: Check that the flue terminal clearances meet Viessmann and local code requirements. Ensure exhaust is not being drawn back into the air intake.
• Check flue seals: Disassemble accessible sections of the concentric flue and inspect inner and outer seals. Replace any hardened, cracked, or missing seals with Viessmann-approved parts.
• Clear air intake: Make sure the air intake is not blocked by debris, snow, or vegetation.
• Room ventilation: For non-room-sealed installations, ensure adequate ventilation openings are present and not blocked.
• Professional combustion check: Have a qualified technician perform a combustion analysis and verify flue integrity before returning the unit to service.