Heater displays F19; unit may shut down due to detected flue blockage.

Safety first: Turn off power and gas. Do not operate the heater until the vent system is verified safe; risk of carbon monoxide.

• Inspect vent termination: Check the outside vent cap for nests, leaves, snow, or other obstructions. Clear any blockage.
• Check vent piping: Follow the vent run from the heater to the termination. Look for crushed sections, sagging, or improper slopes that could trap condensate.
• Internal inspection: Remove the top cover and inspect the flue collector and internal baffles for debris or corrosion that could restrict flow.
• Flue sensor: Locate the Nirvana flue blockage or draft sensor. Inspect wiring and mounting. Replace if testing shows it is stuck open/closed.
• Professional evaluation: If you cannot identify a clear blockage, have a qualified technician perform a full combustion and venting analysis.