Navien furnace intermittently shuts down with E027, especially during long runs

Safety: Turn off power and gas. Allow the unit to cool fully before touching flue components.

• Check water flow: Confirm all valves are open and the circulator pump is operating. Clean the system strainer and bleed air from the system to ensure proper flow.
• Clean burner and heat exchanger: Remove the burner assembly and inspect for soot or debris. Clean the burner surface and inspect the primary heat exchanger for buildup. If heavily fouled, have a technician perform a full combustion chamber cleaning.
• Inspect exhaust temperature sensor: Locate the flue gas temperature sensor on the exhaust outlet. Check wiring and connector. Remove the sensor and inspect for damage or heavy soot. Replace if suspect.
• Verify gas input: Only a licensed technician should adjust gas input. If the unit was recently installed or converted, have a professional verify manifold pressure and combustion using a combustion analyzer.
• Reset and monitor: Restore power and gas, reset the unit, and monitor flue temperature readings (if available in service mode). Persistent E027 requires professional combustion analysis and possible component replacement.