E027
Navien
Exhaust Overheat / Flue Gas Temperature Too High
Boiler shows E027 and locks out, often immediately at startup
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blocked or undersized exhaust vent, Failed exhaust temperature sensor, Excessive firing rate with poor flow, Heat exchanger fouling
How to Fix / Troubleshooting
Safety first: Turn off power and allow the boiler to cool. Hot flue components can cause burns.
- Step 1 – Inspect exhaust vent: Check for obstructions, improper slope, or undersized venting. Correct any installation issues per Navien guidelines.
- Step 2 – Check condensate drainage: Ensure condensate is draining freely. Standing condensate in the vent can restrict flow and overheat exhaust.
- Step 3 – Inspect exhaust temperature sensor: Locate the flue gas temperature sensor on the exhaust outlet. Check for secure mounting and wiring. Replace the sensor if damaged.
- Step 4 – Clean heat exchanger: A dirty or scaled heat exchanger can cause high flue temperatures. Have a technician clean and descale the exchanger.
Do not ignore E027; prolonged high flue temperatures can damage venting and pose safety risks.
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Repair Difficulty
Hard
4/5
Required Part
Exhaust Temperature Sensor
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