Navien furnace display shows normal operation, but the house is overheating.

Safety: Turn off power before working on thermostat wiring or zone valves.

• Check thermostat location: Ensure the thermostat is not near heat sources, direct sunlight, or drafts. Relocate if necessary.
• Verify thermostat calibration: Compare the thermostat reading to an accurate thermometer. Replace the thermostat if it reads significantly off.
• Inspect zone valves: Confirm that zone valves close when there is no call for heat. Replace stuck-open valves.
• Adjust maximum supply temperature: In the Navien installer menu, reduce the maximum supply water temperature to a level appropriate for the emitters (e.g., radiant floors vs. baseboard) to prevent overshooting room temperatures.