Boiler locks out with F.04, no ignition, fan may run continuously

Safety first: Turn off power and allow the boiler and piping to cool before touching any components.

• Check system pressure and air: Verify the system pressure on the boiler gauge is within the range specified by Lochinvar (commonly 12–25 psi for residential hydronic). Bleed air from high points and radiators using bleed valves. Low flow due to air can cause overheating.
• Inspect circulator pump: Confirm the primary boiler circulator (often a Grundfos or Taco pump on Lochinvar systems) is running when there is a call for heat. Listen for motor operation and feel for vibration. If the pump is hot but not circulating, it may be seized.
• Check isolation valves and strainers: Ensure all isolation valves around the boiler and closely spaced tees are open. Clean any Y-strainers or dirt separators that may be clogged and restricting flow.
• Inspect high limit sensor: Locate the outlet water temperature sensor/high limit on the supply header or heat exchanger outlet. Check wiring for damage or loose connections. If the sensor is corroded or physically damaged, it should be replaced.
• Reset and test: After restoring flow and bleeding air, reset the boiler. Monitor supply and return temperatures on the display to ensure normal differential and no rapid temperature spikes.

Professional help: If F.04 persists despite good flow and pressure, a technician should ohm-test the high limit sensor and verify control board readings.