Burner locks out with indication of overheat or safety thermostat trip, furnace very hot to the touch

Safety: Turn off power and fuel supply. Allow the furnace to cool completely before touching any internal components.

Steps:

• Reset high limit (if manual reset type): Locate the manual reset high limit on the furnace or Riello burner. Press the reset button once. If it trips again, do not keep resetting.
• Check air flow: Ensure all supply and return vents are open. Replace clogged air filters. Verify that the blower fan runs when there is a call for heat.
• Inspect flue and heat exchanger: Check for visible blockages in the flue. Look for signs of heavy soot or restricted passages in the heat exchanger. Do not operate the furnace with a blocked flue.
• Verify circulation (hydronic): For boilers, ensure the system pump is running and valves are open so heat can be carried away from the heat exchanger.

Persistent A04 indicates a serious overheating or airflow problem. Have a licensed technician inspect the heat exchanger, flue, and safety controls.